DROUGHT STRESS AND IRRIGATION EFFECTS ON GERMINATION OF HARVESTED SOYBEAN SEED

Drought stress consistently reduces soybean [Glycine mar (L.) Merr.) yield, while irrigation increases yields above nonirrigated levels in the southern USA in most years. Little information is available regarding drought stress and irrigation effects on quality of harvested soybean seed in this region, however. Field experiments using Maturity Group (MG) IV, V, and VI soybean cultivars were conducted in 1990 and 1991 at Stoneville, MS, to evaluate effects of drought and irrigation during reproductive development on subsequent germination of harvested seed. Treatments included a nonirrigated (N1) control and furrow irrigation (I) applied at various times during the reproductive period (R1-R6) in four different experiments. All experiments were grown on Sharkey clay (very-fine, montmorillonitic, nonacid, thermic Vertic Haplaquept), a major soil in the alluvial plain of the lower Mississippi River Valley. Irrigation significantly increased seed yield and number of seed from all cultivars in all experiments. Effect of irrigation on weight per seed was none or small relative to effects on seed yield and number of seed. Drought stress resulted in significantly inferior quality (standard germination < 80%) seed from all cultivars in all experiments. Irrigation initiated during flowering and continued into the seed-rill period significantly increased standard germination of seed from all experiments. Irrigation during the full reproductive period was required to realize both maximum seed yield and maximum germination of harvested seed. The results show that drought stress can result in reduced germination of harvested soybean seed, and that irrigation can be used to overcome this effect.