INFLUENCE OF STRESS DURING SOYBEAN SEED FILL ON SEED WEIGHT, GERMINATION, AND SEEDLING GROWTH-RATE

Water and high air-temperature (AT) stresses that occur during soybean (Glycine max (L.) Merr.) seed fill greatly reduce seed yield, but their effects on seed germination and vigor are less clear. The objective of this study was to determine the effect of water stress optimum and high AT during seed fill on soybean seed yield and individual seed weight, and the subsequent germination and vigor of the seed. At daytime ATs of 27 and 35-degrees-C in 1985 and 29 and 33-degrees-C in 1986, control, moderate, and severe water-stress treatments were imposed by differential irrigation throughout seed fill on greenhouse-grown plants. Water stress intensity, measured by accumulating stress degree days (SDD) during seed fill, increased linearly as the volume of irrigation water declined. The weight and number of seed produced by each plant, and individual seed weight, declined linearly as SDDs accumulated at each AT. Water stress at optimum ATs reduced seed number more than individual seed weight, but water stress at high ATs reduced individual seed weight more than seed number. Water or high AT stress caused fewer larger seed and more small seed to be produced. The germination percentage and vigor of the harvested seeds was reduced by water stress and high ATs, but by a smaller proportion than yield or seed number. Individual seed weight, germination, and seedling growth rate were strongly correlated when reduced by water and high AT stress. Severe stress during seed fill caused soybean plants to exceed their capacity to buffer seed number, shifting seed weight distributions towards a larger proportion of small seed, resulting in poor seed lot germination and vigor.