An Investigation into the Rate and Control of Assimilate Movement from Leaves inPisum sativum

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Abstract
Export studies were made on leaves of Pisum by monitoring the 14CO2-treated source leaf at its surface at frequent intervals. Radiocarbon levels of fresh leaf samples showed a good correlation with results from the more conventional methods of radiocarbon estimation which involve destructive analysis. The rate of export was highest in plants which had been defoliated, except for the source leaf 20 h or more before the start of the export study. Removal of the shoot apex reduced export and progressive reduction in sink capacity was associated with decreased export rates, particularly over short time periods. Export rates were similar in defoliated and non-defoliated plants where the shoot apex and the roots had been excised. This suggested that a decrease in the source resulted in higher export rates from the remaining source only when active sinks were present; this in turn suggests that, at least under these conditions, active removal of photosynthate is more important in controlling export than the photosynthate build-up in the leaf itself. The non-destructive technique enabled comparisons to be made between export curves for individual plants. It was found that in experiments replicated in time, the same relationship between treatments was present on different days and the shape of the export curves was similar but the absolute values for exported radiocarbon sometimes varied considerably.

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