Leaf Net CO2‐Exchange Rate and Associated Leaf Traits of Winter Wheat Grown with Various Spring Nitrogen Fertilization Rates

Abstract

Periods of drought stress may frequently occur in soft red winter wheat (Triticum aestivum L. em Thell.) grown on the southeastern Coastal Plain because of the low water‐retention capacity of the Ap soil horizon. Nitrogen fertilizer applications generally promote greater leaf area development, thereby potentially increasing transpirational water loss and the severity of plant water deficit. This field study was conducted to examine whether early spring N fertilization increases the severity of plant water deficit in wheat grown on the Coastal Plain. Wheat was grown with early spring N rates of 0,34, 67, and 101 kg N ha−1 at one location in 1990 and at two locations in 1991. All plots received 34 kg N ha−1 at planting. Increased N fertilization generally resulted in higher leaf reduced N concentrations (LRN), leaf area indices (LAI, leaf net CO2‐exchange rates (CER), leaf stomatal conductances (gs), and leaf water potentials before anthesis. Leaf CER, LRN, LAI, and gravimetric soil water content (SWC) of all N treatments decreased rapidly during grain fill. Low leaf area index values were obtained for all N treatments at approximately the same time. During grain fill, CER, gs, and SWC were usually lower the higher the N rate applied. Kernel number per head increased and individual kernel weight decreased with increased N. Grain yield was greatest at the 67 kg N ha−1 rate. These results indicate that increasing the rate of spring N applied to winter wheat grown on the southeastern Coastal Plain may increase the severity of plant water stress during grain fill. Reduced CER during grain fill with increased N may contribute to kernel‐weight reductions.