Effect of Relative Size and Distance of Competing Sinks on the Distribution of Photosynthetic Assimilates in Wheat

Abstract

The objective of these experiments was to examine to what extent the size of a storage organ influences its ability to compete for a limited supply of assimilate. The experimental system consisted of wheat plants reduced to one main source of current photosynthate, usually the flag leaf of the main stem, and two main competing sinks, the ears of the first two tillers. The size of the two sinks was varied by the differential removal of grains 9 days after anthesis to give competing sinks comparable in every way except in the number of grains. ¹⁴C was assimilated by the flag leaf 2 days after grain removal, and its distribution determined after a further 2 days.

When the relative size of the two sinks was varied, the grains in the larger sink received more than their pro rata share of ¹⁴C-labelled assimilate from the flag leaf, the bias in favour of the larger sink increasing with increase in its relative size. Grain growth in the smaller sink depended to a greater extent on local sources of assimilate, from ear photosynthesis and stem reserves. The more completely ear photosynthesis was inhibited by DCMU the less was the advantage of the larger sink in securing flag leaf assimilates but, even under the most severe competition, there was a significant bias in favour of the larger sink. Reserve carbohydrates in the stem could not be completely eliminated although they were greatly reduced by early defoliation and inhibition of photosynthesis. Their presence precluded an unambiguous conclusion as to the role of sink size in the competition for assimilates from a common source.

Relative distance of the competing sinks from the source was varied by using one of the three uppermost leaves on one tiller as the source of labelled assimilate. Proximity to source conferred a marked advantage, and greater sink size was particularly important in securing assimilates from distant sources.