WATER AND NITROGEN EFFECTS ON WINTER-WHEAT IN THE SOUTHEASTERN COASTAL-PLAIN .1. GRAIN-YIELD AND KERNEL TRAITS

Understanding how environmental factors affect winter wheat (Triticum aestivum L. emend. Thell.) responses to spring N application is an important component of economically and environmentally sound winter wheat production on the southeastern Coastal Plain of the USA. Increasing the amount of N applied to winter wheat grown in this region has been shown to result in a greater severity of drought stress during grain fill and loner individual kernel weights. This 2-yr field study was conducted to determine whether drought-induced reductions in kernel weight with high spring N rates are the result of decreases in the rate or the duration of kernel growth. Winter wheat was grown with different rates of springapplied N (0, 45, 90 and 135 kg N ha(-1)) under both irrigated and nonirrigated conditions. Increases in the rate of springapplied N resulted in a greater severity of soil water deficits under nonirrigated conditions. Quadratic increases in grain yield and kernel number per square meter occurred in response to increased spring N under both levels of soil water treatment. Grain yield and individual kernel weight responses to irrigation were greater at the higher N rates than at the lower N rates. Over both years, the average increase in individual kernel weight due to irrigation was 3.9 and 13.3% at the 0 and 135 kg N ha(-1) rates, respectively. Similar responses were found for the effective filling period (EFP), where irrigation increased the EFP an average of 3.2 and 14.5% at the lowest and highest spring N rates, respectively. Soil water treatment had no effect on kernel growth rate. Results indicate that high rates of spring-applied N increase the severity of drought stress in nonirrigated winter wheat grown on the Coastal Plain, resulting in reductions in the EFP and, consequently, kernel weight.