LEAF NET CO2-EXCHANGE RATE AND ASSOCIATED LEAF TRAITS OF WINTER-WHEATGROWN WITH VARIOUS SPRING NITROGEN-FERTILIZATION RATES

Periods of drought stress may frequently occur in soft red winter whea t (Triticum aestivum L. em Thell.) grown on the southeastern Coastal P lain because of the low water-retention capacity of the Ap soil horizo n. Nitrogen fertilizer applications generally promote greater leaf are a development, thereby potentially increasing transpirational water lo ss and the severity of plant water deficit. This field study was condu cted to examine whether early spring N fertilization increases the sev erity of plant water deficit in wheat grown on the Coastal Plain. Whea t 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 receiv ed 34 kg N ha-1 at planting. Increased N fertilization generally resul ted in higher leaf reduced N concentrations (LRN), leaf area indices ( LAI), leaf net CO2-exchange rates (CER), leaf stomatal conductances (g (s)), and leaf water potentials before anthesis. Leaf CER, LRN, LAI, a nd gravimetric soil water content (SWC) of all N treatments decreased rapidly during grain fill. Low leaf area index values were obtained fo r all N treatments at approximately the same time. During grain fill, CER, g(s), and SWC were usually lower the higher the N rate applied. K ernel 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 .