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. 2017 Sep 6;7:10647. doi: 10.1038/s41598-017-09584-5

Interactive effect of gibberellic acid and NPK fertilizer combinations on ramie yield and bast fibre quality

Sana Ullah 1, Sumera Anwar 1, Muzammal Rehman 1, Shahbaz Khan 1, Sara Zafar 2, Lijun Liu 1,✉, Dingxiang Peng 1,✉
PMCID: PMC5587721  PMID: 28878353

Abstract

Understanding the effects of different combinations of nitrogen (N), phosphorus (P) and potassium (K) fertilizers and the effects of GA3 (gibberellic acid) foliar spray on the fiber quality and yield of ramie are important for maximizing the economic value of these plants. Three pot experiments were conducted using low NPK (140:70:140 kg/ha), normal NPK (280:140:280 kg/ha), and low NPK + GA3 (10 mg/L) treatments. In each experiment, following fertilizers were applied: no fertilizer (control), N, P, K, NP, NK, PK, and NPK. Ramie was harvested three times from each plant; ramie grown without fertilizers had significantly lower biomass and yield than plants grown with fertilizers. At both normal and low fertilization rates, application of NPK resulted in greater growth and yield than application of N, P, K, NP, NK, or PK. Unfertilized plants produced the thinnest fibres (22-24 µm), with lowest elongation rate (3.0–3.1%) and breaking strength (22.7–23.3 cN). Fibre yield and fibre quality were improved by application of GA3 + fertilizers. Maximum fibre yield was obtained at low NPK + GA3 treatment, resulting in 65–81% more yield than low NPK alone. GA3 with low NPK treatment significantly improved fibre diameter, fibre elongation, and breaking strength compared to both NPK alone and control treatment.

Introduction

An important aspect of agriculture is the cultivation of plants for food, fiber, biofuel, medicine and other products used to sustain and enhance human life. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the development of civilization1–4. In response to the current ecological and environmental problems, the textile industry has increased its demand for eco-friendly natural fibres. Additionally, the use of fully biodegradable “green” composites made from vegetable fibres and non-woody plant fibres for paper production may help to mitigate global warming5. Bast (phloem) fibres are a considerable source of commercial fibres and are obtained from crops such as Linum usitatissimum (flax), Cannabis saliva (hemp), Corchorus capsularis (jute), Hibiscus cannabinus (kenaf), and Boehmeria nivea (ramie). Ramie or China grass (Boehmeria nivea (L.) Gaud.) is a perennial herbaceous plant, mainly grown in China and other Asian countries. The fibres obtained from ramie plants are the longest known plant fibres in nature and attain a length of more than 550 mm6, 7. Ramie fibre has high strength, good durability, moisture absorbance capacity, and high lustre. These characteristics have made ramie fibre suitable for use in the manufacture of a wide variety of textiles and cordage products. Ramie can be blended with other natural and synthetic fibres, including cotton, silk, wool, polyester, and flax8, 9. However, despite the remarkable qualities of this fibre, ramie has received comparatively little attention as an important world crop. However, commercial cultivation of this crop has recently increased in countries such as China, Brazil, and the Philippines10.

Yield and fibre quality are the most important factors to consider in ramie production. As the bast fibre from ramie is extracted from the outer part of the stem, the fibre yield is dependent on the biomass, length, diameter, and thickness of the stem. Fibre from ramie is normally harvested between three and six times each year with an average annual yield of nearly 1200–1800 kg ha−1 of fibre11. Due to the plant’s robust growth and biomass production, the fibre yield of ramie is highly dependent on the availability of soil nutrients. According to Hiroce et al., ramie plants cannot continue to grow without fertilizers after they reach 60 days of age12. The application of fertilizer is crucial for sustaining fibre yield, and optimizing yield requires investigation into suitable fertilization rates13.

Growth regulators play an essential role in the biosynthesis of crop fibres, affecting both the elongation rate and quality. The gibberellins (GA) are natural plant growth promoting hormones that cause the elongation of plant cells. Exogenous application of GAs alters plant growth and affects developmental features. Gibberellins exist in various forms and the bioactive forms are GA1, GA3, GA4, and GA7. One of these forms, gibberellic acid (GA3), promotes growth, especially fibre production and elongation, in hemp, jute, kenaf, cotton, and ramie. The greatest concentrations of GA3 are found in those tissues that are elongating the most rapidly, such as stems, petioles, and, in some crops, flower inflorescences14, 15. The objective of this project was to evaluate several different combinations of low and normal rates of N, P, and K fertilization and the combined effect of a plant growth regulator and a low rate of fertilization on the subsequent growth, yield and fibre quality of ramie.

Results

Influence of treatments on growth

There were three treatment groups: low NPK (N:P:K at 140:70:140 kg ha−1), normal NPK (N:P:K at 280:140:280 kg ha−1), and low NPK + GA3 (gibberellic acid; 10 mg L−1). Each treatment group included control, K, P, PK, N, NK, NP, and NPK treatments. The results from a four-way randomized block analysis of variance within treatment groups showed significant effects of harvest time (H) and fertilization on the main growth parameters of ramie (Table 1). However, the interaction between variables (H × N × P × K) was non-significant. All combinations of N, P, and K resulted in greater ramie growth than control treatments, including biomass, number of stems, stem diameter, and stem weight (Tables 2 and 3). Fertilization with NPK resulted in the maximum plant height, biomass, stem weight, and number of stems. The height of plants fertilized with NPK varied significantly between treatment groups:plant height was 29–72% greater than controls for the low NPK group, 31–83% greater than controls for the normal NPK group, and 54–106% greater than controls for the low NPK + GA3group. Overall, the maximum height, biomass, and stem diameter of ramie plants were attained by low NPK + GA3 plants at the third harvest (H3) and the minimums were observed for control plants at the first harvest (H1). The overall greatest stem fresh weight was found for low NPK + GA3 plants at the second harvest (H2) and the maximum number of stems was recorded for low NPK + GA3 plants at H1.

Table 1.

Four-way randomized block ANOVA evaluating the effect of harvest (H) number, nitrogen, phosphorous, potassium and their interactions on the growth and yield components of ramie.

Source of variation Plant height cm Biomass kg Stem weight Stem NO. Stem diameter mm Raw fiber yield kg Degummed fiber yield kg
Low NPK
H *** *** *** *** *** *** ***
N *** Ns *** *** *** *** ***
P ** *** *** * * * ***
K ** ** *** Ns * ** ***
N × P × K ** Ns Ns * *** Ns **
H × N × P × K Ns * Ns Ns Ns Ns **
Normal NPK
H *** *** *** *** *** *** ***
N *** *** *** *** ** *** ***
P ** *** *** * *** *** ***
K ** *** *** *** *** *** ***
N × P × K *** Ns Ns Ns ** Ns Ns
H × N × P × K Ns Ns Ns Ns Ns * Ns
Low NPK + GA 3
H *** *** *** ** *** *** ***
N *** *** *** *** *** *** ***
P *** *** *** *** *** *** ***
K *** *** *** *** *** *** ***
N × P × K ** Ns Ns Ns ** Ns Ns
H × N × P × K Ns Ns Ns Ns Ns * Ns
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Ns, non-significant; *, significant at p < 0.05; **, significant at p < 0.01; and ***, significant at p < 0.001.

Table 2.

Plant height and biomass of ramie under different treatments at three harvests (H1, H2, and H3).

Treatments Plant height (cm) Plant biomass (kg)
H1 H2 H3 H1 H2 H3
Low NPK (kg/ha)
N0P0K0 (control) 49.0b 49.0c 60.4b 0.11c 0.27c 0.13c
N0P0K140 58.7ab 73.0b 86.2a 0.18bc 0.54ab 0.36ab
N0P70K0 60.3a 76.7ab 83.4a 0.18bc 0.47abc 0.23bc
N0P70K140 67.7a 88.7a 71.4ab 0.25a 0.35bc 0.44ab
N140 P0 K0 60.3a 89.0a 81.6a 0.14c 0.31c 0.27abc
N140 P0K140 61.3a 79.3ab 79.3a 0.22ab 0.33bc 0.31abc
N140P70 K0 59.3ab 82.7ab 79.3a 0.24ab 0.42abc 0.49a
N140P70K140 63.3a 84.3ab 88.3a 0.26a 0.56a 0.46a
Mean 60.0 A 77.7 A 78.7 A 0.19 C 0.41 A 0.34B
Normal NPK (kg/ha)
N0 P0 K0 (control) 56.3c 53.7c 63.3e 0.08d 0.18c 0.11d
N0 P0 K280 64.3abc 88.0ab 88.0abc 0.26bc 0.45ab 0.34c
N0 P140 K0 66.7abc 88.0ab 93.7ab 0.29abc 0.46ab 0.44bc
N0 P140K280 73.0a 77.0b 71.7de 0.33ab 0.39b 0.58ab
N280P0 K0 62.3bc 99.0a 97.3a 0.25c 0.48ab 0.47abc
N280P0 K280 70.3ab 84.3b 83.7bc 0.33abc 0.53ab 0.62a
N0 P140 K0 69.0ab 86.3ab 0.7cd 0.35a 0.61a 0.49abc
N0 P140K280 74.0a 98.3a 96.0a 0.33ab 0.58a 0.59ab
Mean 67.0B 84.3A 84.3A 0.28B 0.46A 0.46A
Low NPK + GA 3
N0 P0 K0 (control) 54.7b 66.0c 58.0e 0.11e 0.24e 0.14c
N0 P0 K140 + GA3 79.3a 92.7b 96.0bcd 0.30d 0.52bcd 0.48b
N0 P70 K0 + GA3 79.0a 94.3ab 88.3d 0.31 cd 0.64abc 0.52ab
N0P70K140 + GA3 82.3a 105.0a 102.0bc 0.35bc 0.65abc 0.56ab
N140P0 K0 + GA3 78.7a 90.7b 91.7 cd 0.32bcd 0.38de 0.57ab
N140P0 K140 + GA3 87.3a 92.7b 106.7b 0.45a 0.68a 0.66ab
N140P70K0 + GA3 81.3a 91.7b 94.7bcd 0.36b 0.49 cd 0.71a
N140P70K140 + GA3 86.0a 101.3ab 119.7a 0.44a 0.67ab 0.73a
Mean 78.6B 91.8A 94.6A 0.33B 0.53A 0.55A
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The low NPK (N:P:K at 140:70:140 kg ha−1), normal NPK (N:P:K at 280:140:280 kg ha−1) and low NPK + GA3 (gibberellic acid; 10 mg L−1) treatment groups. Each of the three treatment groups were further subdivided into (Control, K, P, PK, N, NK, NP, and NPK) treatments.

Plants were harvested on June 20 (1st harvest H1), August 10 (2nd harvest H2), and October 10 (3rd harvest H3), 2015, of ramie, respectively. Data followed by different lowercase letters (a, b, c) in the same column indicate statistically significant differences within a harvest; values followed by different uppercase letters (A, B, C) in the same row indicate significant difference between harvests at p < 0.05 based on LSD test.

Table 3.

Stem weight, number of stem and stem diameter of ramie plants under the low NPK, normal NPK and low NPK + GA3 treatment groups at three harvests (H1, H2, and H3).

Treatments Stem weight (g) NO of stem (plant−1) Stem diameter (mm)
H1 H2 H3 H1 H2 H3 H1 H2 H3
Low NPK
N0P0K0 (control) 22.7b 104.3b 47.7d 2.67b 2.33c 2.67b 4.74c 5.93b 5.93c
N0P0K140 70.7a 180.3ab 175.3abc 7.00a 5.67ab 3.67ab 6.29b 7.77a 8.97a
N0P70K0 79.7a 190.3ab 110.3bcd 6.00a 5.00b 4.00ab 6.73ab 8.20a 8.39ab
N0P70K140 104.0a 166.3ab 185.7ab 6.33a 5.67ab 4.00ab 6.53ab 7.17ab 7.32bc
N140P0K0 62.0ab 175.3ab 90.7 cd 6.67a 7.00ab 5.33a 6.39ab 7.93a 8.42ab
N140P0K140 78.3a 182.7ab 160.7abc 6.00a 6.33ab 5.00a 7.42a 8.20a 8.06ab
N140P70K0 86.3a 200.7ab 211.0a 7.00a 7.33a 5.67a 6.49ab 8.27a 8.80ab
N140P70K140 87.7a 264.0a 249.0a 7.00a 5.67ab 5.33a 7.20ab 8.03a 8.89a
Mean 73.9B 183.0A 153.8A 6.1A 5.6A 4.5B 6.5B 7.7A 8.1A
Normal NPK
N0P0K0 (control) 29.7c 68.3b 42.3e 3.33c 3.33c 2.33c 4.76b 5.70b 6.21b
N0P0K280 110.0b 197.3a 139.7d 6.00ab 5.67abc 4.67abc 7.00a 8.47a 8.66a
N0P140K0 111.7b 198.0a 153.0d 6.67a 4.33bc 3.33bc 7.84a 8.27a 9.22a
N0P140K280 169.3a 171.3ab 266.0ab 6.00ab 4.67abc 5.00ab 7.70a 8.60a 9.45a
N280P0K0 121.7ab 205.0a 187.3cd 4.67bc 6.33ab 4.67abc 6.94a 8.27a 8.81a
N280P0K280 157.7ab 233.7a 235.7abc 6.67a 7.33a 6.00a 7.60a 8.57a 9.09a
N280P140K0 159.7ab 257.7a 204.3bcd 7.00a 7.33a 4.67abc 7.40a 8.20a 9.24a
N280P140K280 168.0a 200.0A 190.8A 7.67a 7.33a 5.67ab 8.18a 8.60a 9.30a
Mean 128.5B 268.7a 298.3a 6.0A 5.8A 4.5B 7.18 C 8.08B 8.75 A
Low NPK + GA 3
N0P0K0 (control) 27.0c 125.3c 54.3d 3.67d 3.33e 2.67e 4.44d 5.77d 6.18c
N0P0K140+GA3 127.6ab 250.0b 213.0c 6.67bc 5.67d 4.67d 6.50c 8.33c 8.25b
N0P70K140+GA3 121.7b 268.3ab 223.3bc 6.00c 7.00bcd 5.33cd 7.39abc 9.27ab 9.62ab
N0P70K0+GA3 172.7ab 330.3a 234.0bc 8.33a 7.67ab 6.00bcd 8.47a 8.97abc 10.05a
N140P0K0+GA3 146.0ab 233.7bc 287.3abc 6.67bc 6.00cd 7.00ab 7.39abc 8.50bc 9.61ab
N140P0K140+GA3 184.3ab 342.7ab 294.3ab 8.00ab 7.33ab 8.33a 8.16ab 9.47a 10.28a
N140P70K0+GA3 178.0ab 280.0ab 357.3a 7.67ab 7.33abc 6.33bc 7.01bc 9.20abc 9.64ab
N140P70K140+GA3 188.0a 368.7a 330.7ab 8.67a 8.67a 7.33abc 8.37a 9.13abc 9.90ab
Mean 143.2B 274.9A 249.3A 6.96A 6.62A 5.96B 7.22C 8.58B 9.19A
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The low NPK (N:P:K at 140:70:140 kg ha−1), normal NPK (N:P:K at 280:140:280 kg ha−1) and low NPK + GA3 (gibberellic acid; 10 mg L−1) treatment groups. Each of the three treatment groups were further subdivided into (Control, K, P, PK, N, NK, NP, and NPK) treatments.

Plants were harvested on June 20 (1st harvest H1), August 10 (2nd harvest H2), and October 10 (3rd harvest H3), 2015, of ramie, respectively. Data followed by different lowercase letters (a, b, c) in the same column indicate statistically significant differences within a harvest; values followed by different uppercase letters (A, B, C) in the same row indicate significant difference between harvests at p < 0.05 based on LSD test.

Influence of treatments on fibre yield

Fibre yield was significantly greater for fertilizer treatments than for controls (Table 4, Fig. 1). However, the interactions between harvest and fertilizer types (H × N × P × K) remained non-significant, with the exceptions of biomass in the low NPK treatment group and raw fibre yield in the normal NPK and low NPK + GA3 treatment groups.

Table 4.

Raw fiber yield and degummed fiber yield from ramie under different the low NPK, normal NPK and low NPK + GA3 treatment groups at three harvests (H1, H2, and H3).

Treatments Raw fiber yield (g) Degummed fiber yield (g)
H1 H2 H3 H1 H2 H3
Low NPK
N0 P0 K0 (control) 10.0b 35.3c 17.0d 2.14d 5.0c 2.37c
N0 P0 K140 29.7ab 91.7a 42.3bd 4.28c 13.3ab 4.78b
N0 P70 K0 31.7ab 71.0ab 33.0 cd 4.94bc 10.7ab 5.73b
N0 P70 K140 37.7a 60.7bc 64.7ac 5.50b 9.3bc 8.30a
N140 P0 K0 33.7a 80.0ab 62.0ac 5.25b 10.0bc 4.83b
N140 P0K140 35.7a 68.7ab 73.7ab 4.85bc 8.7bc 8.84a
N140 P70 K0 36.0a 76.7ab 83.7a 4.92bc 10.3b 8.87a
N140 P70 K140 47.0a 81.0ab 79.0ab 6.70a 15.7a 9.80a
Mean 32.7 C 70.6 A 56.9B 4.8 C 10.4 A 6.7B
Normal NPK
N0 P0 K0 (control) 15.7c 30.3c 17.7d 2.38c 5.0c 2.61d
N0 P0 K280 41.7bc 81.3ab 49.7c 8.65ab 11.3b 6.16c
N0 P140 K0 46.3ab 87.0ab 55.0c 6.86b 12.3b 6.67bc
N0 P140K280 64.3ab 69.3abc 95.0a 8.81ab 12.0b 10.72a
N280 P0 K0 48.0ab 76.7ab 60.7bc 6.95b 11.7b 6.32c
N280 P0 K280 55.3ab 61.7bc 88.7ab 8.32ab 13.0ab 9.10ab
N280 P140 K0 70.3a 99.3ab 79.7ac 9.54a 15.0ab 9.15ab
N280 P140 K280 66.7ab 109.7a 108.7a 9.42a 17.3a 10.41a
Mean 51.0B 76.9 A 69.4A 7.62B 12.2A 7.6B
Low NPK + GA 3
N0 P0 K0 (control) 13.7e 49.0d 27.0d 2.27 f 6.33d 3.74b
N0 P0 K140 + GA3 51.7d 95.0c 82.3c 8.27d 13.3c 9.56a
N0 P70 K0 + GA3 48.0d 98.3bc 84.7c 6.95e 14.3c 9.35ab
N0 P70 K140 + GA3 78.0b 125.7ab 106.7c 10.44bc 17.7bc 11.21a
N140 P0 K0 + GA3 66.3c 92.3c 85.0c 9.22 cd 13.7c 10.78a
N140 P0 K140 + GA3 90.7a 148.0a 108.7c 11.42ab 21.0ab 11.69a
N140 P70 K0 + GA3 82.7ab 103.3bc 149.3a 11.11ab 15.0c 12.92a
N140 P70 K140 + GA3 88.0ab 147.3a 130.3b 11.98a 22.3a 12.99a
Mean 64.9C 107.4A 96.7B 8.96C 15.5A 10.3B
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The low NPK (N:P:K at 140:70:140 kg ha−1), normal NPK (N:P:K at 280:140:280 kg ha−1) and low NPK + GA3 (gibberellic acid; 10 mg L−1) treatment groups. Each of the three treatment groups were further subdivided into (Control, K, P, PK, N, NK, NP, and NPK) treatments.

Plants were harvested on June 20 (1st harvest H1), August 10 (2nd harvest H2), and October 10 (3rd harvest H3), 2015, of ramie, respectively. Data followed by different lowercase letters (a, b, c) in the same column indicate statistically significant differences within a harvest; values followed by different uppercase letters (A, B, C) in the same row indicate significant difference between harvests at p < 0.05 based on LSD test.

Figure 1.

Figure 1

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Degummed ramie bast fibres obtained from plants treated with (a) a low rate of NPK fertilization (N:P:K at 140:70:140 kg ha−1), (b) a normal rate of NPK fertilization (N:P:K at 280:140:280 kg ha−1), and (c) a low rate of NPK fertilization + gibberellic acid (10 mg L−1). CK represents controls. K, P, and N indicate fertilization with potassium, phosphorous, and nitrogen, respectively.

The fresh and degummed fibre yields were greatest for the low NPK + GA3 treatment group, followed by the normal NPK and low NPK treatment groups. For all treatment groups, the fresh fibre yield and degummed fibre yield were highest at H2, followed by those at H3 and H1. The fibres with the lowest fresh and dry weights were from control plants. In the low and normal NPK treatment groups, the combined application of NPK resulted in higher fresh and degummed fibre yield than application of K, P, PK, N, NK, or NP. However, in the low NPK + GA3 treatment group, the highest fresh yield was recorded for NK + GA3 treatment.

Influence of treatments on fibre quality traits

The measures of fibre yield and quality, including the fibre breaking strength, elongation rate, and diameter, were positively affected by fertilizer treatment (Fig. 2). Fibre diameter increased with the application of fertilizers. The thinnest fibres were from unfertilized plants (22–24 µm), and the thickest fibres were from plants in the low NPK + GA3 treatment group that received the NP treatment (47.6 µm). The lowest elongation rate was observed for fibres from unfertilized plants. The maximum elongation rate was observed for fibres from plants in the low and normal NPK treatment groups that received NK treatment and for fibres from plants in the low NPK + GA3 treatment group that received NP treatment.

Figure 2.

Figure 2

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Diameter, elongation rate, and breaking strength of ramie fibre under different NPK combinations for low NPK (N:P:K at 140:70:140 kg ha−1), normal NPK (N:P:K at 280:140:280 kg ha−1) and low NPK + GA3 (gibberellic acid; 10 mg L−1) treatment groups (n = 3 ± SE). Different letters (a,b,c) indicate statistically significant differences among treatments at p < 0.05 based on LSD tests.

The lowest breaking strength was observed for fibres from unfertilized plants, and the highest breaking strength was observed for fibres from plants in the low NPK + GA3 treatment group that received NK treatment, followed by those from the low NPK treatment group that received N and NP treatments.

Discussion

In the present study, ramie responded positively to NPK fertilizers and foliar application of GA. Ramie fibres mainly consist of secondary phloem fibres and the economic value of this plant is based on the amount of fibre produced. Increasing the plant height, biomass, stem diameter, stem weight, and number of stems per plant ultimately increases the bast fibre yield of ramie. Among various combinations of N, P, and K fertilizers tested, the combined application of NPK was the most effective in increasing the fibre yield and fibre quality traits of ramie in all pot experiments. It is well known that N, P, and K are essential nutrients for plant growth. These nutrients are utilized in large amounts because N is an essential component of nucleic acid and protein synthesis, P is used in energy compounds (ATP and ADP) and nucleic acids, and K helps in the transport of water and nutrients through the xylem and is involved in the activation of many enzymes16. In the present study, fertilizer treatments that did not include N, such as the K, P, and PK treatments, produced shorter plants with less biomass and stem weight than the NK, NP, and NPK treatments. Ullah et al., has also reported that treatment with combined NPK (150–75–150 kg ha−1) maximizes plant characteristics that affect ramie fiber yield17. Among the essential plant nutrients, N plays the most important role in improving agricultural production17, 18. N application promotes the growth and fiber yield of ramie by increasing plant chlorophyll, soluble protein, and proline content; reducing MDA content; and enhancing gas exchange parameters and antioxidant enzyme activity19. It is possible, however, that it is the interaction between nutrients, rather than their absolute concentration, that is most important for maximizing fertilizer use efficiency20.

In the present study, ramie plants that received a normal rate of NPK fertilization attained greater height, biomass, number of stems, and stem weight than those that received a low rate of NPK fertilization. The recommended fertilization rates for ramie vary with the soil type, growing conditions, and ramie genotype. For example, 90:60:60 kg ha−1 N:P:K is recommended for ramie growth in clay loam soil21.

As the stems of ramie plants are the main source of fibres, an increase in stem biomass and diameter results in increased fibre yield17. In the present study, the treatments that resulted in the lowest number, weight, and diameter of stems (controls and fertilizer treatments that did not contain N) also resulted in the lowest raw and degummed fibre yield. Similarly, treatments that resulted in the maximum number, weight, and diameter of stems (NPK, NP, and NK treatments) also resulted in the highest fibre yields. These results are in-line with previous reports of a linear relationship between yield measures, such as dry yield, total aboveground biomass and bast fibres, and plant characteristics, such as stem number, plant height and stem basal diameter22.

In the present study, harvest time also significantly affected the production of ramie fibre. The second harvest (H2) was the most productive, resulting in the greatest fibre yield and stem biomass. This contrasts with results reported by Angelini and Tavarini, who found that higher and thicker stems, with higher bast fibre production per hectare, were obtained from the first ramie harvest than from subsequent harvests22.

In the present study, the application of N in combination with P, K, or PK resulted in the highest quality fibres. Fibre breaking strength was increased significantly with fertilizer application and the maximum breaking strength was recorded for fibres from plants in the low and normal NPK treatment groups that received NP treatment. Breaking strength did not increase further by the addition of K. The maximum fibre diameter was obtained for plants in the low NPK group that were treated with NPK and plants in the normal NPK group that were treated with NP. These results contrast with those of Liu et al., who concluded that application of N to ramie plants had the greatest effect on growth and fibre yield, whereas supplemental K had discernible effects on fibre quality15.

The addition of GA3 to fertilized plants in the present study enhanced all recorded growth traits, such as plant height, biomass, stem weight, stem diameter, and the number of stems. High IAA/low GA3 concentrations have been shown to have an inhibitory effect on stem elongation, whereas low IAA/high GA3 concentrations promote rapid internode elongation23. GA3 promotes stem elongation by increasing the physiological levels of auxin, either by increasing auxin production or decreasing the destruction of auxin24. Spraying ramie with GA may also promote growth and yield by increasing endogenous GA content, eliminating oxidative stress, and maintaining cellular integrity25.

We found that the application of GA3 to plants resulted in greater production of fibre than fertilizer alone, regardless of the rate of fertilization. The observed increase in fibre yield with the application of GA3 can be attributed to improved growth, development of chloroplasts, and intensification of photosynthetic efficiency26. Plants treated with GA3 had greater stem weight, more bark, and less wood deposition than plants not treated with GA3. These are all desirable features for bast-producing plants.

GA affects the differentiation of primary phloem fibre and increases the length of bast fibres by increasing internode length. In C. blumei, high levels of GA3 result in long phloem fibres with thin walls and the length of differentiating internodes is correlated with the length of primary phloem fibres23. The increase in the length of fibres treated with GA3 in the present study is likely associated with the observed increase in plant height and with increases in intermodal length.

In addition to relatively long fibres, plants in the low NPK + GA3 treatment group that were treated with NPK had fibres that were greater in diameter than plants in the low and normal NPK treatment groups that were treated with NPK. Fibre elongation rate was also maximized by spraying with GA3 and fertilizing with NPK. Similarly, in transgenic kenaf and populus trees that over express gibberellic acid, the increased GA has a positive impact on fibre number, length, diameter, and wall thickness27.

The breaking strength of fibres from plants in the low NPK + GA3 treatment group that were treated with NPK was greater than that for fibres from control plants and those treated with NPK alone. The strength of fibres was likely increased by increases in their length and diameter. The flexural strength of hemp fibres decreases significantly with decreasing fibre length28. Similarly, long okra fibres are stronger than short fibres because unlike long cells, short fibre cells require many weak connecting points in order to form fibre strands29. According to Withanage et al., enhanced bioactive GA is extremely important for increasing the length of kenaf fibre and can be obtained by over expressing the Arabidopsis thaliana gibberellic acid 20 oxidase gene (AtGA20ox) in transgenic kenaf plants27.

The quantity and quality of ramie bast fibre were significantly affected by harvest, rate of NPK fertilizer, and foliar application of GA3. Plant height, biomass, stem weight, stem diameter, number of stems, fibre yield, fibre elongation rate, fibre diameter, and fibre breaking strength were improved by fertilizer application. The application of NPK at a normal rate of fertilization was more successful in enhancing these traits than application of NPK at a low rate of fertilization or the application of N, P, or K alone. The maximum fibre yield and fibre quality traits were observed for plants treated with a low rate of NPK fertilization and foliar application of GA3. Therefore, spraying ramie plant canopies with GA3 and providing NPK fertilizer at a low rate can enhance fibre yield while reducing the requirement for normal fertilizer doses.

Materials and Methods

A pot experiment was carried out in a greenhouse at Huazhong Agricultural University, Wuhan, China. Rhizome segments (15 cm) obtained from the roots of the normal yield biannual ramie cultivar, Huazhu-5, were obtained from the experimental base at Huazhong Agricultural University. Pots (60 cm diameter) were filled with soil containing 11 g kg−1 of organic matter, 40 g kg−1 total N, 0.18% total P, and 60 g kg−1 total K with EC: 2 dS cm−1 and pH: 5.8. The rhizome segments were planted in the pots on March 25, 2015. Plants were harvested on June 20 (H1), August 10 (H2), and October 1 (H3), 2015, by cutting stems 10 cm above the soil.

NPK fertilizer and exogenous application of GA3

The prepared pots were separated into low NPK, normal NPK and low NPK + GA3 treatment groups (Fig. 3). Each of the three treatment groups was further subdivided into K, P, PK, N, NK, NP, and NPK treatments. In the low NPK groups, fertilizer concentrations were 140, 70, and 140 kg ha−1 for N, P, and K respectively. In the normal NPK group, fertilizer concentrations were 280, 140, and 280 kg ha−1 for N, P, and K, respectively. Controls received no fertilizer. P was applied as a single dose in the form of calcium super phosphate (14% P2O5) at planting. N, in the form of urea (46% N), and K, in the form of potassium chloride (54% K2O), were applied in three doses: at planting (40%), in June (30%) after the first harvest, and in August (30%) after the second harvest. For the NPK + GA3 treatment group (n = 28), 10 mg L−1 GA3 was sprayed over the canopy three times. The first dose (50%) was sprayed in April (10 days after planting), and subsequent doses were sprayed 10 days after each harvest, with 30% sprayed in June and 20% sprayed in August. Each treatment was replicated four times, arranged in a randomized complete block design.

Figure 3.

Figure 3

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Growing ramie in a greenhouse under different fertilizer conditions: low NPK (N:P:K at 140:70:140 kg ha−1), normal NPK (N:P:K at 280:140:280 kg ha−1), and low NPK + GA3 (gibberellic acid; 10 mg L−1).

Plant growth and fibre evaluation

Before each harvest, the effective number of stems in each pot was counted and plant height was measured from the root neck to the upper most part of the stalk. After each harvest, the remaining plants in each pot were allowed to re-grow until the next harvest. Stem diameter (mm) was measured at a height of 15 cm above soil surface using a digital Vernier calliper (ST22302, SG tools, Hangzhou, China). Plant biomass was measured by weighing both stems and leaves and stems were weighed again separately after removing all leaves. The fibre layer of each stem was decorticated (peeled from the pith), the epidermis was removed, and raw fibres were weighed to calculate fibre yield. Then, 20 g of decorticated fibre was boiled for 1 h in an Erlenmeyer flask containing 100 mL of degumming solution (1 g NaOH and 0.05 g EDTA). The degummed fibres were bleached with 2% H2O2 and 0.1% Tween-80 for 1 h at 94 °C in a water bath, washed with distilled water, and dried and combed (Fig. 2). Fibre diameter (µm) was measured using a computerized fibre fineness tester (Model No. YG002C, Changzhou, China) connected to an optical microscope. Fibre breaking strength (centi newtons, cN) and elongation rate (%) were determined using a fibre strength tester (YG004, Nantong Hongda Experiment Instruments, Qidong, China), following the Chinese National Standards (GB 5882–86).

Statistical analysis

All data were subjected to analysis of variance (ANOVA) using the statistical software CoStat Version 6.303 (CoHort, USA). The effects of harvest time (H), nitrogen (N), phosphorus (P), potassium (K), and their interactions (H × N × P × K) were analysed by a four-way randomized block ANOVA. Means and standard errors were calculated and graphs were prepared using Microsoft Office Excel (2007).

Acknowledgements

This research was supported by the National Natural Science Foundation of China (31571717) and China Agriculture Research System project (CARS-19-E12) Fundamental Research Funds for the Central Universities (2662015PY059).

Author Contributions

L.L. and D.P. supervised and designed the project. S.U. performed the experiment and collected data. M.R. and S.K. helped in conducting experiment. S.A. analyzed data and S.A. and S.Z. revised the manuscript.

Competing Interests

The authors declare that they have no competing interests.

Footnotes

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Lijun Liu, Email: liulijun@mail.hzau.edu.cn.

Dingxiang Peng, Email: pdxiang@mail.hzau.edu.cn.

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