Irrigated spring wheat and timing and amount of nitrogen fertilizer. II. Physiology of grain yield response

Abstract

In order to understand the mechanism by which the timing and amount of N fertilizer affect grain yield of wheat, a large experiment comprising 32 N treatments (from nil to 320 kg N ha −1, applications from seeding to booting) was conducted using a high-yielding semidwarf spring cultivar grown under low natural fertility at Griffith, New South Wales (34°S). The weather was close to normal and supplemental spring irrigations eliminated the possibility of water stress. Crop management was otherwise optimal and there was no disease or lodging. Grain yield ranged from 170 to 750 g m −2 across N fertilizer treatments the response to N was not reduced with applications as late as the onset of stem elongation. Grain yield was very closely correlated with kernels m −2 and with total biomass production. Kernel number was in turn closely correlated with spike dry weight (g m −2) at anthesis and with crop dry weight accumulation during the spike growth phase commencing 1 week before fag leaf emergence. Kernels per unit spike weight was unaffected by N but dry matter partitioning to spikes as increased by early N stress. Numerical components of kernel number did not give a useful quantitative explanation of these N responses. Delay in N application up until the onset of stem elongation did not limit N response because N uptake and leaf area responded quickly enough for close to full radiation interception to be reached before the onset of spike growth. Crop dry weight accumulation, generally directly related to absorbed photosynthetically-active radiation (PAR A), was also assisted by small increases in radiation-use-efficiency (RUE, g top dry matter MJ −1 PAR A) with higher N content in foliage. Peak RUE values (mean 2.86±0.03 g MJ −1) were close to other published values for wheat. The apparent recovery of N fertilizer was little affected by N amount and timing (average 63%). N uptake continued throughout grain filling in almost all treatments but especially those with late applications. N-utilization efficiency (grain yield/N uptake), however, declined with large N doses and with applications after the onset of stem elongation: values ranged from 30 to 60 g g −1. Utilization efficiency declines were largely associated with increases in grain N concentration and, to a lesser extent, reinforcing decreases in N harvest index.