EFFECTS OF SIMULATED ACID RAIN ON PRODUCTION, MORPHOLOGY AND COMPOSITION OF EPICUTICULAR WAX AND ON CUTICULAR MEMBRANE DEVELOPMENT

Abstract

SummaryLeaves of Phaseolus vulgaris L., Vicia faba L., Pisum sativum L. and Brassica napus L. were exposed to simulated rain from emergence to full expansion at seven pHs between 5.6 and 2.6, applied at 2 mm h−1 in amounts and at intervals representative of ambient rain in southwestern England. Leaf expansion was unaffected by rain pH greater than 30 in V. faba, and pH 2–6 in the other species. Macroscopic foliar injury was induced at pH 3.4 in non‐wettable leaves of P. sativum and B. napus which had relatively large deposits of crystalline epicuticular wax, and at pH 3.0 in wettable leaves of P. vulgaris and V. faba which had small deposits of amorphous wax. Foliar injury was greatest in species with crystalline wax. Wax production was affected by simulated acid rain in all species and was often accompanied by changes in wax composition. Wax quantity was reduced on leaves of P. vulgaris exposed to rain at pH 4.6, while wax on leaves of P. sativum and B. napus had increased numbers of smaller plates and tubes, respectively, per unit area at pH 4.6. Leaves of B. napus, stripped of crystalline epicuticular wax at about 20% full expansion, regenerated less wax at pH 3.4 than at pH 5.6. The thickness of the cuticular membranes in leaves of P. vulgaris, V faba and P. sativum exposed to simulated acid rain at pH 4.2 decreased by 28 to 35 % compared with those exposed to pH 5.6 rain. Membrane ultrastructure was also altered in leaves of P. vulgaris. Taken together, these changes could have important consequences for leaf wettability, rainfall retention, foliar uptake of chemicals and host‐parasite interactions.