Physiological and agronomic effects of regulated-deficit irrigation on soybean grown under arid climatic conditions

Drought is one of the most severe environmental stresses affecting soybean growth and development, especially in arid and semi-arid areas. The aim of this experiment is to evaluate the effect of regulated deficit irrigation during the vegetative stages on soybean plants and determine the amount irrigation water can be reduced without affecting the physiological parameters, the crop phenology, and the yield of the soybean crop. The field experiments were conducted during two irrigation crop seasons (2021 and 2022) in Louata, Morocco. The results showed that regulated deficit irrigation regimes during the vegetative stages was combined with high temperatures and low air humidities during the beginning of flowering and the pod filling stage during 2021 in comparison with 2022, especially for 25 % CWR (crop water requirements). Regulated deficit irrigation regimes reduced the stomatal conductance by 46 % and 52 % respectively during the first and second growing seasons by limiting CO 2 intake for the Calvin cycle. The stomata closure increased the leaf temperature and affected the functioning of the photosynthetic apparatus by damaging the chlorophyll pigments and impairment of electron transport chains in chloroplasts. The transition from regulated deficit irrigation to 100 % CWR at the beginning of flowering (R1) compensated for the photosynthetic loss, improved the growth and development of soybean plants and enhanced the yield and its components for 50 % and 75 % CWR. The adaptative mechanism such as the remobilization of the carbon reserved in the stems and leaves (vegetative tissues) to the grains improved the grain yield by 36.7 % during 2021 and by 32.2 % during 2022 and. This consequently improved the water use efficiency, the water productivity of soybean for 50 % and 75 % CWR and contributed to water saving with an average of 60 mm per growing season.