SELECTIVE PERMEABILITY OF CUTICLES OVER STOMATA AND TRICHOMES TO CALCIUM CHLORIDE

Using a leaf disc method rates of penetration of ⁴⁵CaCl₂ across upper and lower surfaces of apple, pear, broad bean and corn leaves were studied at 20 °C and 100% humidity. The donor solutions contained 5 g l^-1 CaCl₂ and 0.2 g l^-1 Glucopon 215 CSUP as wetter. A small droplet (5 µl) was applied to each leaf disc, allowed to dry within 1 h and penetration was followed from the hydrated salt residue on the leaves for up to 12 h. The following observations demonstrate a significant involvement of stomata in foliar penetration of salts: (1) Penetration across astomatous apple and pear leaf surfaces was too slow to be measured, whereas half times of penetration across stomatous surfaces ranged from 0.5 to 9 h. (2) Rates of penetration in the light were higher than in the dark. (3) Initial penetration during droplet drying was zero in non-stomatous surfaces while in stomatous leaf surfaces it ranged from 19 to 69% of the dose, depending on species, leaf surface and illumination. It is suggested that stomatal infiltration was involved in initial penetration, while later on, when penetration proceeded from a hydrated salt residue, penetration across the cuticle was the major pathway. When silver nitrate was used as a tracer, black silver precipitates were formed in the cuticle over and inside guard cells, as well as in trichomes and in the cuticle surrounding the base of the trichomes. It is suggested that CaCl₂ and other ionic species may penetrate into leaves via the same routes.