WHITE CLOVER SEED PRODUCTION .2. SOIL AND PLANT WATER STATUS ON YIELD AND YIELD COMPONENTS

White clover (Trifolium repens L.) seed yield is reduced by environmental conditions that favor vegetative growth and that consequently reduce flower density. White clover seed yields in western Oregon can be highly variable and often are low. The objective of this study was to quantify the effects of soil and plant water status on white clover flower production, seed yield, and seed yield components. Five supplemental irrigation treatments were applied in 1990 and 1991 to first- and second-year white clover grown on a Woodburn silt loam (fine-silty, mixed, mesic Aquultic Argixeroll) near Corvallis, OR. Seed yield in 1990 was greatest when water application was delayed until 68% of the available soil-water was used by the crop. This treatment restricted vegetative growth but maintained an even flush of flower production during the reproductive phase of growth. In 1991, all irrigation treatments yielded the same or less than the nonwatered control because vegetative stolen growth between the planted rows reduced the number of flowers produced by greater than 50%. In both years, a twice weekly irrigated treatment that matched evaporative demand increased vegetative growth and reduced seed yield. A single irrigation application later than the optimum 68% soil-water depletion increased total above ground phytomass, but decreased seed yield due to an interruption in flower production compared to irrigation at 68% soil-water depletion. Flower density was the yield component that most affected seed yield in both years. In 1990, effective control of soil-water depletion increased seed yield 69% and potential seed yield 4%, compared to the nonirrigated control. Under conditions that promote stolen development prior to the second reproductive season, aggressive vegetation management may be needed to increase inflorescence density and seed yield.