Abstract
Stomata, each delimited by a pair of guard cells, are crucial for gas exchange in terrestrial plants. Guard-cell apoplastic sucrose concentration was proposed to be a signal that integrates the information from bulk-leaf apoplastic sucrose concentration and leaf transpiration rate. In apoplastic phloem loaders, the bulk-leaf apoplastic sucrose concentration is ~2 mM during the photoperiod, but is concentrated to >150 mM in the guard cell apoplast by transpiration, which could diminish aperture size by up to 3 µm. However, guard-cell apoplastic sucrose accumulation is greatly reduced by decreased leaf transpiration rate. Here, two approaches were used to study the relationship between the bulk-leaf apoplastic photosynthate concentration and the guard-cell apoplastic photosynthate concentration. Firstly, a symplastic phloem loading plant, dwarf basil (Ocimum basilicum cv Minimum), was used because it has a naturally low bulk-leaf apoplastic photosynthate concentration. As typical for symplastic phloem loaders, dwarf basil predominately transported Raffinose Series Oligosaccharides (RSOs) in the phloem instead of sucrose. 14C-mannitol fed via the leaf petiole accumulated around guard cells, indicating an open leaf apoplast. Fluctuations in guard-cell contents of K+ and starch, and the leaf conductance were typical, establishing this as the first symplastic phloem loading model species for guard-cell research. The sum of sugar (RSOs + sucrose + glucose + fructose) concentrations in the bulk-leaf apoplast was <0.3 mM. The upper limit of RSOs (stachyose + raffinose) in the guard-cell apoplast was 10 mM, and sucrose, glucose, and fructose in the guard-cell apoplast were not detectable (p>0.2 compared with concentration zero). Therefore, symplastic phloem loaders lack stomatal osmotic regulation by bulk-leaf apoplastic photosynthate. Secondly, with the apoplastic phloem loader broad bean, leaf photosynthesis rate was lowered by shading and leaf transpiration remained constant by lowering ambient CO2 concentration. With shading/low-CO2 treatment, bulk-leaf apoplastic sucrose concentration decreased to 0.4 mM, one third of the control value, whereas the guard-cell apoplastic sucrose concentration decreased to ~40 mM, less than one-fourth of the control value. This phenomenon indicates that in apoplastic phloem loaders, sucrose accumulation in the guard-cell apoplast is a direct function of the bulk-leaf apoplastic sucrose concentration and, thus, the rate of photosynthesis.
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