Water and Nitrogen Effects on Winter Wheat in the Southeastern Coastal Plain: II. Physiological Responses

The application of N to winter wheat (Triticum aestivum L. emend. Thell.) generally increases leaf reduced N concentration (LRN), thereby potentially increasing the rate and duration of leaf photosynthesis during grain fill. For nonirrigated winter wheat grown on the southeastern Coastal Plain of the USA, however, increasing the rate of spring-applied N has been found to result in greater plant water deficits and lower 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 with 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 CER with irrigation, as opposed to without irrigation, resulted in greater 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 resulted in deviations from this relationship under nonirrigated conditions. The dates of initial decline in LRN of the flag leaf and leaf area index (LAI) were similar for all treatments and in both years. Only under irrigated conditions did increases in spring N rate delay the complete loss of leaf area and photosynthetic activity during grain fill. Compared with 1992, the rate of decline in LAI, LRN, and CER during grain fill was more rapid in 1993, when air temperatures were warmer. Results indicate that supplemental water is needed on the southeastern Coastal Plain for high rates of spring-applied N to increase the rate and duration of leaf photosynthesis in winter wheat during the grain-filling period. South Carolina Agric. Exp. Stn. Technical Contribution no. 3551