SOME PHYSIOLOGICAL RESPONSES OF COTTON TO WATER STRESS AND VARIOUS NITROGEN LEVELS

Field investigations were conducted for two summers in rain-sheltered plots to: (1) determine the effects of nitrogen nutrition on stomatal resistance and leaf water potential of non-stressed and stressed cotton plants; (2) evaluate the effects of water stress and nitrogen status on nitrate reductase activity of cotton plants; (3) describe the assimilate utilization in developing cotton bolls as influenced by water stress and nitrogen nutrition; and (4) study the effects of both nitrogen and water deficits upon floral bud and flower production and ultimate yield of cotton.;Early morning leaf water potentials were more negative in stressed plants compared to non-stressed plants. There was a decreasing trend of early morning leaf water potentials as the season progressed due to decreasing availability of soil water.;Afternoon values of leaf water potentials were lower than mid-morning values. The smaller values in the afternoon were attributed to greater evaporative demand and the failure of soil hydraulic conductivity to maintain soil water potential conditions.;Activity of nitrate reductase appeared to be directly related to the amount of nitrogen applied and inversely related to the drought duration. Inhibition of enzyme activity occurred when leaf water potential between non-stressed and stressed plants differed by a value of 2.5 to 6 bars. Apparently, the decreased activity of the enzyme due to water stress was attributed mainly to decreased nitrate uptake.;Leaf diffusive resistance of field-grown cotton plants was not affected by nitrogen treatments. However, values of stomatal resistance appeared to be directly related to the duration of water deficit. Within a given day, afternoon values of stomatal resistance were greater than mid-morning values. Slight increases in stomatal resistance were observed as leaf water potentials decreased although complete stomatal closure due to water stress was not observed. The data indicated that stomata of field-grown cotton were relatively insensitive to water stress, at least within the range of leaf water potentials observed in this study.;The data indicated that the fate of a developing boll may be governed in part by the amount of photosynthate available. There is a gireater risk of losing more bolls if additional vegetative sinks are produced to compete with the bolls for the available assimilates.;Cumulative square (floral bud) and flower production were directly related to the amount of nitrogen applied and inversely related to the duration of drought period. Consistent decreases in cumulative net number of fruiting forms (squares and flowers) in stressed plants was attributed mainly to decreased production of fruiting sites as a consequence of low available soil water and/or high evaporative demand rather than an increase in square and flower abscission.;Lint, seed and seed cotton yield increased with increasing rate of applied nitrogen and decreased with stress period. The increase in yield and yield components was attributed mainly to a greater number of harvestable bolls; although weight per seed, lint weight and number of seeds per boll increased consistently. Conversely, the reduction in yield and yield components of stressed cotton plants was due mainly to a decreased number of harvestable bolls although consistent decreases in boll attributes due to drought were also evident.