EFFECTS OF FERTILIZER N AND SOIL MOISTURE ON YIELD, YIELD COMPONENTS, PROTEIN CONTENT AND N ACCUMULATION IN THE ABOVEGROUND PARTS OF SPRING WHEAT

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The accumulation of aboveground dry matter (DM) and nitrogen (N) by spring wheat (T. aestivum L. cv. Manitou) grown on stubble land in lysimeters at two moisture levels (irrigation and natural rainfall) and seven rates of N was measured at five sampling dates. With irrigation, DM increased exponentially with time and N fertilization. This also occurred on dryland except between shot blade and anthesis when DM accumulated more slowly and plants lost 20% of their N at application rates > 61.5 kg N/ha. Rainfall after anthesis increased grain yields of dryland crops fertilized with > 61.5 kg N/ha more than those receiving less N because the former plants still had residual fertilizer N available to them. Grain yield response to N fertility followed the law of diminishing returns on irrigated land, but on dryland the relationship fitted a logarithmic growth curve. Grain yield when neither water nor N was added was 1,600 kg/ha; it increased by 71, 47 and 300% when water, 164 kg N/ha, and water plus 164 kg N/ha, respectively, were applied. On dryland, grain protein was 15.4% with no N applied and 17.0% at rates > 61.5 kg/ha; on irrigation, it increased from 14.1 to 15.7% with increasing N levels. Number of heads and kernels and kernel weight were increased by irrigation but only the two former parameters were increased by N. Dry matter accumulation was related to N concentration in plants by: DM = (%N)−k where k was < 1. N accumulated in plants at a faster rate than DM. The maximum rate of N accumulation was not affected by moisture; it was highest (4.7 kg N/ha/day) at a fertilizer rate of 123 kg N/ha. Irrigated plants recovered one-half or more of the fertilizer N, and dryland plants recovered one-quarter to one-third. Fertilizer recovery decreased with increasing fertilizer N. At maturity more than 70% of the N in the aboveground plant parts was located in the grain; N fertilizer had little effect on this porportion but drought during flowering retarded translocation of assimilates to the grain.