SOURCE-SINK RELATIONSHIPS AND GRAIN MASS AT DIFFERENT POSITIONS WITHIN THE SPIKE IN WHEAT

Studies manipulating the source-sink relationships during postanthesis in wheat have shown inconsistent results, concluding that wheat yield could be either sink- or source-limited. Most of the studies reported data of grain mass as an average of all grains preventing the recognition of different sensitivities to the availability of assimilates of grains placed at different positions within the spike. The response in growth and final mass of grains from the base, middle and apical spikelets to increases in assimilate availability was studied by modifing source-sink relationships during the grain growth period of wheat. In addition, a quantitative analysis of the responses in grain mass to manipulations in the source-sink relationships was attempted. Two wheat cultivars, 'Condor' (semi-dwarf) and 'Thatcher' (standard-height) were grown in 1992 at the experimental station of The University of Melbourne under two different photoperiods between emergence and heading at field conditions to have two different environments during grain-filling period (early and late). Twenty main shoots in each plot were chosen and detillered at anthesis. Ten days after anthesis, the source-sink relationship was altered in half of them by removing all the spikelets along one side of the spike. Grain growth (averaged over all grains per spike) in both cultivars and under the two grain-filling periods appeared to be largely insensitive to the increase in the availability of assimilates. Neither grain growth rate nor the effective period of grain-filling was significantly affected by potentially doubling the source capacity per grain by trimming. Similarly, there was no effect of trimming on grain mass and its components when grains from apical, central or basal spikelets were analysed separately for the two grain-filling periods. This lack of effect of changes in source-sink relationships on grain growth would imply that the assimilate availability in control plants in this study was sufficient to fully satisfy grain growth requirements. These results were similar when grains of different size (those at different position within the spike) were individually considered, suggesting that there is no association between the size of the grains in the control treatment and their response to changes in the assimilate availability. Comparing these results and many others from the literature, in which there was not agreement about the fact that grain yield was source- or sink-limited, and using a quantitative approach, we suggest that during post-anthesis period, grain yield of wheat is either sink-limited or co-limited by both source and sink but never source-limited.