Nitrogen uptake, water use, grain yield and protein content in wheat

Abstract

Effects of N application and water supply on yield (Y) and protein content (PC) in the semi-dwarf cultivar, ‘Condor’, were investigated in 1984, 1985 and 1987. Treatments included rainfed and irrigated crops (treatments R F and I, resp.). Nitrogen treatments were nil (N 0) and 150 kg N ha −1 (N 150) applied at sowing. Yield ranged from ca. 200 g m −2 to 700 g m −2, and in many cases approximated that predicted by a water-limited productivity function (WLPF) describing the maximum yield attainable depending on seasonal rainfall. Such cases included rainfed crops which received N at sowing (treatment R FN 150). Otherwise, yield was less than predicted by the WLPF, and was clearly limited by factors other than water when seasonal water supply, including irrigation, exceeded ca. 500 mm. Nitrogen uptake (NU) ranged from 4 g N m −2 to ca. 20 g N m −2. The N uptake of treatment R FN 150 was ca. 12–15 g N m −2 each season. By contrast, NU decreased from ca. 12 g N m −2 to 4 g N m −2 with season in the non-fertilized treatments, R FN 0 and IN 0. This was attributed to depletion of soil N reserves. Nitrogen uptake in treatments IN 150 also decreased with season (from ca. 18 g N m −2 to 11 g N m −2), and indicated that the annual application of 15 g N m −2 did not satisfy the potential crop requirement for N under irrigation. In treatments R F, the efficiency of N uptake ( ϵ up) generally ranged between 0.20 and 0.42 g N (g N applied) −1. Nitrogen application increased NU under irrigation, increasing ϵ up to ca. 0.5 g N (g N) −1, and to a maximum of 0.73 g N (g N) −1 in 1985. Yield in treatments other than R FN 150 increased with NU according to Y=966[−exp(−0.058 NU)] This relationship implied a decrease in yield per unit NU ( ϵ util) with increase in NU. Mean yield in treatment R FN 150 was ca. 60% of that expected on the basis of NU, with consequent effects on ϵ util. A comparison of observed measures of yield with those expected on the basis of the WLPF and the yield-NU function allowed an appraisal of factors limiting yield. Water stress restricted growth in treatment R FN 150, and NU was governed in part by a reduced rate of growth. Nitrogen uptake restricted growth and yield in treatments other than R FN 150. Nitrogen accumulation in the grain increased linearly with NU. Nitrogen harvest index (HI N) was ca. 0.6 in treatments R FN 150, and ca. 0.7 in treatments other than R FN 150. Protein content ranged between ca. 8% and 15%. Maximum PC was found in the grain of treatments R FN 150 despite the lower HI N. In treatments other than R FN 150, where PC increased almost linearly with increase in grain N accumulation, the generally-accepted inverse relationship between PC and yield was not seen. However, negative effects of water stress on yield in treatments that were otherwise able to assimilate a minimum of ca. 10 g N m −2 resulted in a negative relationship between PC and yield. The expression of the inverse relationship between PC and yield therefore depended on fertilizer N treatment and soil N status. In treatments where NU was less than ca. 10 g N m −2, PC was confined to the range 8 to 10%, and yield, in all but one instance, did not exceed ca. 350 g m −2. The data suggest that the downward trend in PC of Australian wheat over the last 15 years may be ascribed to over-cropping and its effects on site fertility.