Control of Import and Export of Photosynthate in Leaves

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Abstract
The flow of photosynthetic assimilate in leaves was traced using radioactive 11CO2. In younger leaves the direction of flow can change in response to changes in assimilate potential in the rest of the plant whereas inolder leaves it cannot. The irreversibility of flow in older leaves is apparently a result of their having a loading process able to maintain a sieve tube assimilate potential higher than elsewhere in the plant. Systematic shifts in the responses of leaves to a succession of short term changes in sink demand are best understood in terms of radial exchanges between the sieve tubes and storage pools of limited capacity in the surrounding tissue. These take place throughout the length of the sieve tubes and may give rise to flows in opposite directions, at the same time, at different points in the same sieve tube. Behaviour indicates that flow between different pools of assimilate in the plant can be thought of as being a functionof differences in their assimilate potentials, with changes in potential eliciting changes in flow.

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