We examined the influence of drought stress during grain filling on grain yield to investigate changes in assimilates in sink and source organs. When plants were subjected to drought stress from the start of grain filling until harvest, the photosynthetic rate rapidly decreased. Grain dry mass during maturation was not significantly different between the control and drought-stressed plants. Under drought stress conditions, starch content in source organs (peduncle, leaf, petiole, stem, and root) was significantly lower than in corresponding organs of control plants; the greatest difference was seen in leaves. Consistent with this observation, α-and β-amylase activities in leaves significantly increased within the first 6 d of drought stress. We conclude that in cowpea subjected to drought stress during grain filling, the grain yield is maintained, despite a dramatic decrease in photosynthetic rate, by translocation of photoassimilates from source organs.
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