The aim of the study was to determine the response of soybean [ Glycine max (L.) Merr.] to drought at various stages of development in a sub-humid environment of Turkey. Drought-stress treatments was applied to plants in 2005 and 2006 by withholding irrigation at six critical stages: completely vegetative (fifth trifoliate) (T 2), flowering (T 3), podding (T 4), seed fill (T 5), full bloom+podding (T 6), and podding+seed fill (T 7). Growth and production was compared in each treatment to full irrigated (T 1) and non-irrigated (T 8) controls. Each drought treatment reduced shoot biomass and seed yield compared to well-watered plants, but only non-irrigated plants or plants droughted at vegetative or flowering stages produced fewer seed pods and seeds. Seed protein and oil content was highest among treatments when plants were droughted during the seed filling stage. Yield increased exponentially with crop water use and ranged from 2.1-2.5 tons ha -1 in non-irrigated plants to 3.5-4.0 tons ha -1 in the well-watered controls. However, plants droughted during the vegetative stage of development produced the highest yield per unit of irrigation water applied (that is, irrigation water use efficiency). This research results will be useful for maximizing soybean production and/or seed quality when irrigation water is limited.
