Guard-cell apoplastic sucrose concentration - a link between leaf photosynthesis and stomatal aperture size in the apoplastic phloem loader Vicia faba L

• Published in: Plant, cell and environment cell and environment, 2007-05, Vol.30 (5), p.551-558
• Main Authors: KANG, YUN, OUTLAW, WILLIAM H. JR, ANDERSEN, PETER C, FIORE, GIORDANO B
• Format: Article
• Language: English
• Subjects: apoplast; Biological and medical sciences; Carbon Dioxide - physiology; carbon‐dioxide uptake; conductance; Darkness; Fructose - metabolism; Fundamental and applied biological sciences. Psychology; Glucose - metabolism; Metabolism; Phloem - physiology; Photosynthesis - physiology; Photosynthesis, respiration. Anabolism, catabolism; Plant Leaves - physiology; Plant physiology and development; Plant Transpiration - physiology; stomata; Sucrose - metabolism; transpiration; Vicia faba - metabolism; Vicia faba - physiology; water loss
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/j.1365-3040.2007.01635.x

In broad bean (Vicia faba L.), an apoplastic phloem loader, the sucrose concentration increases up to ~ 2 mM in the leaf apoplast and up 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 μm or ~ 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 ~ 80% (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.