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Effect of N Fertility on Plant Water Relations and Stomatal Responses to Water Stress in Irrigated Cotton¹

Cotton (Gossypium hirsutum L.) was grown in Phoenix, AZ, in 1981 and 1982. In both years it was irrigated with either 10 or 15 cm of water per application; N was supplied at three rates within each water treatment. Stomatal conductances of the most recently expanded leaves were followed during irrigation cycles to determine effects of N fertility on stomatal responses to water stress. Differences in N status between treatments were small in 1981 but were much greater in 1982. In July 1982 (early in the fruiting period), stomata of low-N plants closed as leaf turgor neared zero, but stomata of high-N plants remained open well past this point. As the season progressed, the differential response to water stress was lost as stomata of high-N plants became more responsive to declining water potentials. Nitrogen had little effect upon predawn osmotic potential (determined by pressure-volume procedures as the water potential for zero turgor), but low N eliminated the diurnal osmotic cycling seen in high-N plants. Diurnal oscillations of osmotic potential in high-N plants had an amplitude of 0.4 MPa early in the fruiting period, then declined to zero only to reappear as fruits matured and vegetative growth resumed. Nitrogen concentrations of the most recently expanded leaves also increased during regrowth. Seasonal changes in stomatal responses to water stress seemed unrelated to either osmotic potentials or overall leaf N status. The loss of the differential response to stress did coincide roughly with a decline in petiole nitrate-N concentrations to very low levels in all treatments. Seedcotton yields were significantly affected by water in 1981 and by water and N in 1982. In the 2nd year, full irrigation increased yields only on high N; on low N, full irrigation did not affect yield but greatly decreased water-use efficiency. The effects of N fertility on stomata explain in part the complex effects of N on water use.

Key Words: Gossypium hirsutum L. • Stomatal conductance • Osmotic adjustment • Water-use efficiency • Leaf area index

² Plant physiologists, USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ 85040.

Received for publication January 23, 1984.