Short-term effects of manipulating the source:sink ratio of white clover (Trifolium repens) plants on export of carbon from, and morphology of, developing leaves

The source:sink ratio of clonal white clover ( Trifolium repens L.) plants was manipulated by shading or removing leaves, and the consequences for carbon export from, and for the weight, area and net photosynthesis of, developing leaves were determined. When treatments were imposed just before young leaves usually change from C sinks to C sources, no effect on the point at which the sink-to-source transition occurred was observed, Leaves exported a similar proportion of the C they fixed, irrespective of stresses imposed upon the rest of the plant. However, differences in the destination of exported C were observed. More C moved to the stolon apex, and less to the stolon tissue itself, from leaves at Carlson stage 0.8 (leaflets about 60% unfolded) when mature leaves were removed or shaded. When 2 out of 3 mature leaves were removed from a stolon, short-term responses such as a 10% increase in net photosynthesis of the residual mature leaf, and greater export of C from this leaf to the apex, partially compensated very young leaves for loss of C supply. The result was that, when these young leaves were fully unfolded, they had similar surface area to those which had developed on undefoliated plants, but weighed nearly 20% less. Thus the immediate response to defoliation was an increase in the speeific leaf area cm² (g of dry weight)^?1] of new leaves, and the assimilation rate (net photosynthesis×area) of these leaves remained unchanged.