Zinc Requirement for Stomatal Opening in Cauliflower

Zn deficiency induced increases in epicuticular wax deposits, lamina thickness, degree of succulence, water saturation deficit, diffusive resistance, and proline accumulation and decreases in carbonic anhydrase activity, water potential, stomatal aperture, and transpiration in the leaves of cauliflower (Brassica oleracea L. var botrytis cv Pusa) plants. Restoration of Zn supply to the deficient plants increased stomatal aperture, transpiration, and carbonic anhydrase activity significantly within 2 h. However, leaf water potential in the Zn-deficient plants did not recover within 24 h after resupply of Zn. The guard cells in epidermal peels from the Zn-deficient leaves had less K+ than those from the controls. Stomatal aperture in the epidermal peels from Zn-deficient leaves was 64% less than in the controls when the epidermal strips were floated on 125 mM KCl. Supplementing the ambient medium 25 mM KCl with ZnCl2 enhanced stomatal aperture in both control and Zn-deficient peels, and the effect was significant in the latter. The observations indicate involvement of Zn in stomatal opening, possibly as a constituent of carbonic anhydrase needed for maintaining adequate [HCO3-] in the guard cells, and also as a factor affecting K+ uptake by the guard cells.