The Weights of Individual Grains of the Wheat Ear in Relation to Their Growth Potential, the Supply of Assimilate and Interaction Between Grains

Abstract

The work reported here was done to explore the extent to which the mature weight of a grain is determined by (i) its potential for growth, defined as its intrinsic capacity to accumulate dry matter, and (ii) the resistance to assimilate transport imposed by the vascular system of the ear. Estimation of growth potential was attempted by observing the effects of systematic patterns of grain removal on the mature weights of grains remaining, these being compared with weights of matched grains from intact ears. Resistance to transport of assimilate was inferred from the apparent order of priority between grains for the supply of assimilate, as revealed by comparing their weights when assimilate supply was either normal, or reduced by plant shading. When neighbouring grains were removed, those remaining usually grew larger to an extent that indicated growth potential appreciably in excess of that utilized in intact ears under the most favourable conditions. Although grains within a spikelet of an intact ear attain quite different weights, the experiments suggested that their differing potentials for growth seemed to play only a minor role in this, and that the major influence was the relative ease with which assimilate could reach the grains; this depended largely on the distance of the grains from the spike rachis. Comparing between spikelets, the difference found in intact ears between grains in the same spikelet location tended to persist when some grains were removed from each spikelet, indicating a possible role of growth potential as a controlling influence. This may be partly due to the sequence of morphogenesis, established as early as the double ridge stage. Although the removal of competing grains within a spikelet usually enhanced the growth of the one remaining, this was not always so; there was evidence from one experiment that removal of competing grains towards the spikelet apex represented the removal of some beneficial influence. The bearing of the results on possible limitations to grain yield are discussed.