FLOWER AND POD DEVELOPMENT IN WATER-DEFICIENT SOYBEANS (GLYCINE-MAX L MERR)

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 (PSI(w)) 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 PSI(w) of buds, flowers, and pods were about 0.2 to 0.3 MPa lower than leaf PSI(w) at midday. The PSI(w) of the reproductive structures decreased in concert with leaf PSI(w) when water was withheld. Turgor, estimated from measurements of PSI(w) and PSI(s), was maintained at low PSI(w) by osmotic adjustment at all flower stages. Upon re-watering, PSI(w) of buds, anthesis-flowers, and expanding pods returned to control values within 1 d. The PSI(w) 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 PSI(w) varies with raceme position and stage of flower development. Flowers in mid-raceme positions are particularly vulnerable to low PSI(w). Early ovary expansion is the critical stage in soybean reproductive development under drought conditions.