Water deficits during flowering decrease the number of seed-bearing pods in soybean (Glycine max L. Merr.). Failure to set pods may indicate an inherent sensitivity to low tissue water potential (ψw) at critical stages of flower development. To test this, we measured the water status and pod set of flowers exposed to water deficits at various stages of development from bud to early ovary (pod) expansion. In well-watered (control) plants, the ψw of buds, flowers, and pods were about 0·2 to 0·3 MPa lower than leaf ψw at midday. The ψw of the reproductive structures decreased in concert with leaf ψw when water was withheld. Turgor, estimated from measurements of ψw and ψ$$$, was maintained at low Ww by osmotic adjustment at all flower stages. Upon rewatering, ψw of buds, anthesis-flowers, and expanding pods returned to control values within 1 d. The ψw of pods that had just formed but were not ‘set’ recovered more slowly. Water deficits imposed when flowers reached anthesis, pod set, or early pod expansion decreased the percentage of flowers that initiated pod expansion and maintained pod development. Inhibition of development was most pronounced in the anthesis-stress treatment in which only one-third as many flowers at mid-raceme positions set pods, compared to controls. None of these pods developed beyond 20 mm in length. The results indicate that sensitivity of soybean flowers to low ψw varies with raceme position and stage of flower development. Flowers in mid-raceme positions are particularly vulnerable to low ψw. Early ovary expansion is the critical stage in soybean reproductive development under drought conditions.
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Westgate et al. (1993) studied this question.