Plant-microbe interactions: Wetting of ivy (Hedera helix L.) leaf surfacesin relation to colonization by epiphytic microorganisms

Leaf wettability, cuticular wax composition, and microbial colonization of upper and lower leaf surfaces of ivy (Hedera helix L.) was investigated for young and old leaves sampled in June and September. Contact angles of aque ous buffered solutions measured on young leaf surfaces ranged between 76 de grees and 86 degrees and were not dependent on the pH value of the applied droplets. Contact angles measured on old leaf surfaces were up to 32 degree s, significantly lower than on young leaf surfaces. Furthermore, contact an gles were significantly lower using aqueous solutions of pH 9.0 compared to pH 3.0, indicating the influence of ionizable functional groups on leaf su rface wet ting properties. Observed changes in leaf wetting properties did not correlate with different levels of alkanoic acids in cuticular waxes. H owever, microscopic examination of the leaf surfaces indicated the influenc e of epiphytic microorganisms on wetting properties of old leaves, since th eir surfaces were always colonized by epiphytic microorganisms (filamentous fungi, yeasts, and bacteria), whereas surfaces of young leaves were basica lly clean. In order to analyze the effect of epiphytic microorganisms on le af surface wetting, surfaces of young and clean ivy leaves were artificiall y colonized with Pseudomonas fluorescens. This resulted in a significant in crease and a pH dependence of leaf surface wetting in the same way as it wa s observed on old ivy leaf surfaces. From these results it can be deduced t hat the native wetting properties of leaf surfaces can be significantly mas ked by the presence of epiphytic microorganisms. The ecological implication s of altered wetting properties for microorganisms using the leaf/atmospher e interface as habitat are discussed.