Effects of simulated acid rain on epicuticular wax production, morphology, chemical composition and on cuticular membrane thickness in two clones of Sitka spruce [Picea sitchensis (Bong.) Carr.]



Sticklings (rooted cuttings) of two Sitka spruce [Picea sitchensis (Bong.) Carr.] clones were exposed to simulated rain of 4 pH values (5.6, 4.2, 3.4, 2.6). Rain was applied at 2 mm h−1 for 1 h, three times weekly, for 11 wk starting 1 wk prior to bud break. A range of physicochemical cuticular characteristics were investigated on current-year needles following 2, 6, and 11 wk of exposure. Cuticular effects were initiated well above the threshold for visible injury (pH 2.6). The magnitude and direction of cuticular response differed significantly between clones. Amount of epicuticular wax recovered was not affected by the pH of the simulated rain. Wax structure was greatly modified at pH ≤ 3.4 on needles from one clone. Changes in wax structure were accompanied by changes in chemical composition, formation of amorphous deposits with increasing rain acidity coinciding with decreases in nonacosane-diols and increases in estolides. Cuticular membrane thickness was increased significantly (P > 0.001) in one clone and decreased in the other due to simulated rain pH. Such alterations to physico-chemical cuticular characteristics would be expected to have consequential effects on needle surface properties such as needle wettability and retention of acidic rain.