Foliar Absorption in Prunus domestica L. I. Nature and Development of the Surface Wax Barrier

Abstract

Surface wax concentration, distribution, ultrastructure and wettability, as affected by developmental temperature, physiological age and seasonal changes, were studied on leaves of Prunus domestica. Surface waxes covered both leaf surfaces in an amorphous sheet, which extended over the guard cells on the abaxial surface. This sheet may have been thin or discontinuous above adaxial anticlinal walls. A secondary structure of wax ridges was superimposed on the amorphous sheet. Neither surface was readily wetted by water or by solutions of standard organic surfactants. The critical abaxial surface tension was estimated as 22-25 mNm^-1 by Zisman plot, confirming that close-packed, oriented, methyl groups are exposed at the wax surface.

Surface wax concentration was inversely proportional to temperature during leaf development. At any time, surface waxes differed little among leaves of different physiological ages, but wax concentration increased during the season on leaves of similar physiological age reaching a maximum on the abaxial surface in mid-summer and thereafter remaining constant. Results are discussed in terms of polar pathways through the cuticle, stomatal penetration and spray application strategies.