AN ANALYSIS OF EFFECTS OF NITROGEN DEFICIENCY ON GROWTH AND YIELD OF A WESTERN AUSTRALIAN WHEAT CROP

A wheat crop was grown on a nitrogen-deficient sandy soil. Urea was supplied at rates of 0 (N0), 56 (Nx), and 336 (N2) kg nitrogen/ha. In general, the relative growth rate (R) decreased with time. During the first half of the growing season R for N2 >N> N0, but in August deficient plants recoveredrapidly. The recovery was associated with the emergence of the second tiller, an increase in net assimilation rate and nitrogen uptake, and a decrease in nitrogen stress. There was no evidence that the recovery was due to an increase in mineralization of soil nitrogen. Nitrogen stress in treatment N0 decreased from 48% in July to 14% in September. By contrast, stress in treatment N2 increased rapidly from 6 to 23% during July and fell to 5% by September. Flower initiation in N0 plants was delayed 14 days and ear emergence was delayed 8 days compared with N2 plants. The grain yields of N0 and Ni plants were 30 and 60% respectively of those of N2 plants. The decrease was due mainly to differences in the number of heads per sq metre, although nitrogen deficiency also reduced the number of grains per head and the weight per grain. The duration of photosynthetic activity after anthesis was not affected by nitrogen treatments. Harvest index was highest on the N0 treatment and similar on Nj and N2 treatments. © 1969 CSIRO. All rights reserved.