Improvement of soybean physiological traits and yield under the end season drought stress conditions through the foliar spray of nutrient elements and polyamine

Soybean is one of the most important products in Iran, due to the production of high-quality oil and protein-rich meal. Soybean is highly sensitive to water deficit stress at the flowering and grain filling stages, so the end-season drought stress can have negative effects on soybean yield and yield components. Therefore, increasing soybean tolerance to drought stress is very important. Due to the importance of the soybean plant and the existence of drought stress in Iran, Improving the agronomic characteristics and yield of this crop under drought stress as well as favorable conditions through agronomic treatments is essential. Considering the positive effects of nutrients availability on the plant's physiological traits and yield,  this study aimed to investigate the effect of foliar application of nutrient elements and polyamine on agronomic and physiological characteristics of soybeans under the end-season drought stress.

To investigate the effect of fertilizer on physiological and yield characteristics of soybean (Glycine max) under the end-season drought stress, a split-plot experiment was conducted in a randomized complete block design (RCBD) with three replications at the Agricultural and Natural Resource Research Center of Ardabil in 2019. The treatments were irrigation (normal irrigation and omitting irrigation at the flowering stage) as main plots and foliar applications (control, nutrient elements containing polyamine, nutrient elements without polyamine) as sub-plots. Foliar application of nutrient elements without polyamine and nutrient fertilizer containing polyamine were applied one month after planting (at the vegetative stage). Drought stress was applied through irrigation stopping at the flowering stage (50% flowering). One week after drought stress, chlorophyll index, relative water content (RWC), and electrical conductivity (EC) were measured. Furthermore, the plant height, number of branches, pod per plant, grain per pod, 100-grain weight, biological and grain yield per unit area, harvest index, and oil percentages and oil yields were measured at the physiological maturity stage.

 The results showed that drought stress decreased the chlorophyll index, leaf relative water content (RWC), and grain and oil yield of soybean seeds, and significantly increased the electrolyte leakage. Furthermore, plant height, the number of branches, pods per plant, grain per pod, biological yield, and 100-grain weight were significantly decreased under drought stress treatment compared to the normal condition. Under drought stress, foliar application of nutrient elements without polyamine caused an increase of about 43.05 percent in the number of branches, 45.11 percent in the number of pods per plant, 38.84 percent in biological yield, and 40.09 percent in grain yield, as compared to the control. Under normal irrigation conditions, foliar application of nutrient elements containing polyamine significantly increased the number of branches (23.6 percent), the number of pods per plant (11.45 percent), grain yield (24.88 percent), and biological yield (20.20 percent), as compared to the control. It seems that nutrient elements without polyamine improved the soybean yield and yield components through the provision of suitable nutritional conditions during the seed formation and filling period. Furthermore, under normal irrigation conditions, the application of polyamine with the nutrient fertilizer improved the growth characteristics and yield of soybeans compared to the nutrient fertilizer without polyamine, which indicates the synergistic effects of polyamine and nutrient element.

The results showed that drought stress during the flowering stage significantly reduced the chlorophyll index and relative water content (RWC) of the leaves and increased electrolyte leakage in soybean plants and finally reduced yield and yield components of soybean. Under drought stress, foliar application of nutrient elements without polyamine significantly increased the chlorophyll index, RWC, and reduced EC of the soybean leaves, which resulted in the improved agronomic and physiological characteristics and yield of soybean plants under end-season drought stress conditions. Therefore, foliar application of nutrient elements with or without polyamine under favorable and unfavorable environmental conditions had a positive and significant effect on the physiological traits and yield of soybean, through providing better nutritional conditions for soybean plants.