Physiological Factors Limiting Grain Size in Wheat

Abstract

The effects on grain size of changing the supply of assimilates, by thinning before anthesis or by shading the plants or by halving the ears either early or late in grain growth, were studied in two glasshouse experiments with Kleiber spring wheat (Triticum aestivum L.), in 1976 and 1977. Late treatments had no effect, presumably because little grain growth occurred thereafter. Thinning the plants before anthesis increased, and shading the plants soon after anthesis decreased grain size. Halving the ears soon after anthesis increased the size of the remaining grains, but grain weight per ear decreased. The effect on grain size of halving the ear tended to be smaller under conditions more favourable for photosynthesis, except when the plants were thinned before anthesis. Shading decreased the total amount of nitrogen per culm and the proportion of total nitrogen recovered in the ear. Halving increased the retention of nitrogen in the stem of unshaded shoots and had no effect on nitrogen distribution within shaded shoots. In 1977 halving the ear increased the rate of dry matter accumulation in the grain throughout the grain filling period, but in 1976 the increase in dry weight was faster in the grains of halved ears only during early grain growth. Later the grains in halved and intact ears increased in dry weight at the same rate, even though the supply of photosynthate and the capacity of the grains (as measured by volume) were greater in the halved ears. These results are discussed in relation to the influence on final grain weight of assimilate supply and the storage capacity of the grain.