Effects of nutritional status on the responses to high temperature of 2 wheat cultivars that differed in their sensitivity to high temperature were investigated in plants grown in pots in environmentally controlled growth rooms. The availability of nitrogen to the grains was altered by changing the amount and timing of the nitrogen supplied to the plants, and also by trimming the ears. Single grain weight was significantly decreased in the plants grown at 30/25°C compared with those grown at 20/15°C. The effects of nitrogen level or trimming on the response of grain filling to temperature did not seem to be mediated through alterations in the availability of carbohydrates within the grains. Neither N level nor trimming had substantial effects on the deposition of starch, but both treatments altered the accumulation of protein, and the responses of protein accumulation to the effects of temperature.

At the lower temperature, increasing the nitrogen supply resulted in greater single grain weight, more protein per grain, and higher grain protein percentage. At the higher temperature, raising the supply of N increased none of these attributes; the rate of protein accumulation was not accelerated by raising the temperature as much at high as at low N, and high N reduced the duration of protein deposition. There was a linear and positive relationship between the amounts of amino acids in the grain and the rate of accumulation of protein in the endosperm, and the effects of N level and trimming were associated with changes in the concentration of amino acids in the grains. High temperature conditions appear to lower the supply of amino acids to the grain in plants well supplied with nitrogen.

There was a positive relationship between the rate of protein deposition and the amount of amino acids in the grain. Although raising the level of nitrogen resulted in an increase in amino acid levels in the grain at low temperature, there was no such increase at high temperature. Trimming, however, did increase grain protein and the rate of protein deposition at high temperature as well as the amino acid levels in the grain. One possible interpretation of this paradox arises from other work showing that under high temperature and/or water deficit there is an inadequate supply of soluble carbohydrates in the shoot to metabolise high levels of nitrogen. Differences in the responses of starch deposition and protein accumulation to N level, trimming, and temperature confirm that these two components accumulate in the grain independently of each other. Consequences of the interactions between nitrogen supply, temperature, and cultivar are discussed in relation to yield and quality.