Field investigations of the response of cotton to water deficits

Abstract

Field investigations of plant water stress on photosynthesis (leaf CO 2 exchange rates), stomatal activity, vegetative and reproductive growth, and yield of cotton ( Gossyppium hirsutum L.) were conducted at Stoneville, MS, in 1976 and 1977, and under mobile rainsheltered plots at Mississippi State, MS, in 1977. A sever drought occurred throughout much of the 1976 growing season and from the day of planting to 70 days after planting at the Mississippi State site. Net photosynthesis was significantly reduced as plant water stress increased at Stoneville in 1976. The rate of decrease was linear from a −20 to −22 bar threshold to the lowest recorded plant water potential of −36 bar. Subsequently stomatal diffusion resistances increased as plant water stress developed with threshold potentials of −20 to −25 bar for the more sensitive upper surfaces and −25 to −30 bar for lower surfaces. Stomatal closure was also observed at plant water potentials of −30 to −36 bar. Calculations of stomatal conductances and diffusion resistances to CO 2 revealed a linear relationship between conductance and calculated rates of photosynthesis. The calculated photosynthetic rates agreed with observed values and supported the contention that at lower plant water potentials reductions in photosynthesis rates can be attributed, for the most part, to stomatal closure caused by more rapid induction of plant water deficits. Several weeks of induced drought at Mississippi State reduced rates of leaf extension, height increase, and vegetative node production as much as 50% at mean daily water potential values of −11 bar. Stomatal conductance was reduced 50% at −27 bar. Flowering was apparently unaffected by the treatments. However, boll periods were shortened by several days by the dry treatments. Boll size was not significantly reduced. Total yields were significantly reduced, however, by either no irrigation with total fruit production. primarily limiting yields, or by rapid stress induction during the peak bloom stage, with square and young boll abscission primarily limiting yields. Yield components, such as the number and weight of seeds was also reduced with no irrigation.