Key findings:

The study found that longer watering intervals significantly decreased plant height, leaf area index (LAI), stem diameter, leaf area duration (LAD), net assimilation rate (NAR), relative growth rate (RGR), and seed protein content in soybean plants. However, spraying with GA100 or NAA25 enhanced these traits, especially under frequent watering.

What is known and what is new?

Known: Water stress reduces plant growth and physiological activities in soybean.

New: Spraying with GA100 or NAA25 enhances morphological and physiological traits, as well as seed protein content in soybean, particularly under frequent watering

What is the implication, and what should change now?

Implication is the findings suggest that using gibberellins (GA) or nephthaline acetic acid (NAA) can mitigate the negative effects of water stress on soybean growth and yield. Farmers and agricultural practices should consider using GA100 or NAA25, especially in areas prone to water stress, to improve soybean productivity.

Soybean Glycine max. (L.) Merrill is a legume belongs to a family (leguminace) that grows in tropical, subtropical and temperate climate. Soybean is a crop which can provide complete protein, containing eight amino acids essential for human consumption. Earlier researchers concluded that, water stress is one of the principal environmental factors controlling leaf area development. A number of studies [1,2] have focused on a decrease in leaf area development in different plants under water stress. Also, water stress causes a series of physiological, biochemical and morphological responses of crops, which finally result in low yield [3]. Plant growth regulators so far have emerged as "magic. 

chemicals" that couldincrease agricultural production atan unprecedented rate and help inremoving and circumventing  many of the barriers imposed by genetics and  environment.  Baz et al., 1984; Kothule et al.,2003 [4,5]  reported that  plant  growth substances of different concentrations i.e.  GA, NAA each at 100  and  200  p.p.m and  when applied exogenously  as foliar  spray improved  morphological characters viz. plant height, number of branches, leaf area,  total dry matter of plant. [6] studied the effects of GA3 on peas and observed  that GA3 tended to increase  protein content of green pods.

Soybean production in Sudan is restricted by soil deficiencies in moisture and plant nutrients. Consequently, adequate levels of irrigation are needed. Adequate water must be available during germination and critical growth periods, therefore, soil moisture depletion is the primary cause of reduced crop growth and yield [7]. These factors interrelate providing an important insight to study their interaction on soybean production. Therefore, the objectives of this study are to investigate interactive effects of water stress and plant growth regulators, namely Gebralinne GA and Nephthaline Acetic Acid NAA, on Morphological , Physiological and Biochemical attributes of soybean. 

To evaluate the impact Plant growth regulators on stressed Soybean (Glycine max l.  Merrill) on  Morphological, Physiological and Biochemical parameters.afield experiment was carried out during winter and summer seasons of 2017and 2018, at the Faculty of Agriculture and Natural Resources Farm, Kassala University, Halfa Elgadidah, Sudan. A soybean cultivar; William was used in this study.  The watering tress treatments were irrigation every 10 (W1) and 20 days (W2), and eight plant growth regulators levels (0,50, 75 and 100 ppm designated as GA₀, GA₅₀ ,GA₇₅ and GA₁₀₀ (gibberellins)  also, NAA₀, NAA₂₀ ,NAA₂₅ and NAA₃₀ corresponding to 0, 20, 25 and 30 ppm, respectively. The experiment was arranged as split plot trail in RCBD with three replications. Plant growth regulators rates were applied on shoots of all observations after 35 days from the sowing date in the two seasons. Ten plants were randomly selected and tagged in each subplot to determine the following parameters: 

Plant height, stem diameter (cm), Leaf area index whereas physiological attributes were   leaf area duration (LAD), net assimilation rate(NAR) and relative growth rate (RGR)  and seed protein content also was determined as following:

Nitrogen percentage was estimated by micro kjeldahal method [8].    Crude protein  percentage    of grain was estimated  by multiplying   the nitrogen percentage by 6.25. 

Data were statistically analyzed for RCBD design of split plot trial using a computer software package. Mean comparisons were worked out by Duncan's Multiple Range Test (DMRT) at 5% level of probability. Correlation analysis was determined by using SPSS computer software package version 20. 

The results revealed that prolonged watering intervals (W₂) significantly reduced the mean plant height, stem diameter and LAI relative to frequent (W₁) watering interval (Fig1). also, the differences between means of these parameters due to growth regulators were  high in both seasons;  in this regard, application of  100 p.p.m of gebraline GA₁₀₀ gave the taller and thicker plants with large LAI, flowed by NAA₂₅ as compared with other levels of hormones in both seasons (Fig 1). Prolonged watering interval significantly inhibited the positive effect of growth regulators level on mean of these morphological attributes (Fig 1).

Fig1: Effects of watering interval and growth regulators on morphological attributes of soybean

Fig2: Effects of watering interval and growth regulators on physiological attributes of soybean

Table1: Effects of watering interval and growth regulators on Biochemical attribute (protein content) of soybean seed during 2017 and 2018 seasons

Reduction in morphological attributes due to prolonged watering interval might be due to negative effect of water stress on cell enlargement and cell division as described by (Mustapha et al.,2014) [9], who concluded that water stress significantly decreased shoot biomass per plant as a result of reduction in leaf area and hastened leaf senescence. Also, Pirevallou et al., (2010) [10] attributed the reduction in plant height as a result of shortening stem internodes under water stress. The increase in these characters due to application of plant growth regulators might be attributed to the fact that GA and NAA promotes the growth of intact plants. The promotion of growth either in terms of increase in plant height or the Leaf area has been thought to be by increasing plasticity  of the cell wall  followed by hydrolysis of starch  to sugars which lowers the water  potential of  cell, resulting  in the entry of water into  the cell causing elongation.

These osmotic driven  responses  under the influence of GA might  have  attributed for an increase in photosynthetic  activity, accelerated translocation and efficiency of utilizing photosynthetic products,  thus resulting in increased  cell elongation  and  rapid cell  division  in growing portion. The highest values of RGR, NAR and LAD were recorded in W1xGA100 treatment in both seasons and the lowest values were recorded in lower hormone rates (control and NAA20) under prolonged water interval (W2) particularly in the first season (Fig 2). The longer leaf area duration under frequent water interval might explain the higher RGR, NAR and dry matter accumulation. In this regard, Saleem et al.,(2009) [11] reported that, greater LAD values of the irrigated soybeans during reproductive growth stages were due to the longer period during which the assimilates were converted to leaf area production. The overall enhancement of growth under GA at high levels possibly might explain the increases in LAD, CGR and NAR observed in this study. The increase of protein content as a result of application of GA3 could be attributed to the fact that, the gibberellic acid promoted cell elongation, the cambium activity and Stimulates protein synthesis. This result agreed with results reported by (Khatun et al., 2016) [12] who stated that, the different plant growth regulators (GA3, NAA ) showed significant effect on seed protein content in soybeans. Also, Pat et al.,  (1981)  observed that foliar application  of  NAA with  concentration 10 ppm  resulted in higher  protein  content of  soybean by 6%. In conclusion, based on results Spraying  soybean plants with GA100 or NAA25 significantly increased morphological traits and particularly under frequent watering regime which was associated with significant increases in characters like(CGR, NAR and LAD) and seed protein content. However spraying soybean with GA100and NAA25 lessened the negative effects of water stress.

No funding sources 

None declared

The study was approved by the Institutional Ethics Committee of University of Kassala

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