Effect of water stress on the epicuticular wax composition and ultrastructure of cotton ( Gossypium hirsutum L.) leaf, bract, and boll

Abstract

The epicuticular waxes (EW) of leaves, bracts, and bolls of cotton ( Gossypium hirsutum L.) were studied with respect to total accumulation, chemical composition, and their ultrastructure under well-watered and water-stressed conditions. Leaf, bract, and boll accumulated 91.71 μg cm −2, 74.18 μg cm −2, and 152.58 μg cm −2 of wax, respectively, under well-watered conditions, while water stress increased the wax concentration to 154.60 μg cm −2, 108.91 μg cm −2, and 158.53 μg cm −2 in the leaf, bract, and boll, respectively. Water stress increased the number and levels of long-chain, higher molecular weight alkanes in the leaf and bract wax, whereas the water-stressed boll wax contained only the long-chain alkanes. Among the long-chain alkanes, n-octacosane (2.70%), n-nonacosane (1.76%), n-triacontane (2.10%), dotracontane (4.60%), and n-tetratriacontane (24.50%) were the chief constituents of the water-stressed leaf wax. The bract wax contained n-octacosane (3.30%) and n-triacontane (38%). n-Octacosane (21.70%), n-nonacosane (8.60%), and n-triacontane (27.90%) were the long-chain alkane constituents of the boll wax. Alkanes constituted 65.94%, 51.00%, and 58.20% of the total wax in the water-stressed leaf, bract, and the boll, respectively, whereas wax from well-watered leaf, bract, and boll contained 10.53%, 25.70%, and 91.30% alkanes, respectively. Scanning electron microscopy revealed that the adaxial and abaxial surfaces of the leaf and bract, and the boll exterior surface had analogous wax morphology under both water-stressed and well-watered conditions. The results suggested that water stress increased the levels of long-chain alkanes in the epicuticular wax in which n-tetratriacontane was the major wax constituent of the water-stressed leaf, whereas the water-stressed bract and boll contained n-triacontane as the major wax component.