Alleviation of Drought Stress in Soybean [Glycine max (L.) Merril] by Foliar Application of Thiourea on Productivity and Profitability under Rainfed Condition

Drought is a critical environmental factor that imposes water stress on crops and a major constraint on plant growth and productivity (Rampino et al., 2006). It is the most damaging abiotic stress affecting modern agriculture (Zhang et al., 2006). Scarcity of water may become more severe in the future with changing global climate. A lack of sufficient moisture leading to drought stress is a common phenomenon in rainfed areas, brought about by uneven and erratic rain and poor irrigation (Wang et al., 2005). Drought hinders the global production of soybean (Glycine max L. (Merril), which provides 71 per cent and 29 per cent of the world¢s protein and oil consumption, respectively (Wijewardana et al., 2019).
       
In India average productivity of soybean is low due to sink limited nature, flower drop, poor adoption of improved technology, soil moisture stress, high temperature stress, erratic rainfall and biotic stresses like pests and diseases (Engels et al., 2017). To address these constraints, several measures can be taken, such as ensuring the supply of good quality seeds, dealing with biotic stresses through resistant varieties of soybean like JS-93-05 and MACS 1188. Economic yield reduction due to drought stress at various growth stages has been reported in many field crops such as, soybean (Samarah et al., 2006) and maize (Monneveux et al., 2006). However, to meet the growing demand for food, it is necessary to increase soybean yield, even in environments with low water availability (Rosa et al., 2021).
       
The productivity of soybean in India is quite low and it faces several constraints related to its physiology, management and external stresses. Some of the notable reasons for the low productivity of soybean include sink limited nature, leaf fall, flower drop, poor availability of quality inputs, poor adoption of improved technology, soil moisture stress, high temperature stress, erratic rainfall and biotic stresses like pests and diseases. Out of all the environmental stresses that soybean crop faces, it is believed that irregular rainfall is the main cause of yield reductions (Engels et al., 2017). Drought stress causes a decrease in the soybean plant’s leaf area index, chlorophyll content and relative water content. Additionally, it results in an increase in the level of osmolytes in the plant (Dong et al., 2019).
       
To address these constraints, several measures can be taken, such as ensuring the supply of good quality seeds, dealing with biotic stresses through resistant varieties of soybean and improving abiotic stress and nutrient management through the use of bioregulators like thiourea. Thiourea, an organosulfur compound, is a well-known bio-regulator for plants that can help mitigate abiotic stresses and improve the nutrient management system in soybean cultivation (Makadia, 2018). Thiourea has been shown to enhance the plant¢s defense against fungal and bacterial diseases. It can activate various defense mechanisms such as lignification, production of phytoalexins and strengthening of the cell wall, which ultimately reduces the severity of the disease. Thiourea can help alleviate abiotic stress such as drought, salinity and heavy metal toxicity (Srivastava et al., 2017). It has been shown to improve plant growth and photosynthesis under stress conditions. In view of this, bioregulators (thiourea) are innovative agricultural techniques used to protect plants, minimizing the adverse effects caused by environmental stress (Abhishek et al., 2021). Therefore, an alternative to improve crop yield under rainfed condition by application of bioregulators like thiourea is/are important in order to mitigate stress in soybean production.
       
Thiourea is an analogue of urea which provides sulphur (42% S) in addition to the nitrogen (36% N) and helps the crop to alleviate the damage caused by several biotic and abiotic stresses. Exogenous application of thiourea also stimulates defense mechanisms under abiotic stress. It helps to reduce the accumulation of reactive oxygen species (ROS) and also increases the activity of antioxidant enzymes, which ultimately leads to improved plant tolerance to abiotic stress (Hassanein et al., 2015). Further, at physiological level, this is directly associated with enhanced photosynthesis, increased metabolite translocation and co-ordinated regulation of plant’s source to sink relationships and also higher productivity (Pandey et al., 2013).
       
Taking all these above factors into account, the current study was conducted over three years to assess how various concentrations of foliar spray of thiourea, applied at various phases of crop growth, would impact on growth, physiological parameters, yield parameters and economics in two soybean varieties under rainfed conditions.

Experimental site
 
Field experiments were conducted during the kharif season of 2020, 2021 and 2022 at AICRP on Soybean, MARS, University of Agricultural Sciences, Dharwad under rainfed conditions. The details of experimental site characteristic and crop management practices (Table 1) and meteorological data of study area is presented in Fig 1.
 

 

 
Experimental design and treatment details
 
The experiment involved two varieties of soybean and five different foliar sprays (control, water spray and three concentrations of thiourea which were arranged using a factorial randomized block design. The two varieties (Factor A) used were V₁-JS 93-05 and V₂-MACS 1188. While the five foliar sprays (Factor-B) were control (F₁), water spray (F₂), thiourea at 250 ppm (F₃), thiourea at 500 ppm (F₄) and thiourea at 750 ppm (F₅). Water or thiourea foliar spray was conducted at varying concentrations during 25 and 55 days after sowing of crop.
 
Collection of data on growth, yield and its components of soybean
 
Crop was raised with recommended package of practices. Observations were recorded on plant height, number of branches, dry matter production, number of pods per plant and test weight. Grain and straw yield was calculated based on the yield obtained from each net plot and converted into to kg ha^-1. For the estimation of LAI, the leaves of the selected plants were detached at 60 DAS after removal from the plot, area of the leaves were calculated by placing it on levelled land and by using meter tape. Following formula, given by Watson (1947), was utilized for calculation of LAI:
   
Representative samples are also utilized to measure the PAR and chlorophyll content. Chlorophyll content was measured by using SPAD-502 by punching the leaves in the SPAD meter. PAR was measured by using a crop analyser. The difference between incident solar radiation and transmitted solar radiation is recorded as intercepted solar radiation.
       
Calculation of (Relative water content) RWC (%) of leaves according to the formula given by Weatherley, (1950):
   
Statistical analysis
 
The statistical analysis of data on various recorded parameters of growth, yield, physiological parameters and economics was done by using Factorial randomized block design as described by Gomez and Gomez (1984).

Effect on growth attributes of soybean
 
Pooled data presented in Table 2, reveled that, use of two different varieties and the foliar spray of thiourea resulted in significant variation in growth parameters. Among the varieties, MACS 1188 recorded significantly higher plant height (69.39 cm), number of branches per plant (5.73) and total dry matter production (68.39 g plant^-1) as compared to JS 93-05 (51.05 cm, 5.28 and 54.52 g plant^-1, respectively) at 60 DAS (Table 2).
 

       
Among the foliar spray, thiourea @ 750 ppm at 25 and 55 DAS recorded significantly higher plant height (66.09 cm), number of branches per plant (6.01) and total dry matter production (65.97 g plant^-1). However, it was on par with thiourea @ 500 ppm. Significantly the lowest growth attributes were recorded in control (55.60 cm, 5.02 and 55.03 g plant^-1) respectively) at 60 DAS (Table 2). But interaction of varieties and foliar spray of thiourea could not bring any significant variation. Similar findings were also obtained by (Meena and Bhati 2016) and (Premaradhya et al., 2018).
       
The increase in various growth parameters can be attributed due to the presence of two macronutrients in thiourea (Nitrogen and Sulphur). Nitrogen improves the activity of growth-promoting hormone, which leads to better cell division and growth (Meena et al., 2023). In addition to that, Sulphur aids to the formation of protein which is the building block of living organisms (Jhanji and Dhingra, 2018). Further Better photosynthetic ability and partitioning of photosynthates in variety MACS 1188 resulted in higher dry matter production. An increment in dry matter production due to foliar spray of thiourea is related to the improvement in plant height, better interception of PAR and mainly due to the improvement in growth influencing parameters and processes. The enzyme and protein stabilising nature of thiourea and its ability to increase the activity of nitrate reductase activity also contributed to the increase in dry matter of the crop (Anitha et al., 2006) and (Sachin et al., 2019).
 
Physiological parameters of soybean
 
Among the varieties, MACS 1188 recorded significantly higher LAI (3.22), PAR (1060.4), chlorophyll content (39.8%) and RWC (82.4%) as compared to JS 93-05 (2.96, 1010.2 mmol/m²/s, 36.4 % and 75.5% respectively) at 60 DAS on pooled basis (Table 2). Foliar spray of thiourea @ 750 ppm at 25 and 55 DAS recorded significantly higher LAI (3.28), PAR (1124.6 m mol/m²/s), chlorophyll content (42.58%) and RWC (78.4%). However, it was on par with thiourea @ 500 ppm. Significantly the lowest physiological parameters were recorded in control (2.56, 974.5 mmol/m²/s and 34.2% and 65.7% respectively) at 60 DAS (Table 2). But interaction of varieties and foliar spray of thiourea could not bring any significant variation.
       
The foliar spray of thiourea, helps maintain turgor pressure in cells and might result in increased LAI (Meena et al., 2023; Verma, 2019). Further directly provides nutrients to the growing part of the crop which ultimately better absorption of nutrients and then the formation of chlorophyll in the plant system is positively influenced. The similar results were collaborated with the findings of Amanmmula et al., (2014) also observed that water-soluble fertilizer significantly increased the PAR interception and chlorophyll content. Increased intercepted PAR and chlorophyll content will eventually result in an increased net photosynthetic rate and, thus, better yield. Apart from that, the foliar application of thiourea is also known to regulate several key steps of photosynthesis (Pandey et al., 2013) and (Choudhary et al., 2017). Further, leaf relative water content (RWC) is an important indicator of water status in plants; it reflects the balance between water supply to the leaf tissue and transpiration rate (Lugojan and Ciulca, 2011).
 
Yield attributes and yield of soybean
 
Significantly higher yield attributes viz., number of pods per plant (63.62), seed yield per plant (30.65 g) and test weight (13.82 g), seed yield (2726 kg ha^-1), straw yield (3912 kg ha^-1) and biological yield (6638 kg ha^-1) was recorded in variety of MACS 1188 than JS 93-05 on pooled basis (Fig 2 and Table 3).
 

 

       
Pooled data presented in Fig 3 and Table 3, reveled that, foliar spray of thiourea @ 750 ppm at 25 and 55 DAS recorded significantly higher yield parameters viz., number of pods per plant (69.83), seed yield per plant (31.28 g) and test weight (15.24 g) and seed, straw and biological yield (2716, 4008 and 6724 kg ha^-1, respectively) followed by thiourea @ 500 pm. Whereas, significantly lowest yield attributes (52.91, 25.97 g and 12.03 g respectively) and yields (2250, 3346 and 5596 kg ha^-1) respectively). The application of thiourea through foliar spray at 500 and 750 ppm rates led to a significant improvement in the seed yield of soybean crop, with increases of 14.41 and 17.15 per cent respectively, when compared to the control.
 
 
       
Foliar application of thiourea directly provides nutrients to the growing part of the crop. It ultimately results in higher photosynthetic activity, better absorption of nutrients and then the formation of chlorophyll in the plant system and improved functioning of several chloroplastic enzymes under the influence of nitrogen and sulphur present in it. Sulphur present in thiourea also protected the plants from several abiotic and oxidative stresses which led to better functioning of the plants under normal or stress conditions (Meena et al., 2016). Increase in number of branches per plant, total dry matter production and number of pods per plant etc., ultimately led to the improvement in yield of the soybean crop. The similar results were obtained by Anitha et al., (2004) and Bangar et al., (2019) and Meena et al., (2023).
 
Economics
 
The higher gross, net returns and benefit cost ratio (Rs. 1,48,528 ha^-1, 1,09,933 ha^-1 and 3.8, respectively) were observed in variety MACS 1188 than variety JS 93-05 on pooled basis (Table 3). Among the foliar spray, significantly highest gross (Rs. 1,47,980 ha^-1) net returns (1,07,652 ha^-1) and B:C (3.7) was observed with thiourea @ 750 ppm, which was equally effective as the application at 500 ppm. Both of these treatments showed better results than the control, water spray and thiourea @ 250 ppm. The higher returns in thiourea application which directly related to the yields. This finding is consistent with a similar study by Bangar et al., (2019) and Meena et al., (2023).

From the results of experiment (3 years data) it was determined that MACS 1188 variety of soybean produced higher growth, yield and economic returns compared to JS 93-05 variety. The application of thiourea through foliar spray at 500 and 750 ppm led to a significant improvement in the seed yield of soybean crop, with extent of 14.41 and 17.15 per cent respectively, when compared to the control. Therefore, it can be concluded that by using MACS 1188 variety of soybean and foliar spray of thiourea at 500 ppm and 750 ppm at 25 and 55 DAS is a commendatory option and most remunerative for farmers to improve the productivity and profitability of soybean in Northern Transition Zone of Karnataka.

Authors are grateful to University of Agricultural Sciences, Dharwad (Karnataka) and Indian Institute of Soybean Research, Indore for providing facilities in smooth conduct of the experiment.

All authors declared that there is no conflict of interest.