The interactive effect of high temperature and water deficit stress on nitrogen fixation, photosynthesis, chlorophyll fluorescence, seed yield and quality in soybean (Glycine max)

Drought and heat stress are important abiotic stresses restricting plant growth, while the two stresses often occur at the same time in nature, little is known about when these stresses occur in combination. Therefore, attempts were made to understand the impact of water stress imposed under different temperature conditions on various physiological parameters such as nitrogen fixation, photosynthesis, chlorophyll fluorescence, seed yield and quality. Cultivar JS 97–52 was grown in pots under polyhouses maintained at day/night temperatures of 30/22, 34/24, 38/26 and 42/28 °C. At each temperature, pots were divided into two sets, one set was unstressed while second was subjected to water stress at reproductive stage (beginning of seed fill). An increase in temperature and water stress caused the reduction in CO2 exchange, which ultimately affected the nitrogen fixation. Nitrogenase activity was significantly declined in the nodules with an increase in temperature. There was a concomitant decline in leghemoglobin, heme-chrome and ureids content in the nodules which ultimately resulted in the reduction of nitrate reductase activity, chlorophyll content and total free amino acid under high temperatures. A decline in nitrogen and carbon fixation ultimately caused a reduction in seed yield and quality. Water stress when imposed at different temperature further aggravated the effects of temperature, and the combination of water stress and high temperature had more detrimental effect. Thus, understanding the mechanisms by which plants acclimate to water deficit combined with high temperature is crucial for identifying stress tolerant soybean cultivars and to pave the way for obtaining cultivars with higher productivity under combined stress conditions.