Influence of water deficit on leaf cuticular waxes of soybean (Glycine max [L.] Merr.)

The objective of this study was to evaluate leaf cuticular wax constituents across a broad selection of soybean (Glycine max [L.] Merr.) cultivars and their response to drought stress. Water deficit was imposed on 18 soybean cultivars by withholding irrigation for 10 d after the postflowering stage (R2 and R3 periods), and the effect on leaf waxes and seed yield was assessed by comparison with a well-watered control. Leaf cuticular waxes were dominated by alkanes and triterpenoids, averaging 28% and 39% of total wax amount, respectively, with primary alcohols being the next most abundant class. The components of soybean leaf cuticular waxes were quite similar, but there were quantitative differences between the cultivars studied. Compared to well-irrigated plants, all drought-treated cultivars except DC exhibited a significant increase in wax amount. When expressed as an average across all cultivars, drought treatment caused a 30% increase in the total wax amount, with a corresponding 59% increase in alkanes, a 16% increase in primary alcohols, a 15% increase in triterpenoids, and a 26% increase in the total of unknowns. In all cultivars, the major alkane constituents were the C27, C29, C31, and C33 homologues, whereas the major primary alcohols were the C30 and C32 homologues, and drought exposure had only minor effects on the chain length distribution within these and other wax classes. Triterpenoid constituents were identified as 3-keto-olean-12-ene, lupenone, lupeol, alpha-amyrin, and beta-amyrin, and each of these showed small quantitative changes after drought. Drought stress caused a large decrease in seed yield but did not affect 100-seed mass, showing that soybean responds to postflowering drought by reducing seed numbers but not seed size. Seed yield was inversely correlated with wax amount after drought treatment, indicating that drought induction of leaf wax deposition does not contribute directly to seed set. This study sheds new light on our understanding of the relationship between soybean leaf wax induction and seed development in a water-limiting environment.