Wetting of the upper needle surface of Abies grandis : influence of pH, wax chemistry and epiphyllic microflora on contact angles
Wetting of the upper needle surface of Abies grandis Lindl. by aqueous solutions of different pH values was investigated. With increasing needle age, contact angles decreased significantly from about 75° on current-year needles to values lower than 30 ° on 4-year-old needles. On older needles, contact angles were significantly lower, by more than 10°, when aqueous solutions of pH9.0 were used compared with those of pH3.0. On the surfaces of older needles, contact angle titrations were carried out, contact angles being measured with aqueous solutions covering a pH range from 3.0 to 11.0. Measured titration curves showed clear inflection points around pH 7.0, indicating the existence of ionizable carboxylic groups in the interface between needle surface and atmosphere. The evidence seems convincing that the pronounced pH dependence of wetting is mainly due to the presence and/or activity of epiphyllic micro-organisms, whereas the cuticular wax composition of Abies grandis needles does not appear to contribute significantly to this phenomenon. Thus, the results presented here allow the general conclusion that changes of contact angles measured on leaf surfaces may not always be due to changes in the leaf surface chemistry and/or the fine structure of leaf surface waxes, but may also be due to increased amounts of epiphyllic micro-organisms significantly altering the leaf surface wetting properties.