Drought stress disrupts biological nitrogen fixation and starch accumulation compromising growth and yield of cowpea plants.

Water scarcity is one of the most important abiotic stresses in biological nitrogen fixation, often limiting the yield of leguminous plants. This study aimed to evaluate the effect of soil moisture on biological nitrogen fixation, cellular osmotic adjustment and cowpea yield. Water restriction started 20 days after sowing with different levels (20, 40, 60, and 80 %) of soil water retention capacity (SWRC). Cowpea plants did not complete life cycle under 20 % SWRC. Plants growing under SWRC less than 60 % decreased stomatal conductance, transpiration, photosynthetic rates and leaf pigments such as chlorophyll and carotenoids. Drought stress caused chlorosis in leaf tissues, reducing sucrose and total sugars in shoots and transport of these metabolites to the roots. Drought stress disrupted number and weight of nodule per plants affecting nitrogen compounds (ureides, nitrate, ammonia and amino acids) accumulation in roots and shoots of cowpea plants. Well-watered plants showed better biological nitrogen fixation, photosynthetic rates and starch accumulation in the leaves. Less stomatal conductance and transpiration rates induced by drought impaired nodulation, biological nitrogen fixation, dry mass of roots and shoots, growth and yield of cowpea plants. These results suggest that stomatal conductance, transpiration rates and nodulation were the main physiological responses limiting growth and yield of cowpea plants grown in soil moisture below than 60 % SWRC.