The Influence of Leaf Position and Defoliation on the Assimilation and Translocation of Carbon in White Clover (Trifolium repens L.) 1. Carbon Distribution Patterns

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Abstract
The assimilation of carbon (C) by, and distribution of 14C from, leaves at each end of an unbroken sequence of ten mature leaves on the main stolon of clonal plants of white clover (Trifolium repens L.) were measured to identify intra-plant factors determining the direction of C movement from leaves. Leaves at two intermediate positions were also measured. Localized movement of 14C to sinks at the same node as, or at the one to two nodes immediately behind, the fed leaf accounted for 40–50% of the total 14C exported by all measured leaves. A further 50–60% of exported 14C was therefore available for more-distant sinks, and the direction of translocation of this C was determined by the relative total strength or demand (number x size x rate of activity or growth) of sinks forward of, or behind, the leaf in question. Thus 85% of the 14C exported from the youngest measured leaf moved toward the base of the stolon, while about 60% of the 14C exported from the oldest leaf moved acropetally. Defoliating plants to leave just one mature leaf on the main stolon (at any one of the same four positions studied in undefoliated plants), and no leaves on branches, resulted in: (1) increased net photosynthetic rate in all residual leaves: (2) increased% export of fixed C from one of the four leaves; (3) increased export to the main stolon apex from all except the eldest leaf; (4) increased export to branches from three of the four leaves; and (5) decreased export to stolon tissue and roots from all leaves, within 3 d of defoliation. These responses would seem to ensure the fastest possible replacement of lost leaf area and, thus, restoration of homeostatic growth. The observed patterns of C assimilation and distribution in both undefoliated and defoliated white clover plants are consistent with the general rules of source-sink theory; the distance between sources and competing sinks, and relative sink strength, emerge as the most important intra-plant factors governing C movement. These results emphasize the need to consider plant morphology, and the modular nature of plant growth, when interpreting patterns of resource allocation in clonal plants, or plant responses to stresses such as partial defoliation.

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