In broad bean (Vicia faba L.), an apoplastic phloem loader, the sucrose concentration increases up to similar to 2 mM in the leaf apoplast and up to similar to 150 mM in the guard-cell apoplast during the photoperiod. This high concentration in the guard-cell apoplast results from transpiration and is sufficient osmotically to reduce stomatal aperture size by up to 3 mu m or similar to 25% of the maximum aperture size. In this paper, we investigated a parallel and required role for high bulk-leaf apoplastic sucrose concentration, which correlates with high photosynthesis rate. An empirically determined combination of lowered light intensity and lowered CO2 concentration reduced the photosynthesis rate to nominally one-fifth of the control value without a significant change in transpiration. This reduction in photosynthesis caused the sucrose concentration in the leaf apoplast - the immediate source pool for guard cells - to decrease by 70% (to 0.4 mM). In turn, sucrose concentration in the guard-cell apoplast decreased by similar to 80% (to similar to 40 mM). These results complete the required evidence for a non-exclusive, transpiration-linked, photosynthesis-dependent passive mechanism for the modulation of stomatal aperture size. In an ancillary investigation, hexoses in the bulk-leaf apoplast decreased when photosynthesis was lowered, but their concentrations in the guard-cell apoplast of control plants indicated that their osmotic contribution was negligible.
