ABSTRACT
In wheat, as in many other crops, the increase in genetic yield potential has resulted, so far, mainly from a rise in the harvest index, that is, in the proportion of above-ground assimilates partitioned to the grains, the harvested sink organs. At all stages of crop growth, the photosynthetic activity of the source organs has a pronounced effect on subsequent demands by the sink organs: that is, there are feed-forward effects of source on sink size. Feed-forward controls do not bear a simple relationship to the supply and demand for assimilates, and there are many examples in which less direct, possibly hormonal, controls have been invoked. The translocation of photosynthetic assimilates between sources and sinks is via the phloem, and the question therefore arises as to whether the growth of sink organs may at times be limited by phloem capacity as much as by source size.
