WATER STATUS AND LEAF-AREA PRODUCTION IN WATER-STRESSED AND NITROGEN-STRESSED COTTON

The combined effects of water and N deficits on leaf area production a nd water relations in cotton (Gossypium hirsutum L.) have not been inv estigated thoroughly. A study was conducted to evaluate the responses of leaf area production and water relations in cotton plants exposed t o water and N deficits during the preflowering stage. Two N treatments -nutrient solution with 12 mM of N (N-supplied) and N-free nutrient so lution (N-starved)-were applied to plants when the third true leaf was visible. Two irrigation treatments-daily irrigation (well watered) an d no irrigation (water stressed)-were superimposed on each N treatment once the plants were moved into the test chambers when they reached a leaf area of 0.050 +/- 0.002 m(2). Leaf area and leaf water and osmot ic potentials were measured at the end of each day-time period. Under well-watered conditions, leaf turgor potential in N-starved plants was about 0.3 kJ kg(-1) lower than in N-supplied plants. When plants were exposed to water deficits, the decline of leaf water potential relati ve to soil water content was greater in N-starved plants than in N-sup plied plants. Nitrogen deficiency delayed and decreased the magnitude of osmotic potential adjustment and, therefore, reduced the capacity o f leaves to maintain turgor. Water and N deficits decreased whole-plan t cumulative leaf area about 50 and 40%, respectively, through decreas ed daily production of mainstem and branch leaves and decreased final area of individual mainstem and branch leaves. Nitrogen deficiency ind uced a higher sensitivity of leaf growth inhibition to water deficits.