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. 2024 Dec 28;14:31243. doi: 10.1038/s41598-024-82602-5

Analysis of canopy light utilization efficiency in high-yielding rapeseed varieties

Xiaolu Xiao 1, Bo Duan 1, Fangyuan Huang 1, Ximin Zhi 1, Zhan Jiang 1, Ni Ma 1,✉
PMCID: PMC11682425  PMID: 39732880

Abstract

The photosynthetic mechanism responsible for the differences in yield between different rapeseed varieties remains unclear, and there have been no consensus and definite conclusions about the relationship between photosynthesis and yield. Representation of the whole plant by measuring the photosynthetic performance at a single site may lead to biased results. In this study, we comprehensively analyzed the main photosynthetic organs of four high-yielding rapeseed varieties at the seedling, bud, flowering, and podding stages. The canopy photosynthetic parameters were derived by measuring the photosynthetic area, net photosynthetic rate, and chlorophyll content, and canopy photosynthetic capacity was used to evaluate the light utilization efficiency of different rapeseed varieties to establish the relationship between canopy photosynthetic traits and yield. The results showed that there were significant differences in photosynthetic traits among different parts of rapeseed plants. The photosynthetic trait parameters of the whole plant differed significantly when represented by leaves at different positions among different varieties, and different rapeseed varieties exhibited significantly different sensitivity to light intensity. The whole-plant study showed that the canopy photosynthetic capacity was the highest and second highest at the seedling and bud stage, respectively, both of which were closely and positively correlated with rapeseed yield, and ZY501 had higher canopy photosynthetic capacity than other varieties at these two stages due to its larger canopy photosynthetic area. Canopy chlorophyll content was also positively correlated with canopy photosynthetic capacity. These results indicated that investigation of photosynthetic characteristics at single sites in rapeseed might lead to biased results of photosynthetic capacity in different varieties, and provided a new evaluation index for studying the light utilization efficiency of rapeseed. Our results also clarified that canopy photosynthetic area has significantly greater contribution to canopy photosynthetic capacity than canopy photosynthetic efficiency, and provided a theoretical basis for investigating the photosynthesis mechanism underlying high crop yield.

Keywords: Rapeseed, Canopy photosynthetic area, Canopy photosynthetic efficiency, Canopy photosynthetic capacity, Yield

Subject terms: Leaf development, Plant morphogenesis, Photosynthesis, Plant development

Rapeseed (Brassica napus L.) is one of the most important oil crops in the world. Rapeseed production in China accounts for nearly 20% of world production and about half of the domestic edible vegetable oil1. However, the self-sufficiency rate of domestic vegetable oil in China has been only about 30% for a long time. From 2021 to 2022, the rapeseed oil production in China accounted for only 50.9% of the total domestic oilseed oil production2. It is of great strategic importance to develop the rapeseed industry and improve China’s oilseed production capacity for ensuring the security of vegetable oil supply, particularly increasing the rapeseed yield per unit area3.

Photosynthesis is the material basis for the formation of crop biomass and yield, and improvement of light utilization efficiency is one of the main ways to improve yield4. Theoretically, the light utilization efficiency of plants can reach 4.6–6.0%5, while the actual light utilization efficiency of current crop cultivars still has much room for improvement, which is also a key direction for crop design and improvement4,6. Photosynthetic area and efficiency are both important parameters in determining the light utilization efficiency. In the process of early crop domestication, larger leaves might increase the allocation of resources to the aboveground part of plants, thereby improving the light energy use efficiency and plant growth rate7. An increase in leaf area tends to increase the leaf area index (LAI), which is a potential mechanism to promote photosynthesis throughout the canopy8,9. Increasing the leaf area and hence the photosynthetic area may be an important strategy to enhance rapeseed yield through varietal improvement. Photosynthetic efficiency significantly affects crop yield10,11. For instance, bioengineering to improve photosynthetic efficiency in soybeans could promote fruiting, increase the number of seeds per plant, and significantly improve the yield by about 33%12. However, another study in rice reported a negative correlation between photosynthetic efficiency and biomass accumulation among different rice varieties13. These inconsistent results about the effect of photosynthetic efficiency may be related to the site of measurement or the method to assess photosynthesis. To date, most studies of plant photosynthesis have been focused on a specific leaf position, such as the functional leaves of rice14, maize15, rapeseed16 and other crops, and there are also differences in the selection of functional leaves for the same crop15–18. Assessment of the photosynthetic performance by measurement at a particular leaf position may lead to biased results in evaluating the photosynthetic capacity of an individual plant. In addition to functional leaves, leaves from other parts may also have great contribution to crop yield. For example, photosynthesis capacity in the lower canopy leaves of maize was significantly positively correlated with the yield, and also a key indicator for the formation of high maize yield under dense planting conditions19. The photosynthetic efficiency of leaves in the central canopy of soybean was found to play an important role in yield improvement by influencing the number of pods12. Therefore, it is highly necessary to comprehensively consider the whole-plant photosynthetic traits when evaluating the photosynthetic capacity of a plant. In addition, photosynthetic area and photosynthetic rate sometimes show a negative correlation with each other20,21, and the variation in light utilization efficiency may not be fully explained by changes in a single index, while can be explained by a combination of multiple effective indices.

Different from rice, maize, and other crops, whose main photosynthetic source for yield is the leaves, rapeseed has an obvious turnover of photosynthetic organs from leaves to pods, and long-stalked, short-stalked, and sessile leaves in different parts at different reproductive stages from seedling to flowering. Accumulation of photosynthetic products from the three types of leaves before flowering is mainly used for the construction of individual configurations, laying the foundation for yield22. Photosynthesis of silique walls accounts for 60–70% of rapeseed yield23,24. It is obviously inappropriate to compare the photosynthetic characteristics of different rapeseed varieties at a single leaf position due to its complex canopy configuration.

To dissect the mechanism underlying the differences in yield among different rapeseed varieties from the perspective of light utilization efficiency, we selected four high-yielding rapeseed varieties widely planted in the Yangtze River Basin for comprehensively analyzing their main photosynthetic organs at the seedling, bud, flowering and podding stages. By measuring the photosynthetic area, net photosynthetic rate, and chlorophyll content in different parts of the plant, we determined the corresponding parameters of canopy photosynthetic traits and capacity to assess the light utilization efficiency of each variety, as well as established the relationship between canopy photosynthetic traits and the yield. Moreover, we evaluated the adaptation of rapeseed varieties to light intensity at the seedling stage. The results may provide a new evaluation index for studying the light utilization efficiency of rapeseed, and preliminarily reveal the photosynthetic mechanism for the formation of high yield in different rapeseed varieties, providing a theoretical basis for investigating the photosynthetic mechanism of high-yielding crops.

Materials and methods

Experimental materials

The rapeseed varieties used in the experiment were Zhongyouza 501 (ZY501), Zhongyouza 39 (ZY39), Zhongyouza 19 (ZY19), and Dadi 199 (DD199), four high-yielding hybrid rapeseed varieties widely grown in the Yangtze River Basin, among which ZY501 set a new record for high yield in the Yangtze River Basin. The materials are all commercial varieties supplied by the Oil Crops Research Institute, Chinese Academy of Agricultural Sciences.

Experimental design

Polyethylene plastic pots were used for the experiment, with each containing 5 kg of soil mixture (soil: organic matter = 2:1, v/v). A basic fertilizer was applied before sowing with N 0.2 g, P2O5 0.15 g and K2O 0.2 g per kg of soil mixture. The experiment was carried out in a controlled growth chamber with a photoperiod of 16 h (06:00–22:00 h) and a day/night temperature 25/18 °C at the Oil Crops Research Institute, Chinese Academy of Agricultural Sciences. Each variety was planted in eight pots and the seedlings were fixed at the three-leaf stage, leaving three uniformly growing seedlings in each pot. The water content of soil was maintained at about 70% of the field moisture capacity throughout the life of rapeseed. Phenotypic and photosynthetic parameters were determined at the seedling stage (eight-leaf stage), bud stage (upon the appearance of buds), flowering stage (flowering of 75% of inflorescences) and podding stage (15 days after final flowering) of rapeseed, and four pots were selected and harvested individually at maturity for determination of individual plant biomass and yield.

Measurement of leaf and silique wall area at different positions

The green leaf area at all leaf positions of the whole plant was determined at the seedling, bud, and flowering stage. It was determined with the straightedge method by measuring the leaf length (leaf tip to leaf base, excluding petiole) and leaf width (the widest point in the perpendicular direction of the main leaf vein), and calculated as the leaf area = leaf length × leaf width × 0.7525. The silique wall area was calculated using Clarke’s method as Sa = π × d × (h1 + 1/3 × h2)26, where Sa is the silique wall area, h1 = 0.8 H, h2 = 0.2 H, with H indicating the length of the rapeseed pod, and d representing the average width of the pod. At the podding stage, three pods were selected from the upper, middle, and lower part of each branch for area measurement. The average value represented the area of a single pod of that branch, which was then multiplied by the number of pods on each branch to obtain the total area of silique walls of that branch. Each treatment was replicated four times. Since the leaves and pods are the main photosynthetic organs, their area was used to represent the photosynthetic area of rapeseed at different reproductive stages.

Measurement of chlorophyll content in leaves and silique walls at different positions

Four round leaf discs (6 mm in diameter) were randomly punched from each green leaf with a single-hole punch. One pod was selected from the upper, middle, and lower part of each branch at the podding stage and the silique wall was punched. The leaves and silique walls were extracted with 95% ethanol overnight in the dark until the samples turned white, and then the contents of chlorophyll a and chlorophyll b were determined at wavelengths of 665 nm and 649 nm using 95% ethanol as a blank, and summed to obtain the total chlorophyll content27. Four replicates were measured for each treatment.

Measurement of net photosynthetic rate in leaves and silique walls at different positions

The net photosynthetic rates of rapeseed leaves and silique walls at saturated light intensity (light intensity set at 1200 µmol·m− 2·s− 1 for leaves and 1500µmol·m− 2·s− 1 for silique walls) were determined using a LI-6800 portable photosynthesis measurement system (LI-COR Inc., Lincoln, NE, USA). The CO2 concentration was set at 400 µmol·mol− 1, a flow rate at 500µmol·s− 1, and the leaf chamber temperature and humidity at 25 °C and 60%, respectively. The green leaf area at all leaf positions of the whole plant was determined at the seedling, bud, and flowering stage and three pods selected from the upper, middle and lower parts of each branch were measured at the podding stage. Net photosynthetic rates of all green leaves at the seedling stage were also determined at different light intensities of 300, 600 and 1200 µmol·m− 2·s− 1, which were labelled as LL, ML, and HL, respectively. Each treatment was replicated four times.

Calculation of canopy chlorophyll content and net photosynthetic rate

At the seedling, bud, and flowering stages, the ratio of leaf area at different leaf positions to the total leaf area was used as the weight, and at the podding stage, the ratio of silique wall area of each branch to the total silique wall area was used as the weight. The chlorophyll content and net photosynthetic rate of each leaf position or branch were multiplied by the corresponding weights of each position and then the summed values were used to represent the canopy chlorophyll content and net photosynthetic rate. This method can reduce the influence of uneven leaf area distribution on the evaluation of canopy characteristics to a certain extent. Each treatment was replicated four times.

Calculation of canopy photosynthetic capacity

The photosynthetic capacity was represented by the value of the photosynthetic area multiplied by the net photosynthetic rate. The photosynthetic area of each leaf position or branch was multiplied with the net photosynthetic rate of the corresponding part, and the obtained values were summed to represent the canopy photosynthetic capacity of rapeseed at different growth stages. Each treatment was replicated four times.

Measurement of biomass and yield at maturity

At maturity, four pots were selected and the rapeseed plants were harvested individually to measure the yield and plant biomass.

Data processing and statistical analysis

Microsoft Office Excel 2019 was used to process the data. One-way ANOVA analysis was performed using IBM SPSS Statistics (Version 20.0; IBM SPSS Inc., USA). Multiple comparisons were made using Duncan’s test at a significance level of P < 0.05, and data were presented as the mean of four replicates. Data plots and correlation analysis plots were generated using Origin software (Version OriginPro 2017; OriginLab Inc., USA).

Results and analysis

Photosynthetic area at each leaf or branch position of different rapeseed varieties

The distribution of photosynthetic area of rapeseed during the whole life cycle is shown in Fig. 1. The results revealed that the coefficient of variation (CV) of photosynthetic area at the seedling, bud, flowering, and podding stages were 37.01%, 39.06%, 58.29%, and 82.17%, respectively (Fig. 1), indicating that the photosynthetic area of rapeseed has an obviously uneven distribution and the unevenness degree increases with the progression of growth. Taking the seedling stage as an example (Fig. 1A), the photosynthetic area represented by the functional leaf area among different varieties followed the order of ZY39 > ZY501 > ZY19 > DD199 (top second leaf) or ZY501 > ZY39 > DD199 > ZY19 (top third leaf). These results demonstrated that the photosynthetic area of the whole plant differed significantly when represented by leaves at different positions. Therefore, it is highly necessary to comprehensively consider the photosynthetic performance of each position to avoid biased results caused by different performance of different leaf positions.

Fig. 1.

Fig. 1

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Photosynthetic area distribution of different rapeseed varieties at the seedling (A), bud (B), flowering (C), and podding stages (D). The axis labels (1, 2, 3 …… ) in (A–C) denote top first leaf, top second leaf, top third leaf, ……; and the axis labels (1, 2, 3 ……) in (D) denote top first branch (main branch), top second branch, top third branch.

Chlorophyll contents in leaves and silique walls of different rapeseed varieties at different leaf or branch positions

The average chlorophyll contents in the leaves and silique walls at the seedling, bud, flowering, and podding stages were 1.98, 1.95, 1.35, and 0.66 mg·dm− 2, with CV of 23.71%, 27.03%, 28.02%, and 8.78%, respectively (Fig. 2), indicating that the growth stage and leaf or branch position significantly affect the content and distribution of chlorophyll. At the seedling stage, the chlorophyll content in the top second and third leaves followed the order of ZY501 > ZY39 > DD199 > ZY19 and ZY501 > DD199 > ZY39 > ZY19, respectively (Fig. 2A). These results indicated that the chlorophyll content of the whole plant represented by different leaf positions was significantly different. Therefore, it may be inaccurate to represent the whole plant chlorophyll content with the chlorophyll content in single leaf positions.

Fig. 2.

Fig. 2

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Chlorophyll contents of different rapeseed varieties at each leaf or branch position.

Net photosynthetic rate in leaves and silique walls of different rapeseed varieties at each leaf or branch position

The net photosynthetic rate at saturated light intensity varied significantly among different leaf or branching positions. At the seedling, bud, flowering, and podding stages, the average net photosynthetic rate was 18.04, 14.42, 8.97, and 5.54 µmol·m− 2·s− 1, with CV of 21.39%, 36.93%, 20.02% and 7.43%, respectively (Fig. 3). Taking the seedling stage as an example (Fig. 3A), the net photosynthetic rate in different varieties at the same leaf position followed the order of ZY501 > ZY39 > DD199 > ZY19 (top second leaf, top third leaf) and ZY39 > ZY501 > DD199 > ZY19 (top fourth leaf), indicating that the photosynthetic efficiency was different at different leaf positions.

Fig. 3.

Fig. 3

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Net photosynthetic rate of leaves and silique walls at each leaf or branch position of ZY19 (A), DD199 (B), ZY501 (C), and ZY39 (D).

Response of net photosynthetic rate at each leaf position to light intensity

As shown in Fig. 4, the net photosynthetic rate exhibited the same first increasing and then decreasing trend from the top to the bottom of leaf position in all rapeseed varieties at low, medium, and high light intensities (300, 600 and 1200 µmol·m−2·s−1). The CV at different leaf positions of all varieties gradually increased with increasing light intensity. Specifically, the CV of ZY19, DD199, ZY501, and ZY39 rose from 24.13%, 18.73%, 27.87%, and 27.27% at low light intensity to 31.43%, 28.97%, 44.45%, and 36.44% at high light intensity, which were increased by 30.30%, 54.63%, 59.48%, and 33.61%, respectively, indicating that the photosynthesis of different rapeseed varieties has different sensitivity to light intensity. The sensitivity to light intensity followed the order of ZY501 > DD199 > ZY39 > ZY19, in which ZY501 and DD199 had significantly higher sensitivity to light intensity than ZY39 and ZY19.

Fig. 4.

Fig. 4

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Response of net photosynthetic rate at each leaf position to light intensity. LL, ML, and HL in the figure represent low light intensity (300 µmol·m-2·s-1), medium light intensity, (600 µmol·m-2·s-1), and high light intensity (1200 µmol·m-2·s-1), respectively. Different lowercase letters indicate significant differences among leaf positions (P < 0.05).

Canopy structure and canopy photosynthesis parameters of different rapeseed varieties

To avoid biased results caused by representing the whole plant traits by the performance of individual leaf traits, we analyzed the whole plant traits. With the progression of plant growth, the canopy photosynthetic area of rapeseed reached the maximum at the podding stage, followed by the bud stage, and then the flowering and seedling stages. Compared with that at the seedling stage, the canopy photosynthetic area increased by 37.40%, 18.72% and 107.91% at the bud, flowering and podding stages, respectively, and was the maximum in ZY501 among different varieties from the seedling to the flowering stage (Fig. 5A). The parameters such as canopy chlorophyll content and canopy net photosynthetic rate all gradually decreased with the progression of plant growth. Compared with those at the seedling stage, the canopy chlorophyll content and net photosynthetic rate at other three stages were reduced by 8.58%, 44.01%, 70.15% and 32.87%, 58.66%, 71.45%, respectively, and ZY501 had the maximum canopy chlorophyll content at the seedling and bud stages and canopy net photosynthetic rate at the bud stage (Fig. 5B–C). These results indicated that canopy traits of rapeseed are highly variable at different reproductive stages, and the asynchronous changes in canopy photosynthesis-related traits are not conducive to the interpretation of changes in canopy light utilization efficiency.

Fig. 5.

Fig. 5

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Analysis of canopy photosynthetic area (A), canopy chlorophyll content (B), canopy net photosynthesis rate (C, D) of different rapeseed varieties. LL, ML, HL in (D) represents different light intensities. Different lowercase letters indicate significant differences between treatments (P < 0.05).

Under different light intensities, the canopy net photosynthetic rate of all varieties followed the order of ZY39 > ZY501 > ZY19 > DD199, and increased significantly with increasing light intensity. At medium and high light intensities, the canopy net photosynthetic rate of ZY39 was significantly higher than that of ZY19 and DD199, and that of ZY501 was also significantly higher than that of DD199 (Fig. 5D). These results indicated that light intensity has no obvious effect on the changing trend in net photosynthetic rate of rapeseed, but increasing light intensity could increase the difference in net photosynthetic rate between different varieties.

Canopy photosynthetic capacity of different rapeseed varieties

Photosynthetic area and efficiency together determine the light utilization efficiency of plants. Here, we evaluated the canopy photosynthetic capacity by combining the canopy photosynthetic area and canopy net photosynthetic rate to assess the light utilization efficiency of different rapeseed varieties. At different light intensities, the canopy photosynthetic capacity of all varieties followed the order of ZY501 > ZY39 > DD199 > ZY19, with ZY501 showing the highest photosynthetic capacity at all light intensities (Fig. 6A). With the progression of plant growth, the canopy photosynthetic capacity of rapeseed first decreased and then increased, with the highest value at the seedling stage and the lowest value at the flowering stage. Specifically, compared with that at the seedling stage, the canopy photosynthetic capacity decreased by about 7.88%, 51.30%, and 41.01% at the bud stage, flowering stage, and podding stage, respectively. The slight decrease in canopy photosynthetic capacity at the bud stage relative to that at the seedling stage could be mainly attributed to the increase in canopy photosynthetic area, while the relatively more dramatic decreases at the flowering and podding stages could be ascribed to significant decreases in canopy net photosynthetic rate. At the seedling stage, ZY501 had the highest canopy photosynthetic capacity, mainly due to the lager canopy photosynthetic area, which was about 9.45%, 26.24%, and 47.62% higher than that of ZY39, DD199 (P < 0.05), and ZY19 (P < 0.05), respectively. At the bud stage, ZY501 also had significantly higher canopy photosynthetic capacity than other varieties, while there was no significant difference at the flowering stage, and ZY19 had the highest canopy photosynthetic capacity at the podding stage (Fig. 6B).

Fig. 6.

Fig. 6

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Analysis of canopy photosynthetic capacity of different rapeseed varieties under different light intensities (A) and under saturated light intensity at different growth stages (B). Different lowercase letters indicate significant differences between treatments (P < 0.05).

Individual plant biomass and yield of different rapeseed varieties

We determined the biomass of individual rapeseed plants at maturity. The results showed that ZY39 had the highest individual plant biomass, which was significantly higher than that of ZY19 and DD199, and was not significantly different from ZY501, and there was no significant difference among ZY501, ZY19, and DD199 (Fig. 7A). Determination of the yield of individual plants revealed that ZY501 had the highest individual plant yield, which was significantly higher than that of ZY19 and DD199, and there was no significant difference among ZY19, DD199, and ZY39 (Fig. 7B).

Fig. 7.

Fig. 7

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Analysis of individual plant biomass and yield of different rapeseed varieties. Different lowercase letters indicate significant differences between treatments (P < 0.05).

Correlation analysis between canopy parameters and yield of different rapeseed varieties

We performed a correlation analysis of various canopy parameters and yield of different rapeseed varieties. The results showed that there was a significant positive correlation between biomass and yield. Plant photosynthetic characteristics and yield had the most significant correlation at the seedling stage, when canopy photosynthetic area, canopy net photosynthetic rate, canopy photosynthetic capacity, and canopy chlorophyll content were all positively and significantly correlated with the yield, followed by the bud stage, when the canopy photosynthetic area, canopy photosynthetic capacity, and canopy chlorophyll content all showed positive correlations with the yield. No significant correlation was observed between canopy photosynthetic traits and yield at the flowering and podding stages. Moreover, canopy photosynthetic capacity also showed correlations with canopy photosynthetic area, canopy net photosynthetic rate, and canopy chlorophyll content at the seedling and bud stages (Fig. 8).

Fig. 8.

Fig. 8

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Correlation analysis between canopy parameters and yield. MQ, TQ, HQ, and JQ denote the seedling stage, bud stage, flowering stage and podding stage, respectively; LA, PA, Pn, PC, and Cha denote canopy photosynthetic area, canopy silique wall area, canopy net photosynthetic rate, canopy photosynthetic capacity, and canopy chlorophyll content, respectively.

Discussion

Advantages and disadvantages of studying photosynthetic characteristics by crop functional leaves

Leaves are main photosynthetic organs for crops, and leaves at different positions may vary in photosynthetic function. Functional leaves of crops are regarded as the main representative parts in the study of photosynthetic characteristics, which can reflect the growth state of crops to a certain extent. For example, a study of the response of maize ear leaves to high temperature stress revealed the mechanism for high temperature to influence the photosynthetic characteristics of maize canopy28. Flag leaf morphology was found to determine the ultimate yield potential of wheat29, and mining of flag leaf morphology-related genes may provide a theoretical basis for high-yield breeding of wheat30. Functional leaves provide convenience for studying the photosynthetic characteristics of crops with obvious advantages of high efficiency and speed, but they also have some limitations at the same time. For example, rapeseed has a complex canopy structure and succession of photosynthetic organs. Almost all leaves will wither at the podding stage, when the silique wall becomes the main photosynthetic organ to contribute to most of the rapeseed yield31,32. In addition to that of functional leaves, the photosynthesis of leaves in other canopy parts also plays an important role in crop yield formation 12,19. In this study, we investigated the photosynthetic characteristics of top second leaf, top third leaf or top fourth leaf of rapeseed. The results showed that the comparison of photosynthetic parameters of varieties differed at different positions, indicating that it may be inaccurate to evaluate the photosynthetic capacity of rapeseed based on single leaf positions.

Effect of leaf area on canopy light utilization efficiency

LAI is one of the most important factors affecting canopy photosynthesis33,34. An increase in leaf area leads to an increase in LAI, which may be a potential mechanism to improve canopy photosynthesis8,9. LAI has a direct impact on plant canopy light interception, thereby affecting canopy light utilization efficiency35. Previous studies of chard, cabbage, sunflower, tomato, wheat, maize and other crops have demonstrated that variations in whole-plant carbon gain cannot be explained by changes in photosynthetic rate during crop domestication. After domestication, the leaves become larger to increase the light interception, which is the main reason for the increase in light utilization efficiency and canopy carbon gain36. Some other studies have also shown that leaves grow larger during crop domestication37. Phenotypic analysis of leaves in this study revealed significant differences in leaf area among different varieties and a significant positive correlation between leaf area and yield at the seedling and bud stages of rapeseed. The effect of leaf area on yield is closely related to its positive regulation of canopy photosynthetic capacity.

Effect of photosynthetic efficiency on canopy light utilization efficiency

Photosynthetic efficiency is also one of the most important factors affecting canopy photosynthesis38. A study in maize reported that leaf photosynthetic efficiency is the main determinant of canopy light utilization efficiency, with a 17.5–29.2% contribution in different lines39. Further improvement of crop light utilization efficiency may be achieved by increasing the recovery rate of photosynthetic efficiency after photoinhibition40. The plant canopy has different response to light intensity. A study of 215 rice populations revealed that photosynthetic efficiency under low light is highly heritable and shows great variations among varieties. Enhancement of photosynthetic efficiency under low light is a feasible way to improve canopy photosynthetic efficiency and is a new modification target to improve rice yield41. In this study, a significant positive correlation was observed between canopy photosynthetic efficiency and canopy photosynthetic capacity at the seedling and bud stages, which is consistent with the earlier proposal of increasing photosynthetic efficiency to improve light utilization efficiency. Light intensity was found to have no obvious effect on the changing trend in photosynthetic efficiency among different rapeseed varieties, but higher light intensities could increase the difference in photosynthetic efficiency between different varieties, indicating that the photosynthetic efficiency of rapeseed at medium and high light intensities can serve as an important target for improvement of the canopy light utilization efficiency. The reason for the inconsistency with previous results may be the differences in crop type. The less dramatic variation in photosynthetic efficiency at low light intensities suggests that there is still great potential to improve photosynthetic efficiency at low light intensities in rapeseed.

Influence of canopy light utilization efficiency on crop biomass and yield formation

Canopy light utilization efficiency has a major impact on crop biomass and yield development, and its improvement is an important way to further increase crop biomass and yield42. A study of wheat varieties in Australia over the past 50 years found that the light interception and radiation use efficiency of the wheat canopy increase with continuous improvement of plant type, which is an important factor accounting for the continuous increase in wheat yield43. Previous studies of 20 rice and 60 wheat varieties reported that crop biomass or yield has no positive correlation with photosynthetic capacity13,44, which contradicts with the current photosynthetic theory, probably because the overall light utilization efficiency of crops is determined by the total canopy photosynthesis rather than by individual leaf photosynthesis41. Canopy light utilization efficiency is not only influenced by plant configuration (such as LAI and leaf angle index) and photosynthetic efficiency, but also affected by the leaf chlorophyll content38. Leaf chlorophyll content is a key determinant of canopy light interception efficiency and drives canopy light utilization45,46. The chlorophyll content in leaves varies greatly between varieties, affecting not only the light absorption but also the photosynthetic efficiency of the leaves47. It has been found that reducing the number of antenna units in photosystem I and photosystem II can effectively promote photosynthetic photochemical reactions, reduce heat dissipation, and improve the light energy use efficiency of the canopy, which together contribute to increases in crop biomass and yield potential48. In this study, the parameters of photosynthetic capacity were used to evaluate the canopy light utilization efficiency of rapeseed. The results showed that there was a significant positive correlation between photosynthetic capacity and yield at some critical stages (the seedling and bud stages) that determine the yield of rapeseed, which is in agreement with the results of previous studies, and different varieties showed consistent performance in photosynthetic capacity under different light intensities. Canopy chlorophyll content showed a positive effect on canopy photosynthetic capacity, which is contradictory to the previous finding that reducing leaf chlorophyll content would improve light distribution in the crop canopy and thus increase the canopy light utilization efficiency5,49. This is probably due to the fact that the canopy light utilization efficiency of the rapeseed is mainly determined by the upper canopy leaves and the chlorophyll content of the upper canopy leaves directly affects the photosynthetic efficiency, while the light distribution in the lower part of the canopy has a smaller contribution to the light utilization efficiency.

Conclusions

By comparing the photosynthetic traits of individual photosynthetic organs and the whole canopy of high-yielding rapeseed varieties, we found that functional leaves at a certain position could not correctly reflect the photosynthetic traits in rapeseed. As an index for evaluating the light utilization efficiency in the canopy of rapeseed, photosynthetic capacity could effectively avoid the asynchronous changes in canopy photosynthetic area and photosynthetic efficiency, and explain the mechanism for high yield formation in rapeseed. Canopy chlorophyll content directly affected canopy photosynthetic efficiency, and canopy photosynthetic efficiency and canopy photosynthetic area were closely and positively correlated with canopy photosynthetic capacity, with the latter having a greater contribution to canopy photosynthetic capacity than the former. Our study provides a new evaluation index for the canopy light utilization efficiency of rapeseed and lays a foundation for elucidating the photosynthetic mechanism underlying high yield in rapeseed.

Acknowledgements

We are grateful to the individuals and teams from the Oil Crops Research Institute, Chinese Academy of Agricultural Science, who directly provided the rapeseed varieties for this study. This work was supported by the National Natural Science Foundation of China (Grant No. 31971855), the Agricultural Science and Technology Innovation Project of Chinese Academy of Agricultural Science (CAAS-ASTIP-2021-OCRI), the Wuhan Knowledge Innovation Special Program (2023020201020400), the China Agriculture Research System (CARS-12) and the Hubei Provincial Natural Science Foundation of China (2024AFB442).

Author contributions

X.X.: Data curation; formal analysis; investigation; methodology; validation; sisualization; roles/writing—original; writing-review & editing. B.D.: Data curation; investigation; methodology. F.H.: Methodology. X.Z.: Methodology; software. Z.J.: Methodology; software. N.M.: Funding acquisition; project administration; Writing—review & editing; supervision.

Data availability

This manuscript contains all data generated or analyzed during the current study. Other necessary data of this study can be obtained from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

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Data Availability Statement

This manuscript contains all data generated or analyzed during the current study. Other necessary data of this study can be obtained from the corresponding author on reasonable request.

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