Foliar Water Uptake in the Needles of Pinus Torreyana

Abstract In semi-arid climate systems that encounter low clouds or fog, many plants have adapted to capture atmospheric moisture on the foliar surface. The natural habitat of Pinus torreyana, the Torrey pine, is restricted to two locales in coastal Southern California that experience substantial fog and low clouds during the dry months of a Mediterranean climate. In this study, droplet contact angles were used to assess surface wettability, and droplet uptake was used to measure the foliar water uptake rate along a needle. We demonstrated that there are only remnants of epicuticular wax on the needle. The results showed that the needle surface of P. torreyana, including under the base sheath, is hydrophilic and capable of direct water uptake. There are small spatial gradients in the uptake pattern, but the variability is high and the statistical significance is not strong. However, the decrease in uptake at higher contact angles can be described by an ordinary linear regression (r2 = 0.45, p = 4 x 10-8). On average, the foliar uptake of the adaxial surface of a one-year-old needle is 2.88 ± 0.60 x 10-2 mg cm–2s–1. In comparison, the uptake rates by the adaxial surfaces of four local broadleaf plants are between 0.05 and 0.6 of that of P. torreyana.