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 similar semi-arid climate systems that encounter fog or low clouds, many plants can capture atmospheric moisture and are capable of direct foliar water uptake to reduce water stress. In this study, we investigated if the needles of P. torreyana are also capable of direct water uptake. In addition to water immersion, we measured the surface properties along a needle using microdroplets. The droplet contact angle is a measure of surface wettability, and the droplet absorption is a measure of localized foliar water uptake. The results showed that the entire length of the P. torreyana needle, including under the base sheath, is hydrophilic and capable of direct water uptake. The spatial gradients of the wettability and the water uptake along the needle are relatively small. Moreover, the wettability and water uptake increase from young shoots to one-year and two-year old needles. Our results also showed that the decrease in water uptake at higher contact angles can be described by a linear regression. Compared with the leaves of four shrubs in the same habitat, Heteromeles arbutifolia, Malosma laurina, Rhus integrifolia, and Eriodictyon crassifolium, the P. torreyana needles have lower contact angles and higher water uptake rates.
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.
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