title: Guard-cell apoplastic sucrose concentration--a link between leaf photosynthesis and stomatal aperture size in the apoplastic phloem loader Vicia faba L.
abstract: In broad bean (Vicia faba L.), an apoplastic phloem loader, the sucrose concentration increases up to approximately 2 mM in the leaf apoplast and up to approximately 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 microm or approximately 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 CO(2) 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 approximately 80% (to approximately 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.
authors.affiliation: Department of Biological Science, Florida State University, Tallahassee, FL 32306-4370, USA.
authors.forename: Yun
authors.initials: Y
authors.lastname: Kang
authors.forename: William H
authors.initials: WH
authors.lastname: Outlaw
authors.forename: Peter C
authors.initials: PC
authors.lastname: Andersen
authors.forename: Giordano B
authors.initials: GB
authors.lastname: Fiore
mesh_terms: D002245:Carbon Dioxide / Q000502:physiology; D003624:Darkness; D005632:Fructose / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D052585:Phloem / Q000502:physiology; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000502:physiology*; D018526:Plant Transpiration / Q000502:physiology*; D013395:Sucrose / Q000378:metabolism*; D031307:Vicia faba / Q000378:metabolism / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide; D005632:Fructose; D013395:Sucrose; D005947:Glucose