Growth and development characteristics of fruit and vegetative bud outgrowth of Prunus sibirica L. in relation to physiological fruit drop

Abstract

Background

Prunus sibirica L. is one of the most pivotal eco-economic tree species in China’s arid and semi-arid areas. The phenomenon of physiological fruit drop in P. sibirica L. is severe, and understanding fruit growth patterns and drop characteristics is crucial for high-quality cultivar production. However, there are few reports on P. sibirica fruit development and physiological fruit drop.

Results

In this study, we investigated the characteristics of fruit development, vegetative bud outgrowth, and fruit abscission, and explored the dynamic features of sugar metabolism in different tissues during physiological fruit drop and its relationship with fruit drop. The results showed that the fruit and vegetative bud outgrowth of the “Shanxing No. 1” variety exhibited an S-shaped growth pattern with three physiological stages. The flower and fruit drop of “Shanxing No. 1” lasted for about 70 days with an 89.73% total drop rate and three abscission peaks, which could be divided into the flower abscission stage mainly caused by pistil abortion, rapid fruitlet abscission stage, mainly caused by carbohydrate competition between the fruit and vegetative bud outgrowth; and slow fruit abscission stage mostly related to seed abortion. The perspective of the vegetative buds removal experiment further proved the competition between fruitlet and vegetative bud outgrowth simultaneously. During physiological fruit drop, the sugar contents and activities of sucrose metabolism enzymes in different tissues showed regular changes, corresponding to the dynamic law of fruit drop. Sucrose metabolism was mainly dominated by decomposition, and the enzymes involved in sucrose decomposition played a significant role. Acid invertase and sucrose synthase (decomposition direction) were the key enzymes regulating fruit abscission in P. sibirica L.

Conclusions

These results laid a foundation for revealing the physiological characteristics and physiological mechanism of P. sibirica L. fruit development, and also provided a theoretical basis for high-quality and high-yield cultivation of P. sibirica L.

Introduction

Prunus sibirica L. (family: Rosaceae) grows widely in northeast, north, and northwest China, with an area of approximately 1,700,000 ha and an annual seed production of over 192,500 tons [1–3]. As a native species in northern China, P. sibirica L. is a pioneer tree species for ecological construction because it is tolerant to cold, drought, and barren conditions [4]. In addition, Siberian apricot seeds have important economic value, are rich in oil, protein, vitamins, and amino acids, and have great potential for development in food, medicine, and industry [2, 5, 6]. However, low and unstable yields, mainly related to the abnormal development of floral organs and physiological fruit drop, have affected the development of the P. sibirica L.-based industry. Physiological fruit drop is a common phenomenon in fruit development, but the substantial fruit loss significantly impacts yield and affects fruit quality, garnering considerable attention. Previously, scholars have mainly conducted research on pistil abortion [7], male sterility [8], late frost damage [9], and seed composition [10, 11] in P. sibirica L. However, to our knowledge, there are few reports on fruit development, and research is urgently needed to help solve the problem of physiological fruit drop.

Producing fruit is the primary economic purpose of Non-Timber Forest, and understanding fruit growth and development is the basis for breeding and selection of good candidate varieties [12]. The logistic growth curve model is widely used to depict the growth process of fruits and classify their growth and development into various phases. This model has proven successful in simulating fruit growth patterns and helping with cultivation and management strategies for various plants, such as blackberry [13], sweet orange [14], and blueberry [15]. Studies have reported on the growth and development of fresh edible and kernel-used apricots (P. armeniaca L). The fruit growth of some apricot (P. armeniaca L.) varieties, including ‘Heiye’, ‘Saimaiti’, and ‘Jianali’, could be divided into four or five stages [16], and that of other varieties, such as ‘Longwangmao’ and ‘Youyi’, could be divided into two phases: rapid growth and slow growth [17]. Depending on developmental characteristics, management measures at different stages can improve fruit quality. For example, in the early stone hardening stage of apricot (P. armeniaca L.), suitable application of fast nitrogen fertilizer and foliar fertilizer can ensure the development of seed kernels [18].

Studies have investigated the physiological fruit drop of many species, including camellia [19], betel nut [20], grape [21], sweet cherry [22, 23], pecan [24], citrus [25], and lychee [26], etc. Generally, the physiological fruit drop of trees is usually divided into three or four phases. For example, macadamia nuts contain three stages of fruit drop: the first occurs in the first 2 weeks after anthesis when 90% of the flowers fall off; the second stage occurs 3–10 weeks after anthesis when ~ 80% of the initial fruitlet falls off, which is mainly related to the growth of spring tips; the third stage occurs 11 weeks after flowering to maturity [27]. Zeng [27] et al. sprayed forchlorfenuron on the leaves and fruit bunches of macadamia 2 weeks after anthesis, which increased the fruit setting rate and reduced fruitlet abscission.

Prunus is a hysteranthous plant, meaning that vegetative bud begins to grow when the fruit is set. Competition for carbohydrates between shoots and fruit induces fruit abscising. Atkinson [28] and Nzima [29] et al. confirmed the fierce competition for sugar nutrition between fruits and leaves during fruit development. In peaches [30], lychee [31], and macadamia [32], carbon nutrients can be transported between the shoots and fruit. Huang [33] et al. reported that increased in enzyme activity in the direction of sucrose decomposition was related to citrus fruit abscission. Although previous studies have shown that fruit- leaf nutrition competition and young fruit nutritional stress lead to fruit abscission, few reports exist on how sugar metabolism changes in different tissues during young fruit physiological abscission, how it relates to fruit abscission, and which key enzyme activities of sucrose metabolism play a major role.

The objective of this study was to determine the characteristics of fruit abscission in P. sibirica L. “Shanxing No.1” (Fig. 1). To this end, we investigated the development of fruit and vegetative bud outgrowth and the characteristics of physiological fruit drop and explored the relationship between them. Additionally, we measured the content of sugar components and the activity of key enzymes in sucrose metabolism in different tissues during the physiological fruit drop period, analyzed their relationship with fruit drop, and elucidated the physiological mechanism of fruit drop induced by vegetative bud outgrowth. These results provide a scientific basis for developing management measures for producing and cultivating of P. sibirica L.

Fig. 1.

Characteristics of variety ‘Shanxing No. 1’ in this study. A Individual of the 7-year-old ‘Shanxing No. 1’. B Complete flower (left) and pistil abortive flower (right). C Vegetative bud outgrowth at 14 DAFB. D Green fruit stage. E non-abscising fruit (green circle) and abscising fruit (red circle) in first abscission peak. F non-abscising fruit (green circle) and abscising fruit (red circle) in the second abscission peak. G Fruit maturation. Bar = 1 cm

Results

Dynamic change in P. sibirica L. fruit development

P. sibirica L. fruit growth was measured based on morphological changes, such as longitudinal/transverse diameter, fresh weight, and volume. Generally, the fruit longitudinal diameter (FLD), fruit transverse diameter (FTD), fruit thickness (FT), single fruit weight (SFW), and fruit volume (FV) of fruit increased alongside development by displaying an S-shape curve (Fig. 2A, D). In contrast, the changing trend of the fruit shape index was opposite to that of the five diameters (Fig. 2B). These morphological traits fit well with the logistic model, each with an estimated intercept and a quadratic coefficient of 0.99 (Table 1). These changes occurred in three prominent phases: fruit set (S1), fruitlet rapid growth (S2), and slow growth (S3) (Figs. 2 and 3).

Fig. 2.

Dynamic changes of fruit growth of ‘Shanxing No. 1’. A Growth curve of fruit diameters. B Changing curve of fruit shape index. C Amount of growth for fruit. D Dynamic changes of single fruit weight and fruit volume. FLD, Fruit longitudinal diameter; FTD, Fruit transverse diameter; FT, Fruit thickness; SFW, Single fruit weight; FV, Fruit volume

Table 1.

Logistic models fitted for longitudinal, transverse, thickness, fresh weight, and volume of fruit according to R²

Dependent variable, Y	Logistic model	R 2
FLD (mm)	Y = 0.998/(1 + 9.749e−8.581 x)	0.99
FTD (mm)	Y = 0.994/(1 + 31.746 e−10.76 x)	0.99
FT (mm)	Y = 0.996/(1 + 27.626 e −10.609 x)	0.99
SFW (g)	Y = 0.996/(1 + 949.248 e −16.237 x)	0.99
FV (mm3)	Y = 0.947/(1 + 483.491 e−14.397 x)	0.99

FLD Fruit longitudinal diameter, FTD Fruit transverse diameter, FT Fruit thickness, SFW Single fruit weight, FV Fruit volume

Fig. 3.

Non-abscising and abscising fruit status during Siberian apricot fruit development. Bar = 1 cm

Based on these results, the growth rate was slow during the first two weeks of development (S1). The increase in the three diameters followed the order: FLD > FT > FTD (Fig. 2C), all of which were less than 2 mm. The shape index decreased gradually. At the S2 stage, the growth increased exponentially after 14 day after full bloom (DAFB) and stopped at 42 DAFB, lasting approximately four weeks. During this period, the FLD, FTD, and FT increased by 17.97, 23.19, and 19.51 mm, respectively (Fig. 2C). Furthermore, the growth rate of FTD was higher than that of FLD, with the diameter of the former surpassing that of the latter at 35 DAFB (Fig. 2A). The fruit shape index exhibited a cliff-like decline at this stage (Fig. 2B). Afterward, growth became constant (S3 stage), lasting approximately 6 weeks before senescence occurred, which mainly completed the hardening of the nutlet and rapid growth of the cotyledon (Fig. 3).

The fruit changed in terms of SFW and FV, which were significantly correlated (Fig. 2D). Initially, both were not obvious at less than 0.03 g and 0.03 mm³ at the S1 stage. When reaching the S2 stage, FV expanded by 75.76%, and SFW increased sharply by 72.32%. The growth rates of SFW and FV increased gradually from 28 to 42 DAFB, further indicating the main growth period of Siberian apricot. It is worth mentioning that the fruit fresh weight and volume continued to increase significantly between 42 and 56 DAFB in S3 (Fig. 2D). This growth was related to a continuous increase in FTD, stone hardening of the nutlet, and seed kernel formation (Figs. 2A and D and 3). The growth of the abscising fruit was similar to that of the non-abscising fruit, but the diameters were significantly smaller than those of the non-abscising fruit (Fig. 3).

Dynamic change in P. sibirica L. seed development

The growth of P. sibirica L. seeds followed an S-shaped curve, with three distinct stages observable in the seed longitudinal diameter (SLD), seed transverse diameter (STD), seed thickness (ST), single seed weight (SSW), and seed volume (SV), including the first stage of slow growth (0–14 DAFB, S1), the second stage of rapid growth (14–42 DAFB, S2), and the third stage of restrained growth (42–84 DAFB, S3) (Figs. 3 and 4). Seed size fit well with the logistic model, with an estimated intercept and quadratic coefficient of 0.99 (Table 2). From 0 to 28 DAFB, the growth rate followed the order SLD > STD > ST (Fig. 3C). The seed interior was completely water-soaked and gelatinous (Figs. 3 and 5A). During development, it gradually became translucent and gelatinous (Figs. 3 and 5B). At 42 DAFB, the size of the seeds essentially ceased to change until maturity, mainly involving the transformation of its internal structure (Figs. 3 and 5).

Fig. 4.

Dynamic changes of seed growth of ‘Shanxing No. 1’. A Growth curve of seed diameters. B Changing curve of seed shape index. C Amount of growth for seed. D Dynamic changes of single fruit weight and fruit volume. SLD, Seed longitudinal diameter; STD, Seed transverse diameter; ST, Seed thickness; CTD, Cotyledon transverse diameter; CT, Cotyledon thickness; SSW, Single seed weight; SV, Seed volume

Table 2.

Logistic models fitted for longitudinal, transverse, thickness, fresh weight, and volume of seed according to R²

Dependent variable, Y	Logistic model	R 2
SLD (mm)	Y = 0.999/(1 + 8.467 e−9.229 x)	0.99
STD (mm)	Y = 0.985/(1 + 16.956 e−9.726 x)	0.99
ST (mm)	Y = 0.980/(1 + 7.846 e−8.333 x)	0.99
SSW (g)	Y = 0.964/(1 + 1263.696 e −18.091 x)	0.99
SV (mm3)	Y = 0.989/(1 + 596.622 e −16.505 x)	0.99

SLD Seed longitudinal diameter, STD Seed transverse diameter, ST Seed thickness, SSW Single seed weight, SV Seed volume

Fig. 5.

Developmental changes of ‘Shanxing No. 1’ fruit and kernel. A 21 DAFB. B 42 DAFB. C 49 DAFB. D, 56 DAFB. E The kernel of 56 DAFB. F The kernel of 63 DAFB. WG, Water-soaked gelatinous; TG, Translucent gelatinous; C, Cotyledon; CC, Complete cotyledon; N, Nutlet. Bar = 1 cm

The growth of P. sibirica L. seed kernel exhibited two stages: rapid and slow growth (Fig. 4A). Starting at 42 DAFB (early May), tender white cotyledons gradually grew from the tip of the nutlet and began to form a kernel (Fig. 5B), followed by rapid growth and development into two complete cotyledons by 56 d (late May) (Fig. 5D). This was a period of rapid growth, mainly in terms of changes in seed kernel size and morphology, lasting ~ 2 weeks (Fig. 5B–D). Over time, the seeds began to grow slowly, and their volume, shape, and size remained constant. The seed coat gradually turned brown thereafter. Seed kernels at this stage mainly consisted of the accumulation and transformation of inclusions.

Dynamic change in P. sibirica L. vegetative bud outgrowth

As shown in Fig. 6A, the leaf growth of P. sibirica L. can be divided into two phases: rapid and slow growth. From 14 to 35 DAFB, there was a rapid growth period, and the leaf length and width grew to 58.48 mm and 43.46 mm, respectively. As the functional leaf matured after 35 DAFB, the leaf width did not changed, and the leaf length grew slowly. There were three stages in the growth of the new shoots of P. sibirica L., including first rapid, slow, and second fast growth periods. Siberian apricot leaf buds sprout in early April. After leaf bud sprouting, the 1-year shoot (OYS) grew rapidly until 14–35 DAFB (from mid-April to early May), which was the first rapid growth period, reaching the first growth peak (Fig. 6B). The period from 35 to 49 DAFB was the slow growth period of OYS. From 49 to 70 DAFB, the secondary rapid growth of OYS occurred.

Fig. 6.

Dynamic changes of vegetative bud outgrowth of ‘Shanxing No. 1’. A the growth curve of the leaf. B the growth curve of OYS. LL, Leaf length; LW, Leaf width; OYSL, 1-year shoots length, OYSC, 1-year shoots coarseness

The fruit drop pattern of P. sibirica L.

There were three peaks of flower and fruit drop in ‘Shanxing No. 1’ (Fig. 7A), and the cumulative fruit dropping rate (CDR) was 89.87% (Fig. 7B). During the blooming stage, the relative fruit dropping rate (RDR) and CDR of flower drop showed an upward trend, and the number of flowers that dropped at 7–14 DAFB (28.39%) was significantly higher than that at 0–7 DAFB (18.00%) and showed the first drop peak at 14 DAFB in the fruit set stage (S1). The second drop peak occurred at 28 DAFB, with an RDR of 55.62%, significantly higher than that in the other periods (Fig. 7A, C). The third drop peak reached 49 DAFB at a rate of 25.06% (Fig. 7A, D). After the second peak, the RDR was less than 10%. Briefly, flower abscission and physiological fruit drop lasted for approximately 2 weeks and 56 d, respectively.

Fig. 7.

Dynamic changes of fruit drop of ‘Shanxing No. 1’. A Relative fruit dropping rate; B Cumulative fruit dropping rate. C First abscission peak fruit; D Second abscission peak fruit. Lowercase letters represent significant differences at the 0.05 level

According to the morphology of fallen flowers/fruit and RDR, the fruit drop process of ‘Shanxing No. 1’ could be divided into three stages. The first stage was within 2 weeks after anthesis (early and mid-April), which mainly involved flowers (Fig. 4), and the final fruit-bearing rate was ~ 58%. The second stage was the fruitlet rapid abscission period (14–35 DAFB, late April), resulting in ~ 70% of the initial number of fruit to shed (Fig. 7C). The third stage was the fruit slow abscission period (35–70 DAFB, mid-May to early June), during which fruit drop was prolonged. Only ~ 11% of the initial fruit number was lost (Fig. 7D). Therefore, early control of fruitlet drop plays a critical role in yield.

The fruit dropping rates of different crowns were shown in Fig. 8. The RDR during the same period was mostly inner > middle/outside. At the first fruit abscission peak, the RDR at different sites on the crown differed in the following order: inner (67.52%) > middle (59.09%) > outer (53.56%), but the difference was not significant (Fig. 8A). The RDR of the inner crown was significantly higher than that of the middle and outer regions at the second fruit abscission peak (Fig. 8A). The CDR were 95.61%, 89.73%, and 87.48% in the inner, middle, and outside of crowns, respectively. From 0 to 21 DAFB, the CDR of the inner crown was significantly higher than that of both the middle and outside crowns, and there was no significant difference in the number of abscised fruits in different crowns from 49 to 70 DAFB (Fig. 8B).

Fig. 8.

Dynamic changes of fruit drop in different tree-crown locations of ‘Shanxing No. 1’. A Relative fruit dropping rate; B Cumulative fruit dropping rate. Lowercase letters represent significant differences at the 0.05 level

The fruit drop characters of P. sibirica L.

While investigating the fruit dropping rate, abscised flowers/fruits were collected and returned to the laboratory for observation and statistical analyses. The results showed that fallen flowers could be divided into three types: pistil abortion, incomplete flowers, and complete flowers (Fig. 9). At 7 DAFB, the drop type was mainly pistil abortion, accounting for 59.45% (Fig. 9A), and was characterized by abnormal style structure, degeneration, and disintegration of pistils (Fig. 9B). The second most prevalent type was incomplete flowers, accounting for 32.27% (Fig. 9A), which manifested as the length of the style being shorter than that of the stamen (Fig. 9C). Ovaries of this type have not yet expanded because they have not been pollinated.

Fig. 9.

The type of abscised flowers at 7 and 14 DAFB of ‘Shanxing No. 1’. A The proportion of different types of abscised flowers. Lowercase letters represent significant differences at the 0.05 level. B Abscised flowers at 7 DAFB. C Abscised flowers at 14 DAFB. Bar = 5 mm

The proportion that involved the drop of complete flowers was only 8.28%, and the ovary began to expand but was smaller than that of unfallen complete flowers, which may be caused by poor fertilization (Fig. 9A, B). At 14 DAFB, the drop type was mainly complete flowers, accounting for 91.21% (Fig. 9A). The falling flowers had slightly enlarged ovaries, smaller than those of the preserved flowers, with brown surface (Fig. 9C), indicating that poor fertilization may limit ovary development. Incomplete flowers and pistil abortions accounted for only 8.79% (Fig. 9A). In general, pistil abortion and poor pollination were the limiting factors affecting fruit set and promoting abscission in P. sibirica L.

At 21–35 DAFB, 59.39–96.13% of fallen fruit was related to abnormal embryos (Fig. 10A). These fruits were yellow in appearance and the internal seed was full; however, the base of the embryo browned after dissection (Fig. 10C). According to the statistics of seed plumpness at different periods (Fig. 10B), it was found that the embryo fullness of 82.5% of seeds from fallen fruit was more than 70% at 42 DAFB. From 49 to 70 DAFB, the dropped fruit with seed fullness of less than 30% accounted for more than 66% (slow abscission stage), indicating that the physiological fruit drop of P. sibirica L. was closely related to abnormal seed development (Fig. 10D).

Fig. 10.

The type of abscised fruits at 21–70 DAFB of ‘Shanxing No. 1’. A The proportion of different types of abscised fruits. B The plumpness of seed at 42–70 DAFB. C The first abscission peak fruit. D The second abscission peak fruit. Lowercase letters represent significant differences at the 0.05 level. Bar = 1 cm

Combined analysis of fruit drop, fruit development, and vegetative bud outgrowth

The dynamic growth curves of fruit, seeds, and new shoot development showed that 14–35 DAFB was the rapid growth period of new shoots and the rapid expansion period for fruitlets, which was also the main stage of physiological drop. Furthermore, a correlation analysis of fruit and vegetative bud outgrowth traits and fruit dropping rate at 14–35 DAFB and 42–70 DAFB was performed. The results showed that the FLD, FTD, and FT were significantly or extremely significantly positively correlated with 1-year shoot length (OYSL), 1-year shoot coarseness (OYSC), leaf length (LL), and leaf width (LW) in both stages. One point worth noting was the significantly positive correlation between OYSL/LL and CDR during 14–35 DAFB, while there was no significant relationship during 42–70 DAFB (Fig. 11). Therefore, there may be a nutritionally competitive relationship between the early stages of fruitlet and vegetative bud outgrowth development, which may induce physiological fruit drop.

Fig. 11.

The correlation analysis of fruit dropping rate with vegetative bud outgrowth and fruit growth in ‘Shanxing No. 1’. FLD, Fruit longitudinal diameter; FTD, Fruit transverse diameter; FT, Fruit thickness; SFW, Single fruit weight; FV, Fruit volume; LL, Leaf length; LW, Leaf width; OYSL, 1-year shoots, OYSC, 1-year shoots coarseness; CDR: Cumulative fruit dropping rates; RDR: Relative fruit dropping rates. *, **, and *** represent significant differences at the 0.05, 0.01, 0.001 level, respectively

The vegetative bud outgrowth removal experiment substantiated our perspective(Table 3). At 14 DAFB, fruit setting rates for the 25%, 50%, and 75% vegetative bud removal treatments were significantly higher than the non-removal group. At 28DAFB (first abscission peak), the fruit setting rate for the 75% treatment was the highest at 57.13%, followed by the 50% treatment, both significantly higher than that of the non-removing group. The 25% treatment did not differ significantly from the non-removing group. At 49DAFB (second abscission peak), the 50% treatment showed the highest fruit setting rate at 46.20%, significantly surpassing other treatments. The fruit setting rate of the 75% treatment was the lowest at 35.02%, significantly below the non-removal group’s 37.94%. At 70 DAFB, the final fruit setting rate for 75% treatment decreased to 25.90%, a 27.23% reduction, while the 50% treatment obtained a 25.51% enhancement compared to non-removing. This experiment further demonstrated that the competition between fruitlets and vegetative bud outgrowth reduced the P. sibirica L. fruit setting rate. In this study, 50% treatment reduced early-stage nutrient competition, ensuring an adequate photosynthetic supply later, thereby increasing the fruit setting rate.

Table 3.

Effects of leaf bud removing on the fruit setting rate of Siberian apricot

Treatment	14 DAFB (%)	28 DAFB (%)	49 DAFB (%)	70 DAFB (%)
Non-removing	68.56 ± 0.58b	40.93 ± 0.83c	37.94 ± 0.17b	35.59 ± 0.52b
25% removing	71.08 ± 0.79a	41.06 ± 0.47c	38.92 ± 0.31b	36.02 ± 0.18b
50% removing	73.43 ± 1.27a	52.03 ± 1.29b	46.20 ± 1.38a	44.67 ± 1.96a
75% removing	74.00 ± 1.69a	57.13 ± 1.06a	35.02 ± 0.24c	25.90 ± 2.57c

Lowercase letters represent significant differences at the 0.05 level

Dynamic change in the content of sugar components in different tissues during the physiological fruit drop

During the physiological fruit drop stage of P. sibirica L., the sucrose concentrations in the pulp of non-abscising and abscising fruit (PNF, PAF), carpopodium of non-abscising fruit (CNF) initially decreased (21–42 DAFB, S2) and then increased sharply (42–63 DAFB, S3). In contrast, the carpopodium of abscising fruit (CAF) and seed of non-abscising fruit (SNF) showed an upward trend (Table 4). The sucrose concentrations in OYS and leaves exhibited fluctuations, initially increasing, then decreasing, and eventually increasing again. During the fruitlet rapid abscission period (21–35 DAFB), sucrose contents were significantly lower in PNF and SNF compared to OYS and leaves at the same time, indicating that vegetative bud outgrowth (OYS and leaves) was more nutrient competitive than fruitlet. During the slow abscission period (42–63 DAFB), the role of the leaf gradually changed from “sink” to “source” as leaf development matured and the level of photosynthetic capacity increased, which significantly accumulated the sucrose concentration in fruitlet. At 63 DAFB, the sucrose concentration in PNF was 0.87 times that in leaves.

Table 4.

The dynamic changes of sucrose content in different tissues during physiological fruit drop

Tissues	Sucrose/mg·g−1 (FW)
	21	28	35	42	49	56	63
PNF	4.22 ± 0.25c	3.07 ± 0.24e	3.59 ± 0.11d	7.44 ± 0.93e	13.70 ± 1.01c	20.94 ± 1.20a	28.32 ± 1.05a
PAF	3.90 ± 0.45d	2.63 ± 0.22f	3.32 ± 0.21d	5.15 ± 0.49f	5.65 ± 0.40e	16.58 ± 1.57b	23.54 ± 0.94b
CNF	8.50 ± 0.79b	7.68 ± 0.42b	7.65 ± 0.24c	12.20 ± 0.67c	12.80 ± 1.63c	14.99 ± 0.63b	16.16 ± 0.65c
CAF	3.95 ± 0.47e	6.07 ± 0.54c	7.58 ± 1.01c	9.79 ± 0.23d	10.90 ± 0.37d	13.83 ± 0.28c	14.83 ± 0.32d
SNF	2.22 ± 0.25f	3.40 ± 0.30e	3.52 ± 0.19d	3.70 ± 0.14g	5.73 ± 0.69e	10.00 ± 1.00d	16.22 ± 1.09c
SAF	0.26 ± 0.04g	1.27 ± 0.10g	0.26 ± 0.04f	0.79 ± 0.12h	0.29 ± 0.07f	0.08 ± 0.04e	4.01 ± 0.02e
OYS	6.87 ± 0.49c	5.53 ± 0.71d	10.3 ± 1.34b	17.5 ± 0.66b	15.1 ± 1.89b	14.46 ± 0.72b	17.26 ± 1.91c
Leaves	15.1 ± 2.13a	19.6 ± 0.77a	18.8 ± 3.75a	26.0 ± 1.74a	21.9 ± 3.12a	22.3 ± 1.64a	32.67 ± 3.21a

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot

Lowercase letters represent significant differences at the 0.05 level

Table 5 summarized the dynamic changes in fructose concentrations in different tissues and the differences between the tissues. The fructose concentrations in the CNF and SNF showed an increasing trend, consistent with sucrose. OYS and leaves showed a trend of increasing at first (21–42 DAFB) and then decreasing (42–63 DAFB), with a peak value at 42 DAFB, while a more complex trend of increasing–decreasing–increasing was observed in the PNF, with a peak value at 63 DAFB. The key stage in kernel formation was 42–56 DAFB. The process of kernel formation may consume fructose, as evidenced by the decrease in PNF fructose content. The fructose content in the leaves and PNF w higher than that in other tissues. In the early stage (before 42 DAFB), the leaves had higher concentrations than PNF, and in the later stage (after 42 DAFB), the relationship of fructose content between both was reversed with the recovery of leaf function and the end of a large number of fruit drop.

Table 5.

The dynamic changes of Fructose content in different tissues during physiological fruit drop

Tissues	Fructose/mg·g−1 (FW)
	21	28	35	42	49	56	63
PNF	3.24 ± 0.13b	4.17 ± 0.26c	5.48 ± 0.19b	10.4 ± 0.62a	8.94 ± 0.39a	8.17 ± 1.06a	13.0 ± 1.23a
PAF	2.73 ± 0.18c	2.62 ± 0.61d	3.47 ± 0.54d	5.23 ± 0.77b	7.04 ± 0.19b	6.63 ± 0.24b	9.64 ± 1.54b
CNF	2.72 ± 0.17c	2.74 ± 0.24d	4.94 ± 0.74c	6.00 ± 0.44b	6.58 ± 0.93b	7.73 ± 0.67a	8.57 ± 0.67b
CAF	2.02 ± 0.28e	2.36 ± 0.11e	4.56 ± 0.44c	4.83 ± 0.37c	3.46 ± 0.82e	3.04 ± 0.72d	2.33 ± 0.44e
SNF	2.39 ± 0.08d	2.57 ± 0.24d	1.58 ± 0.29e	3.18 ± 0.19d	4.09 ± 0.30d	5.56 ± 0.39c	6.21 ± 0.07c
SAF	0.13 ± 0.05f	0.67 ± 0.15f	0.37 ± 0.04f	0.28 ± 0.08e	0.12 ± 0.05f	0.08 ± 0.01e	0.22 ± 0.01f
OYS	3.24 ± 0.44b	5.24 ± 0.39b	5.95 ± 0.44b	6.40 ± 0.69b	6.02 ± 0.67c	5.46 ± 0.31c	4.59 ± 0.35d
Leaves	4.61 ± 0.62a	6.65 ± 1.10a	7.07 ± 0.47a	9.37 ± 0.46a	8.84 ± 0.92a	6.81 ± 0.36ab	6.47 ± 0.52c

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot

Lowercase letters represent significant differences at the 0.05 level

As shown in Table 6, the glucose concentrations in different tissues varied during the physiological fruit drop stage of P. sibirica L. The glucose concentrations of the CNF and SNF showed an increasing trend, while that in the PNF showed the same pattern as fructose, increasing and then decreasing. The regularity of glucose contents in the leaves and OYS was not obvious. The concentration in non-abscising fruits was higher than that in the other tissues. The glucose content of normal flesh was the highest during the entire physiological fruit drop stage.

Table 6.

The dynamic changes of glucose content in different tissues during physiological fruit drop

Tissues	Glucose/mg·g−1 (FW)
	21	28	35	42	49	56	63
PNF	8.78 ± 0.48a	9.45 ± 1.39a	11.7 ± 0.20a	10.3 ± 0.37a	9.77 ± 0.05a	9.32 ± 0.75a	11.3 ± 0.12a
PAF	5.70 ± 0.81bc	5.49 ± 0.53c	7.30 ± 0.29b	7.22 ± 0.45b	8.33 ± 0.07b	7.71 ± 0.20b	6.00 ± 0.22c
CNF	6.05 ± 0.83b	6.41 ± 0.11b	4.77 ± 0.52d	4.31 ± 0.31e	5.32 ± 0.23c	6.88 ± 0.39b	9.83 ± 0.28b
CAF	5.47 ± 0.08bc	5.68 ± 0.39c	4.00 ± 0.15e	4.02 ± 0.53e	4.76 ± 0.13d	4.19 ± 0.43Bcd	5.83 ± 0.36c
SNF	4.47 ± 0.78cd	4.78 ± 0.37d	5.97 ± 0.21c	6.34 ± 1.56d	6.21 ± 0.79c	6.45 ± 0.89b	8.78 ± 0.79b
SAF	2.86 ± 0.74e	2.56 ± 0.25e	3.83 ± 0.13e	3.54 ± 1.09f	3.62 ± 0.78e	3.65 ± 0.59e	2.73 ± 0.75e
OYS	3.47 ± 0.49e	2.71 ± 0.13e	4.58 ± 0.23d	5.76 ± 0.27d	2.13 ± 0.23e	4.05 ± 0.36cde	4.80 ± 0.20d
Leaves	4.95 ± 0.33cd	6.85 ± 0.94b	5.78 ± 0.30c	7.51 ± 0.48c	4.79 ± 0.31d	5.61 ± 0.36b	5.27 ± 0.12c

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot

Lowercase letters represent significant differences at the 0.05 level

Comparing the difference in sucrose, fructose, and glucose concentrations between non-abscising and abscising fruit about to drop, it was found that the sucrose, fructose, and glucose concentrations in the PNF, CNF, and SNF were significantly higher than those of PAF, CAF, and SAF (seeds of abscising fruit) (Tables 4, 5 and 6). This result indicates that the lack of carbohydrates in fruitlets is related to fruit drop.

Dynamic change in enzyme activities of glucose metabolism in different tissues during fruit drop

As can be seen from Table 7, the change curves of acid invertase (AI) enzyme activity in PNF, CNF, and SNF, OYS, and leaves were consistent with a sharp downward trend at 21–42 DAFB, followed by a gentle upward fluctuating trend at 42–63 DAFB. The highest point of activity appeared before or the peak of fruit drop; for example, the maximum values of CNF, SNF, and leaves were at 21 DAFB, and PNF and OYS were at 28 DAFB. At 21–35 DAFB, the development of fruitlets and growth of new tips originated from the nutrients stored in the previous year, and the sucrose supply of normal fruit and leaves originated from the stored nutrients. Between 21 and 35 DAFB, the tree body played the role of “source”, while the new tips and fruitlets played the role of “sink”. At this stage, AI activity in the vegetative bud outgrowth (leaves and OYS) was higher than in fruitlets (PNF and CNF). With fallen fruit’s stability and the leaf’s gradual growth into functional leaf, AI enzyme activity in the fruit was higher than that in the vegetative bud outgrowths.

Table 7.

The dynamic changes of acid invertase in different tissues during physiological fruit drop

Tissues	Acid invertase/mg·g−1(FW)·h−1
	21	28	35	42	49	56	63
PNF	111.42 ± 5.37d	121.53 ± 6.63c	93.73 ± 4.80b	79.94 ± 10.61ab	88.20 ± 5.08b	107.46 ± 13.43a	106.86 ± 19.80a
PAF	108.09 ± 7.15d	114.20 ± 3.87d	78.09 ± 9.54c	76.90 ± 11.98ab	45.42 ± 7.94d	81.40 ± 2.55b	70.29 ± 4.48b
CNF	154.76 ± 13.20b	148.78 ± 2.24b	95.26 ± 2.32b	81.19 ± 3.51a	85.02 ± 5.10b	67.03 ± 4.76c	70.47 ± 3.44b
CAF	136.87 ± 4.74c	121.74 ± 13.00c	87.93 ± 1.04c	63.86 ± 11.48bc	69.68 ± 7.27c	58.96 ± 3.66cd	67.03 ± 1.83b
SNF	65.04 ± 3.03e	58.74 ± 4.55e	26.59 ± 2.64d	19.27 ± 1.33d	32.53 ± 2.91e	44.48 ± 6.56d	28.68 ± 3.47c
SAF	52.29 ± 1.85f	20.44 ± 2.41f	20.19 ± 0.20e	4.38 ± 1.85e	5.23 ± 0.68f	2.58 ± 1.02e	15.90 ± 0.27d
OYS	168.88 ± 13.24b	181.26 ± 9.54a	74.44 ± 6.54c	57.38 ± 7.71c	65.31 ± 3.27c	34.76 ± 13.20d	27.35 ± 6.41c
Leaves	230.47 ± 12.90a	183.88 ± 27.13a	158.57 ± 15.29a	68.78 ± 2.71b	147.72 ± 7.19a	104.07 ± 8.67a	79.73 ± 7.69ab

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot

Lowercase letters represent significant differences at the 0.05 level

The dynamic changes in neutral invertase (NI) activity in different tissues were similar to those of AI activity (Table 8). The NI activity in PNF decreased from 21 to 35 DAFB (S2), followed by an increase from 42 to 56 DAFB (S3). CNF exhibited a trend of gentle fluctuation after a sharp decline. OYS and leaves showed a decreasing trend (S2) before increasing slowly (S3). No obvious pattern was observed in the SNF. The NI activity of the vegetative bud outgrowth (leaves and OYS) in the fast abscission stage (21–35 DAFB) was significantly higher than that of the fruitlets in the slow abscission stage (42–63 DAFB), and this relationship was reversed after 42 DAFB. This situation also emerged in AI activity.

Table 8.

The dynamic changes of neutral invertase in different tissues during physiological fruit drop

Tissues	Neutral invertase/mg·g−1(FW)·h−1
	21	28	35	42	49	56	63
PNF	137.40 ± 19.94b	95.47 ± 3.00b	95.08 ± 4.60b	191.50 ± 23.19a	183.57 ± 6.90a	146.93 ± 1.59b	128.41 ± 11.70b
PAF	125.76 ± 28.58bc	72.06 ± 5.11c	116.90 ± 16.06ab	185.29 ± 19.39a	160.55 ± 17.21a	167.56 ± 6.42a	153.17 ± 11.60a
CNF	103.52 ± 19.02bc	90.87 ± 12.99b	84.76 ± 8.97bc	75.71 ± 9.62b	76.03 ± 6.88b	79.47 ± 9.81de	78.14 ± 0.92d
CAF	147.98 ± 22.46ab	130.79 ± 25.08a	79.21 ± 4.89c	73.65 ± 11.39b	80.79 ± 7.51b	69.15 ± 7.49e	68.09 ± 9.52d
SNF	68.37 ± 12.52b	90.47 ± 3.00b	43 ± 4.60d	38.82 ± 2.52d	65.06 ± 5.83c	88.97 ± 13.13cd	54.05 ± 2.61e
SAF	17.62 ± 1.16d	7.20 ± 5.11d	3.87 ± 1.60e	12.84 ± 8.97e	10.46 ± 13.7e	5.16 ± 2.04g	31.80 ± 5.55f
OYS	138.17 ± 5.99b	133.19 ± 15.05a	103.65 ± 7.22b	31.05 ± 4.84d	33.96 ± 6.29d	54.60 ± 9.02f	38.99 ± 3.67f
Leaves	178.67 ± 17.48a	152.69 ± 17.04a	132.53 ± 7.69a	45.47 ± 5.70c	56.19 ± 9.21c	99.04 ± 5.83c	87.14 ± 6.04c

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot

Lowercase letters represent significant differences at the 0.05 level

The sucrose synthase (decomposition direction, SS-I) activity displayed a consistent trend of change in different tissues, showing a trend of first increasing and then decreasing in a fluctuating manner, except for OYS, which showed a fluctuating downward trend (Table 9). The SS-I activity peak values of CNF, CAF, SNF, and SAF occurred at the first peak of fruit abscission (28 DAFB), and that of OYS and leaves appeared earlier than fruit at 21 DAFB. Furthermore, the SS-I activity of the latter was higher than that of the former, illustrating that SS-I activity was dominant in the nutritional competition between vegetative bud outgrowth and fruitlet.

Table 9.

The dynamic changes of sucrose synthase (decomposition direction) in different tissues during physiological fruit drop

Tissues	Sucrose synthase (decomposition direction)/mg·g−1(FW)·h−1
	21	28	35	42	49	56	63
PNF	33.47 ± 1.56b	45.08 ± 5.62a	37.99 ± 4.19a	25.64 ± 3.06b	16.29 ± 2.49c	14.65 ± 0.69bc	14.09 ± 4.12 b
PAF	31.75 ± 3.82b	41.14 ± 5.19a	35.95 ± 2.98a	13.24 ± 1.90c	11.77 ± 1.04d	9.65 ± 0.53d	10.77 ± 0.99bc
CNF	33.01 ± 1.50b	43.03 ± 4.72a	39.73 ± 1.90a	27.09 ± 2.01b	12.26 ± 1.90d	17.11 ± 1.65b	12.29 ± 1.23bc
CAF	27.14 ± 1.71c	32.61 ± 4.30b	27.18 ± 4.98b	30.33 ± 1.46b	9.64 ± 0.43e	13.5 ± 0.92c	7.65 ± 1.50cde
SNF	13.85 ± 3.45d	10.35 ± 0.35c	12.94 ± 2.18c	8.43 ± 0.98d	6.75 ± 1.23f	6.03 ± 0.05e	5.75 ± 1.03de
SAF	6.98 ± 0.98e	8.32 ± 4.20d	7.88 ± 1.01d	3.49 ± 0.11e	2.43 ± 0.09g	1.23 ± 0.01f	0.85 ± 0.01f
OYS	55.62 ± 1.62a	44.59 ± 8.35a	39.41 ± 2.82a	34.16 ± 2.90ab	21.67 ± 2.46b	14.63 ± 4.71bc	9.26 ± 2.19cd
Leaves	50.99 ± 2.92a	40.94 ± 1.86a	37.21 ± 3.43a	39.77 ± 2.40a	36.01 ± 3.88a	27.94 ± 7.04a	24.1 ± 1.35a

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot, SS-I Sucrose synthase (decomposition direction)

Lowercase letters represent significant differences at the 0.05 level

The changes in sucrose synthase (synthesis direction, SS-II) activity were roughly the opposite of SS-I’s. It showed a slow decrease (21–35 DAFB), followed by a fluctuating increase (42–63 DAFB) in fruit (PNF, CNF, and SNF). In contrast, it was not obvious in OYS and leaves, with a fluctuating trend at different developmental stages (Table 10). During the 21–49 DAFB stage, SS-II activity was generally lower than that of SS-I activity, indicating that sucrose metabolism was mainly decomposed. PNF’s sucrose phosphate synthase (SPS) activity first decreased and then fluctuated, while CNF increased gradually. The regularity of SPS in SNF, OYS, and leaves was not strong (Table 11). In addition, the activity of SPS was at a low level, similar to that of SS-s but generally lower, indicating that SPS may not play a major role.

Table 10.

The dynamic changes of sucrose synthase (synthesis direction) in different tissues during physiological fruit drop

Tissues	Sucrose synthase (synthesis direction)/mg·g−1(FW)·h−1
	21	28	35	42	49	56	63
PNF	21.71 ± 4.23a	18.53 ± 1.01a	14.84 ± 3.94b	17.40 ± 1.15a	17.28 ± 2.89ab	20.76 ± 1.56a	21.42 ± 4.19ab
PAF	24.44 ± 4.99a	15.08 ± 3.74ab	12.98 ± 4.40b	20.63 ± 1.83a	18.35 ± 3.18ab	23.67 ± 2.89a	24.61 ± 1.15a
CNF	19.11 ± 3.25a	15.91 ± 5.91ab	9.24 ± 2.45b	10.93 ± 2.34b	14.11 ± 4.67bc	21.37 ± 3.82a	26.31 ± 2.73a
CAF	20.49 ± 5.95a	14.33 ± 3.07ab	13.47 ± 1.86b	12.93 ± 1.20b	18.37 ± 2.56ab	16.77 ± 4.19ab	20.85 ± 3.24ab
SNF	15.03 ± 2.75b	14.90 ± 3.40ab	12.49 ± 1.11b	12.89 ± 3.45b	13.54 ± 2.44c	13.37 ± 1.58b	16.76 ± 3.29b
SAF	11.34 ± 2.94bc	9.43 ± 1.89c	7.30 ± 1.24c	4.88 ± 2.22c	2.59 ± 1.00d	0.40 ± 0.01d	0.43 ± 0.01d
OYS	9.14 ± 0.32c	12.47 ± 0.53b	14.50 ± 3.68b	17.92 ± 2.04a	11.57 ± 2.54c	7.15 ± 1.18c	10.92 ± 0.51c
Leaves	10.1 ± 0.32c	16.24 ± 1.14a	21.82 ± 4.69a	20.20 ± 1.61a	20.65 ± 1.67a	14.07 ± 1.96b	16.17 ± 1.42b

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot, SS-II Sucrose synthetase (synthesis direction)

Lowercase letters represent significant differences at the 0.05 level

Table 11.

The dynamic changes of sucrose phosphate synthase in different tissues during physiological fruit drop

Tissue	Sucrose phosphate synthase/mg·g−1(FW)·h−1
	21	28	35	42	49	56	63
PNF	18.97 ± 1.39a	15.49 ± 1.64bc	12.94 ± 2.22b	23.28 ± 1.65a	20.18 ± 2.40a	21.55 ± 3.25a	24.22 ± 4.17a
PAF	12.57 ± 0.85bc	17.84 ± 1.59ab	6.74 ± 2.15c	24.77 ± 3.08a	14.70 ± 1.82b	7.66 ± 0.30c	8.55 ± 1.88d
CNF	10.69 ± 1.46cd	13.25 ± 1.23Acd	15.41 ± 1.83ab	17.62 ± 3.08a	17.07 ± 3.47ab	13.15 ± 1.69b	18.87 ± 2.77ab
CAF	6.42 ± 1.05e	10.97 ± 1.86d	13.58 ± 3.08ab	12.10 ± 2.69bc	14.59 ± 0.91b	19.59 ± 2.16a	15.13 ± 2.13bc
SNF	9.34 ± 1.74d	10.72 ± 1.89d	8.23 ± 1.03c	11.58 ± 2.46bc	13.43 ± 3.49b	12.49 ± 1.13b	14.77 ± 1.19cd
SAF	8.01 ± 1.22d	7.32 ± 1.05e	5.85 ± 0.99d	3.94 ± 0.55d	1.56 ± 0.07c	0.84 ± 0.03d	0.62 ± 0.01f
OYS	17.60 ± 5.00ab	10.16 ± 1.57d	19.02 ± 2.94a	10.90 ± 0.29c	19.72 ± 0.97a	13.23 ± 2.23b	14.17 ± 1.79cd
Leaves	18.18 ± 0.58a	18.39 ± 0.66a	17.70 ± 0.92ab	13.16 ± 0.81b	16.31 ± 1.26b	10.64 ± 0.55b	7.48 ± 0.57e

PNF Pulp of non-abscising fruit, PAF Pulp of abscising fruit, CNF Carpopodium of non-abscising fruit, CAF Carpopodium of abscising fruit, SNF Seed of non-abscising fruit, SAF Seed of abscising fruit, OYS 1-year shoot, SPS Sucrose phosphate synthase

Lowercase letters represent significant differences at the 0.05 level

In analyzing enzyme activities between non-abscising and abscising fruits, AI activity in PNF/CNF was significantly elevated compared to PAF/CAF during 21 to 35 DAFB. Conversely, during 42 to 63 DAFB, SS-I showed significantly higher levels in PNF/CNF than in PAF/CAF, which suggested that the rapid abscission phase of Siberian apricot was associated with AI activity, while the slow abscission phase may be related to SS-I activity. The variability of NI, SS-II, and SPS activity between non-abscising and abscising fruit at different times was complex, implying that NI, SS-II, and SPS in fruit was not the key enzyme leading to abscission.

Discussions

Characteristics in P. sibirica L. fruit growth

Pollination and fertilization occur during the blooming stage, known as anthesis. Successful fertilization and cell division lead to the development of the ovary into a fruitlet, known as fruit set [34, 35]. In the field, it takes approximately 14 d for P. sibirica L. to move from flowers to fruitlets. According to field observations, it took approximately 84 d for P. sibirica L. fruit to ripen from the time of anthesis entirely, which was close to that of kernel-using apricots (P. armeniaca L.), i.e., ‘Youyi’ and ‘Longwangmao’ [17]. Cumulative data showed that P. sibirica fruit displayed an S-shaped curve, where the longitudinal, transverse, thickness, fresh weight, and volume increased exponentially as the fruit developed from 14 to 42 DAFB. A similar growth pattern has been observed in Actinidia eriantha [36], Carissa congesta [37], and Musa AAB [38].

The seed growth curve matched P. sibirica L. fruit development. Cotyledon formation began at the stone-hardening stage. After 2 weeks of rapid growth, the size of P. sibirica L. seed kernels remained unchanged, developing 1 week faster than ‘Longwangmao’ and ‘Youyi’ kernel apricot varieties [17], which may be related to germplasm characteristics. Subsequently, the growth of seed kernel in P. sibirica L. slowed, which is an essential period for accumulating and transforming kernel nutrients [17]. The rapid growth period of the fruit includes embryo formation, and the hard nucleus period corresponds to cotyledon differentiation, expansion of cotyledon inclusion enrichment, and finally, the formation of seed kernel [39]. Therefore, ensuring nutrition during this period is important for increasing kernel yield.

The present study also showed that the fruit shape of P. sibirica L. changed over time. At the beginning of fruit setting, P. sibirica L. fruit had a flat-long-circular shape owing to a high longitudinal-to-transverse ratio (Fig. 1B). As the fruit grew, the longitudinal direction gradually slowed, whereas the transverse direction increased rapidly (Fig. 1C). At S3, the fruit appeared almost flat, wide, and circular. This unique characteristic has also been observed in Carissa congesta L [37]. and A. eriantha [36]. During development, the fruit becomes a sink organ for accumulating photosynthate products from photosynthesis, such as sugars and water [40]. Thus, this is the major contributor to the increase in the longitudinal diameter, transverse diameter, thickness, weight, and volume of P. sibirica L. fruits.

Characteristics of P. sibirica L. fruit drop

Understanding the physiological fruit drop is the premise for formulating fruit protection measures to increase yield. In this study, we observed that in the arid and semi-arid areas of western Liaoning, China, the flower/fruit drop of P. sibirica L. could be divided into three stages. The first stage was the first 2 weeks after anthesis, during which many flowers fell off; this stage was mainly related to pistil abortion and pollination failure [41]. The second stage was 3–5 weeks after full bloom (late April), the main fruit drop stage, when ~ 70% of the initial fruit set fell off. At this stage, the female organs of the fruit appeared to be functioning properly; however, inadequate fertilization hindered the transfer of necessary nutrients to the ovary, causing stunted growth and limited development of the endosperm, resulting in slow fruit development and eventual shedding [42]. This is supported by the fact that sucrose, fructose, and glucose concentrations in the seeds of abscising fruit were significantly lower than those in non-abscising fruit. During the third stage, which lasted 6–10 weeks after full bloom (May), fruit drop occurred sporadically. Embryo development relies on the endosperm for nutrition, but it disintegrates prematurely, causing the young embryo to stop developing or disintegrate, resulting in shriveled seeds (Fig. 8D) [43]. Our classification of the physiological fruit drop stage in P. sibirica L. was similar to that of fresh apricot varieties [18].

Differences in fruit drop at different canopy positions have also received considerable attention. Our findings revealed that the fruit dropping rate was higher for the interior bearing branch of the crown than for the central and external branches, particularly during the rapid shedding period, and that of the central branch was higher than that of the external branch. This result coincided with the previous conclusion of our research group that the yield of different crown parts was in the order outer > central > internal [44]. According to Wagenmakers and Callesen’s report, apple yields are higher in the outer and upper portions of the canopy, whereas lower yields are observed in the inner and lower parts [45]. A study on Zanthoxylum armatum “Tengjiao” documented the same law [46]. Research has demonstrated that the amount of light is linked to crop yield, with the intensity gradually decreasing from the top to the bottom of the tree [47]. The highest light intensity was observed in the outer canopy, followed by the middle crown, and the lowest intensity was observed in the inner crown [48]. Therefore, reducing the fruit dropping rate of the middle and lower branches is vital for actual production.

Combined analysis of fruit development, vegetative bud outgrowth, and fruit drop

Vegetative bud outgrowth and fruit development were strongly linked. During rapid fruitlet growth (14–42 DAFB), vegetative bud outgrowth exhibited accelerated growth between 14 and 35 DAFB. Similarly, the period of slow fruitlet growth (42–84 DAFB) coincided with that of vegetative bud outgrowth (35–56 DAFB). Intense competition manifested in the early vegetative bud outgrowth and fruit formation phases. As fruit development approached the middle stage, the carbon sequestration ability of vegetative buds increased, and nutrient supply surpassed consumption, resulting in reduced competition with fruit development. Nutrients generated through plant photosynthesis are primarily allocated to the secondary growth of vegetative bud outgrowth [31, 49] and the accumulation and transformation of fruit inclusions [18]. The significant positive correlation between OYSL/LL and CDR in the rapid growth stage (rapid abscission period) no longer existed in the slow growth period (slow abscission period), further indicating that elongation of the vegetative bud outgrowth promoted the loss of a large number of fruitlets in the early stage. In addition, the rapid occurrence of fruit drop at the late stage of thinning and the decrease in the fruit set rate after 75% thinning in the field experiment confirmed this. In short, vegetative bud outgrowth and fruit development are affected by the dual factors of tree storage nutrients and production management in the current year [50]. Therefore, fertilizer and water management could be strengthened around the beginning of April to meet the demand for mineral elements for the rapid growth of vegetative bud outgrowth and fruit.

Effect of carbohydrates and sucrose metabolism on fruit drop

Carbohydrates are not only important energy and carbon sources, but also important structural materials. Energy shortages can lead to metabolic disorders, resulting in programmed cell death and fruit loss [26]. It is generally believed that the failure of a specific carbohydrate content in fruit to reach a certain threshold during fruit development is the most likely cause of fruit abscission [51]. Different tree species have different types of sugar accumulation, and the main types of sugars that cause fruit drop differ. In this study, the sucrose, glucose, and fructose contents of about-to-abscise fruit were significantly lower than those of non-abscising fruit, especially during the rapid abscission period (21–35 DAFB). Before leaves become fully functional, the nutrient requirements of fruit and vegetative bud outgrowth primarily on reserves stored in the tree from the previous year. However, the ability of vegetative bud outgrowth (especially leaves) to obtain carbohydrates is often higher than that of fruit [52], and this phenomenon will lead to fruit drop, which is particularly intense during the early stages of fruit development in flower-before-leaf species such as P. dulcis [53]. This may be an important reason why the sucrose and fructose contents of vegetative bud outgrowths were significantly higher than those of fruitlet during the P. sibirica L. rapid abscission period. This conclusion has also been confirmed in litchis [54] and almond [53].

The ability of vegetative bud outgrowth and fruitlet to obtain carbohydrates depends on the activity of sucrose metabolic enzymes (NI, AI, SS, and SPS) [55, 56]. It has been shown that AI and NI provide energy and carbon skeleton for growth and development by breaking down sucrose, e.g., AI located in the plastid ectodomain is involved in the unloading of sink organ sugars from the phloem and determines the sink strength [57]. Another study on the changing trends of sucrose metabolic enzymes during the physiological fruit drop of almond [58] and apple [59] found that AI was closely related to fruit abscission. In our work, the AI and NI activities in the vegetative bud outgrowth were consistently higher than those in the fruitlets during the rapid abscission period, revealing that the carbohydrates competition of the former was stronger than that of the latter, resulting in ~ 70% of the initial fruitlet shedding due to nutritional “starvation” stress. In the slow abscission stage, due to the completion of leaf morphogenesis, photosynthesis gradually recovered, and the leaf transformed into “source”, while fruit continued to absorb nutrients as a strong “sink” for development, which was confirmed by the fact that the AI and NI activities of fruit were generally greater than those of vegetative bud outgrowth. Further comparison of exfoliated and normal tissues revealed that AI in non-abscising fruit was always significant or significantly higher than that in about-to-abscise fruit, indicating that AI was one of the key enzymes regulating the shedding of fruitlets of P. sibirica L.

Another enzyme worth noting is SS-I. The SS-I enzymatic activity of non-abscising fruit was always significantly or significantly higher than that in fruit about to drop and was gradually trending downward. This promoted the slow accumulation of fructose and glucose in fruit, implying that SS-I was also one of the important enzymes regulating P. sibirica L. fruit abscission, which was consistent with the findings of studies on the relationship between sucrose metabolism and fruit drop in almond [58]. In addition, the activity of SS-II was generally higher than that of SS-I, which explains the rapid accumulation of sucrose and the slow accumulation of fructose and glucose at this stage.

Conclusions

In summary, fruit development and new tip growth of P. sibirica L. followed a single S-shaped curve with three periods: ovary enlargement, rapid growth, and slow growth. The fruit/flower drop lasted for ~ 63 d, which was divided into three stages: the flower abscission stage mainly caused by pistil abortion; the rapid abscission stage of fruitlets caused by nutrient competition between the fruit and vegetative bud outgrowth; and the slow abscission stage mainly related to seed abortion. Nutritional competition between vegetative bud outgrowth and fruit leads to an insufficient sucrose supply during fruit development, which cannot break down enough glucose and fructose for development. These results of the growth and development law and fruit drop law of P. sibirica L. fruit obtained in this study provide a basis for formulating of effective fruit protection measures in production.

Materials and methods

Plant material

Prunus sibirica L. plants of the variety ‘Shanxing No. 1’, a significant variety in arid and semi-arid areas of western Liaoning, China, were obtained from the National Forest Germplasm Resources Preservation Repository for P. sibirica L. located in Kazuo County, Liaoning, China (119°24′54″E–120˚23′24″E, 40˚47′12″N–41˚33′53″) and planted in areas of ~ 5 ha. The experimental site belongs to the low hilly region with an elevation of 300–400 m and has a continental monsoon climate with a frost-free period of 144 days, an annual sunshine duration of 2807.8 h, an annual average temperature of 8.7 ℃, and an annual average rainfall of 491.5 mm. Plants grafted with wild common apricot (P. armeniaca L.) as rootstock were planted with 2 × 3 m spacing in 2016 and managed under the same conditions, including pest control and pruning procedures.

Fifty ‘Shanxing No. 1’ plants (6–7 years old) with the same growth potential and unified management were randomly selected (Fig. 1A). Five plants were used to study the dynamic law of fruit development and vegetative bud outgrowth, 15 plants were used to assess the characteristics of physiological fruit drop, and 30 plants were used to study the dynamic characteristics of sugar components and sucrose metabolism in different tissues, including PNF, PAF, CNF, CAF, SNF, SAF, OYS, and leaves during physiological fruit drop.

Investigation of fruit and vegetative bud outgrowths

We labeled one flower-bearing branch (with equal flower counts) on the crown of five selected plants, positioned in the east, south, west, and north directions outside the tree crown. Observations commenced at the full bloom, designated as 0 day. From the full-bloom stage, one non-abscising fruit was collected from each bearing branch every seven days and returned to the laboratory. We measured FTD, STD, FLD, SLD, FT, ST, CT (cotyledon thickness), and CTD (cotyledon transverse diameter) using Vernier calipers, determined SFW and SSW using an electronic balance (Mettler ME204E), and calculated the mean values of fruit, seed diameter and shape index (longitudinal/transverse diameter). FV and SV were estimated by immersing the fruit in a water-filled measuring cylinder (25 mL) and measuring the amount of water displaced by complete immersion [37]. During 7–14 DAFB, the vegetative bud outgrowth grew (Fig. 1C). The length and width of the OYS on the bearing branches were measured using a tape and Vernier caliper, respectively. The length and width of the third to fifth leaves from the base of the OYS were measured. Twenty non-abscising fruits and OYS and 60 leaves were measured at each stage.

Investigation of fruit drop

In a three-replicate experiment, we divided 15 plants into three groups, with five plants per group. From each tree located in the east, south, west, and north, we marked three fruit-bearing branches with over 200 flowers from each tree’s inner, middle, and outer sections in the east, south, west, and north. After full bloom, the number of flowers or fruit remaining on the designated fruit-bearing branch was counted every 7 DAFB. Meanwhile, 100 flowers and fruits that fell off at the slightest touch were collected after surveying fruit drop and brought back to the laboratory to examine their external and interior characteristics (Fig. 1B, E, F), and three biological replicates were set up. The calculation formulas of RDR and CDR were as follows:

Vegetative bud outgrowth removing test

Twelve trees were selected to mark the bearing branches before flowering. Four different treatments were set up: (1) 75% treatment, 75% of vegetative buds were removed from all three trees; (2) 50% treatment, 50% of vegetative buds were removed from all three trees; (3) 25% treatment, 25% of leaf buds were removed from all three trees; and (4) Non-removing treatment, all three trees were left with vegetative buds. The number of flowers was counted during the initial flowering stage, and the number of fruit sets was counted at 14, 28, 49, and 70 DAFB.

Sample collection

Thirty trees were selected for the study of sucrose metabolism in various tissues. After fruit set (14 DAFB), 30 fruit-bearing branchlets with equal amounts of fruit from the central area of the southern crown of each tree were selected for sampling. These branchlets were assigned specific identification numbers. The specific sampling method was as follows: 100 non-abscising fruits (including carpopodium), 100 abscising fruits (including carpopodium), and five 1-year shoots with leaves were collected from the plants on the designated branches at 11:00 a.m. every 7-day intervals from 21 DAFB to 63 DAFB. These samples were placed in an ice box and immediately returned to the laboratory. After the samples were cleaned with deionized water, they were divided into eight tissues: PNF, PAF, CNF, CAF, SNF, SAF, OYS, and leaves, and stored at −80 °C for the determination of sugar components and the activities of the related enzymes.

Measurement of sugar components

The sucrose, fructose, and glucose contents of the different tissues were determined using the method described by Liu et al. [9] with a spectrophotometer (U-5100 UV, Hitachi, Tokyo, Japan). The samples (each 0.5 g) were meticulously ground and diluted with distilled water to achieve a final volume of 5 mL. The homogenate was subjected to an extraction process involving boiling water for 30 min, and centrifuged at a rotational speed of 12,000 r/min for 15 min after cooling to room temperature. The above procedure is repeated twice. The supernatant was collected to determine sugar components. Three biological replicates were performed for each sample.

To assess the sucrose content, 0.2 mL 2 mol/L NaOH was combined with 0.4 mL extracted solution and then boiled for 5 min. Subsequently, 2.8 mL of 12 mol/L HCl and 0.8 mL of 0.1% resorcinol were added, and the mixture was then incubated at 80 ℃ for 10 min. The sucrose concentration was determined by measuring the OD at 480 nm. For the fructose content, 2 mL 0.1% resorcinol and 1 mL 12 mol/L HCl were mixed with 1 mL supernatant and incubated at 80 ℃ for 10 min. Once cooled to room temperature, the fructose content was then determined by measuring the OD at 480 nm. To assess the glucose content, 1.5 mL 3,5-Dinitrosalicylic acid (DNS) reagent was added to extracted solution, followed by boiling for 5 min. Subsequently, the solutions were diluted to a total volume of 25 mL with water. The OD at 540 nm was measured to calculate the glucose concentration. The unit of sugar concentration was mg·g⁻¹ (FW).

Measurement of enzyme activity related to sugar metabolism

The enzyme solution was prepared according to the method described by Keller and Ludlow [60]. The frozen different tissues were ground in a mortar containing an ice-cold medium consisting of 0.05 mol·L⁻¹, Hepes-NaOH (pH 7.5), 0.01 mol·L⁻¹ MgCl₂, 1 mmol·L⁻¹ ethylenediaminetetraacetic acid (EDTA), 2.5 mmol·L⁻¹ dithiothreitol (DTT), 0.05% (w/v) Triton X-100 and 0.1% (w/v) bovine serum albumin (BSA). The homogenate was centrifuged at 12,000 rpm for 1 min. The supernatant was immediately desalted by filtration on a Sephadex Desalting Gravity Column. Three biological replicates were set up for the AI, NI, SS-I, SS-II, and SPS. The unit of all enzyme activity was mg·g⁻¹ (FW)·h⁻¹.

The activities of NI and AI were measured according to Miron and Schaffer with modifications [61]. AI activity was assayed in 0.1 mL of enzyme extract and 1 mL reaction liquid [1% (w/v) sucrose, 0.1 mol·L⁻¹ acetate buffer (pH = 5.5)]. The reaction was incubated at 37 °C for 1 h and then transferred to a boiling water bath for 5 min to terminate the reaction. The reducing sugars were measured using 3,5-dinitrosalicylic acid colorimetry. The enzyme solution, inactivated at 100 °C for 10 min, was used as a control. The reaction tubes were cooled to room temperature, and the absorbance was determined at 540 nm. The process for determining NI is the same as that for AI, except for the different reaction liquid, which contained 1% (w/v) sucrose, 0.1 mol·L⁻¹ phosphate buffer (pH = 7.5), 5 mmol·L⁻¹ MgCl₂, and 1 mmol·L⁻¹ EDTA. Standard curves were fabricated using glucose (0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 mg·mL⁻¹).

The activity of SPS was determined as described by Dali et al. [62]. The reaction mixtures (70 µL), composed of 50 mmol·L⁻¹ Mopes-NaOH (pH 7.5), 15 mmol·L⁻¹ MgCl₂, 25 mmol·L⁻¹ fructose-6-phosphate (Fru-6-P), 30 mmol·L⁻¹ glucose-6-phosphatase (Glc-6-P), 30 mmol·L⁻¹ uridine diphosphate glucose (UDP-Glc), and 40 µL of desalted extract, were incubated at 37 °C for 30 min. Reactions were terminated by the adding 100 µL of 30% (w/v) KOH. The tubes were placed in a boiling bath for 10 min to destroy unreacted fructose. After the mixtures had cooled, 1 mL of a solution containing 0.14% (w/v) anthrone and 13.8 mol·L⁻¹ H₂SO₄ was added. Incubation took place for 20 min at 40 °C. After the mixtures had cooled, the color development was determined at 620 nm. Standard curves were fabricated using sucrose (0.2, 0.4, 0.8, 0.8, 1.0, and 1.2 mg·mL⁻¹).

Determination of the SS-I activity was based on the Lowell et al. method [63] with modifications. Initially, 200 µL enzyme solution was combined with 500 µL of reaction buffer and incubated in a 30 °C water bath for 20 min. Subsequently, 400 µL of the enzyme mixture was mixed with 500 µL of glycine buffer, followed by the addition of 20 µL of guanosine diphosphate glucose (UDPG) dehydrogenase. The reaction was incubated at 30 °C for 15 min. Upon cooling, the nicotinamide adenine dinucleotide (NADH) concentration was determined by measuring absorbance at 340 nm. Standard curves were fabricated using NADH (0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mg·mL⁻¹). The reaction buffer comprised 50 mmol·L⁻¹ Mes-NaOH (pH = 7.0), 50 µmol·L⁻¹ UDP, 30 mmol·L⁻¹ sucrose, and 2.5 mmol·L⁻¹ DTT. The glycine buffer system consisted of 200 mmol·L⁻¹ glycine (pH = 8.7), 1 mmol·L⁻¹ EDTA, 5 mmol·L⁻¹ MgCl₂, and 15 µmol·L⁻¹ NAD⁺.

The SS-II activity was determined according to the method described by Nielsen et al. with modifications [64]. Firstly, 200 µL enzyme extract, 30 µL 50 mmol·L⁻¹ D-fructose solution, 30 µL 50 mmol·L⁻¹ UDPG, and 90 µL reaction buffer [50 mmol·L⁻¹ Mops-NaOH (pH = 8.5)] were thoroughly mixed and incubated in a water bath at 30 °C for 30 min. Subsequently, 100 µL 30% (w/v) NaOH was added, and the mixture was inactivated at 100 °C for 5 min. After cooling, an additional 500 µL 12% resorcinol solution and 500 µL concentrated HCl were added, and enzyme activity was determined by measuring sucrose production at 510 nm after heating at 80 °C for 8 min. The standard curve is the same as SPS.

Statistical analysis

Microsoft Excel 2019 software was used for preliminary analysis of the experimental data. The morphological traits of fruit growth were subjected to logistic regression analysis fitting to the equation Y = k/(1 + ae^–bx), using SPSS 19.0 (IBM, Armonk, NY, USA). The Duncan’s multi-range test was used to distinguish the differences between the means. Correlation analyses between traits during fruit, vegetative bud outgrowth, and fruit dropping rate were performed using the Pearson’s correlation matrix in Origin 2021 (OriginLab, Hampton, MA, USA).

Abbreviations

FLD

Fruit longitudinal diameter

FTD

Fruit transverse diameter

FT

Fruit thickness

SFW

Single fruit weight

FV

Fruit volume

SLD

Seed longitudinal diameter

STD

Seed transverse diameter

ST

Seed thickness

CTD

Cotyledon transverse diameter

CT

Cotyledon thickness

SSW

Single seed weight

SV

Seed volume

LL

Leaf length

LW

Leaf width

OYLS

1-year shoot length

OYLC

1-year shoot coarseness

AI

Acid invertase

NI

Neutral invertase

SS-I

Sucrose synthase (decomposition direction)

SS-II

Sucrose synthase (synthesis direction)

SPS

Sucrose Phosphate Synthase

PNF

Pulp of non-abscising fruit

PAF

Pulp of abscising fruit

CNF

Carpopodium of non-abscising fruit

CAF

Carpopodium of abscising fruit

SNF

Seeds of non-abscising fruit

SAF

Seeds of abscising fruit

OYS

1-year shoots

CDR

Cumulative fruit dropping rates

RDR

Relative fruit dropping rates

DAFB

Day after full bloom

Authors’ contributions

YS: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Writing–original draft. PW: Investigation, Data curation, Methodology. CZ: Writing– review & editing. BL, YK, JY, and TR: Investigation. JC: Conceptualization, Writing– review & editing. SD: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Writing– review & editing.

Funding

This work was supported by the Liaoning Province Wild apricot Germplasm Resource Preservation and Breeding National Permanent Scientific Research Base (grant numbers 2020132519).

Data availability

The authors declare that the data supporting the findings of this study are available within the paper. The datasets used and analyzed during the current study could be available from the corresponding author on request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.