WATER AND NITROGEN EFFECTS ON WINTER-WHEAT IN THE SOUTHEASTERN COASTAL-PLAIN .2. PHYSIOLOGICAL-RESPONSES

The application of N to winter wheat (Triticum aestivum L. emend. Thel l.) generally increases leaf reduced N concentration (LRN), thereby po tentially increasing the rate and duration of leaf photosynthesis duri ng grain fill. For nonirrigated winter wheat grown on the southeastern Coastal Plain of the USA, however, increasing the rate of spring-appl ied N has been found to result in greater plant water deficits and low er leaf CO2 exchange rates (CER) during the grain-filling period. This 2-yr study was conducted to determine the effects of spring N rate on the rate and duration of leaf photosynthesis in winter wheat grown wi th irrigation and to examine the relationship between leaf CER and LRN as affected by spring N rate, soil water treatment, and wheat growth stage. Wheat was grown with different rates of spring-applied N (0, 45 , 90 and 135 kg N ha(-1)) under irrigated and nonirrigated conditions. Leaf CER generally increased with increased spring N under irrigated conditions but decreased under nonirrigated conditions. Higher leaf CE R with irrigation, as opposed to without irrigation, resulted in great er vegetative dry weights during the early stages of grain fill. Leaf CER was closely associated with LRN during grain fill under irrigated conditions, but greater stomatal closure with increased spring N resul ted in deviations from this relationship under nonirrigated conditions . The dates of initial decline in LRN of the flag leaf and leaf area i ndex (LAI) were similar for all treatments and in both years. Only und er irrigated conditions did increases in spring N rate delay the compl ete loss of leaf area and photosynthetic activity during grain fill. C ompared with 1992, the rate of decline in LAI, LRN, and CER during gra in fill was more rapid in 1993, when air temperatures were warmer. Res ults indicate that supplemental water is needed on the southeastern Co astal Plain for high rates of spring-applied N to increase the rate an d duration of leaf photosynthesis in winter wheat during the grain-fil ling period.