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

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.

fertilizer is crucial for sustaining fibre yield, and optimizing yield requires investigation into suitable fertilization rates 13 .
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 GA 1 , GA 3 , GA 4 , and GA 7 . One of these forms, gibberellic acid (GA 3 ), promotes growth, especially fibre production and elongation, in hemp, jute, kenaf, cotton, and ramie. The greatest concentrations of GA 3 are found in those tissues that are elongating the most rapidly, such as stems, petioles, and, in some crops, flower inflorescences 14,15 . 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 + GA 3 (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 + GA 3 group. Overall, the maximum height, biomass, and stem diameter of ramie plants were attained by low NPK + GA 3 plants at the third harvest (H 3 ) and the minimums were observed for control plants at the first harvest (H 1 ). The overall greatest stem fresh weight was found for low NPK + GA 3 plants at the second harvest (H 2 ) and the maximum number of stems was recorded for low NPK + GA 3 plants at H 1.
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 + GA 3 treatment groups.
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 + GA 3 treatment group, the highest fresh yield was recorded for NK + GA 3 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 + GA 3 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 + GA 3 treatment group that received NP treatment. 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 + GA 3 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 (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.
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 enzymes 16 . 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 yield 17 . Among the essential plant nutrients, N plays the most important role in improving agricultural production 17,18 . 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 activity 19 . It is possible, however, that it is the interaction between nutrients, rather than their absolute concentration, that is most important for maximizing fertilizer use efficiency 20 .
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 soil 21 .
As the stems of ramie plants are the main source of fibres, an increase in stem biomass and diameter results in increased fibre yield 17 . 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  In the present study, harvest time also significantly affected the production of ramie fibre. The second harvest (H 2 ) 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 harvests 22 .
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 quality 15 .
The addition of GA 3 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 GA 3 concentrations have been shown to have an inhibitory effect on stem elongation, whereas low IAA/high GA 3 concentrations promote rapid internode elongation 23 . GA 3 promotes stem elongation by increasing the physiological levels of auxin, either by increasing auxin production or decreasing the destruction of auxin 24 . Spraying ramie with GA may also  Table 4. Raw fiber yield and degummed fiber yield from ramie under different the low NPK, normal NPK and low NPK + GA 3 treatment groups at three harvests (H 1 , H 2 , and H 3 ). 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. promote growth and yield by increasing endogenous GA content, eliminating oxidative stress, and maintaining cellular integrity 25 . We found that the application of GA 3 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 GA 3 can be attributed to improved growth, development of chloroplasts, and intensification of photosynthetic efficiency 26 . Plants treated with GA 3 had greater stem weight, more bark, and less wood deposition than plants not treated with GA 3 . 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 GA 3 result in long phloem fibres with thin walls and the length of differentiating internodes is correlated with the length of primary phloem fibres 23 . The increase in the length of fibres treated with GA 3 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 + GA 3 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 GA 3 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 thickness 27 .
The breaking strength of fibres from plants in the low NPK + GA 3 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 length 28 . 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 strands 29   , 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 quantity and quality of ramie bast fibre were significantly affected by harvest, rate of NPK fertilizer, and foliar application of GA 3 . 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 GA 3 . Therefore, spraying ramie plant canopies with GA 3 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.  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 + GA 3 (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.