Calcium in the xylem and its influence on the behaviour of stomata

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Abstract
Calcium is known to play an important role in regulating guard cell turgor and the movements of stomata. The concentrations of calcium ions in xylem sap are often higher than 1 mol m -3 , which would be sufficient to influence, or interfere with, stomatal function if such concentrations were delivered to points of evaporation in the vicinity of the guard cells. This paper presents some recent experimental evidence concerning the way in which a plant’s calcium status affects the amount of free calcium in the xylem, and the effect this may have on the diurnal pattern of stomatal behaviour. Changes in the rhizospheric supply of calcium have a major influence on the concentrations in the xylem. In Commelina communis an eightfold increase in rhizospheric calcium led to an increase in the xylem sap in the shoot of approximately sixfold. Very high concentrations of xylem calcium were associated with reduced stomatal opening, and injection of a pulse of calcium ions into the xylem via a catheter caused stomatal closure. Calcium-induced suppression of stomatal aperture does not inflict permanent damage upon the guard cells, because stomatal aperture recovered quickly when the calcium concentration in the xylem was reduced. The experimental data presented suggest that the amount of calcium delivered by the transpiration stream to points of evaporation needs to be regulated if interference with stomatal behaviour is to be avoided. This regulation is likely to occur in tissues such as the mesophyll. The roots may also play an important part in controlling the delivery of calcium into the xylem and evidence is presented of malfunctioning of the regulatory mechanism in roots when plants are exposed to high calcium in the rhizosphere. Some of the data presented are for a calcifuge, Lupinus luteus , and the possibility is discussed that disturbances in stomatal behaviour contribute to the physiological problems of such plants in the presence of high rhizospheric calcium.
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