Does calcium in xylem sap regulate stomatal conductance?

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Abstract
SUMMARY: Whole plants and individual leaves of Commelina communis and Triticum aestivum were used to study the influence of changes in apoplastic Ca on the regulation of gas‐exchange, in particular stomatal conductance (g) and transpiration (E). Plants grown in nutrient solution at a low concentration of Ca (1 mol m−3) were used in experiments where rapid changes in Ca concentration were induced via the rhizosphere or by direct entry into the xylem. Solutions containing Ca2+ ions were applied directly to the leaf surface and also through the cut end of detached leaves and via a xylem feeding system to attached leaves. Putative Ca‐channel blockers (lanthanum chloride and verapamil hydrochloride), which are believed to deny Ca entry into the cytoplasm, were also used to provide more information about the nature of the action of Ca on the guard cells.In all the experimental systems used, the short‐term application of 8 mol m−3 Ca induced reductions in both g and E, but when longer‐term changes in the calcium concentrations in the xylem sap were induced through changes in Ca supply from the rhizosphere, no change in g was apparent. These experiments raise important questions about how plants with an abundant supply of calcium in the rhizosphere prevent high concentrations of apoplastic calcium from interfering in the stomatal mechanism. The data presented do not preclude the possibility that a pulse of calcium ions delivered into the xylem could be used in root–shoot communication.