Soybean is a very important crop for the Brazilian economy. Due to its C3 characteristics, it has a low efficiency in resource utilization, water and light, compared with others crops. When cultivated in tropical and subtropical areas, soybean crop has to cope during its growing season, with dry spell, which can reduce, even more, its ability to capture and to use the resources water and energy.
Soybean (Glycine max (L.) Merril, cv. Capinopólis) water relations, growth, yield and water use were investigated at Viçosa, MG, Brazil during the growing season 1995/1996. The plants were grown under natural field conditions until 30 days after sowing (DAS) when two regimes of water supply were applied: (1) crops received all the rainfall available throughout the season and irrigation was applied in order to avoid any limitation of water supply; (2) Crop received all the rainfall available throughout the season and irrigation was not applied. The water use efficiency was calculated by each phenological phase by considering the dry mass produced by the water received by the crop in each phase. The results have showed that the water stress on the vegetative phase did not had any effect on the crop water use efficiency, however it has decreased its Leaf Area Index by around 40%. The water stress at the flowering phase had a great effect in increasing the crop water use efficiency but it had a great effect on the crop productivity. Taking in consideration the whole season, the Water use efficiency of the water-stressed treatment was much greater than the irrigated one. The differential effects of the water stress on the Leaf Area Index, crop yield and dry matter partitioning were also analyzed.