Effects of SA Induction on Leaf Cuticular Wax and Resistance to Sclerotinia sclerotiorurn in Brassica napus

Cuticular wax on the surfaces of higher plants is believed to be the initial defense system to various pathogens. However, whether or how wax deposition in Brassica napus is involved in the resistant mechanism of infecting Sclerotinia sclerotiorum, is still unclear. In the current experiment, two Brassica napus cultivars with different disease resistances, Zhongshuang 9 (resistant) and Yuyou 19 (susceptible), were selected to analyze the dynamics of disease indices, contents and crystal structure of leaf cuticular wax, activities of defense enzymes, and gas exchange indices under the conditions of SA induction and Sclerotinia sclerotiorurn inoculation. The results showed that SA induction significantly reduced the disease index of Zhongshuang 9, increased its resistance, while SA had no significant influence on disease index of Yuyou 19. When the plants were inoculated with S. sclerotiorurn, the activity of PAL in leaf of Zhongshuang 9 increased while that of Yuyou 19 decreased. The activities of PAL and POD in plants with SA induction were significantly higher than those in plants treated with only S. sclerotiorurn inoculation for both cultivars. The content of total wax in leaf of Zhongshuang 9 was significantly lower than that of Yuyou 19. SA induction increased the contents of total wax and wax constituents for Zhongshuang 9, reduced the amounts of rod crystalloid and increased the amounts of plate crystalloid, and increased the leaf area covered with waxes. The contents of waxes for Yuyou 19 were changed insignificantly by SA induction. However, the changes of the wax crystalloid structure of Yuyou 19 by SA induction were similar with those of Zhongshuang 9, except for a lower leaf area covered with waxes. Conclusion is that both defense enzymes and leaf cuticular waxes are involved in the process of increasing resistance induced by SA for resistant cultivar Zhongshuang 9.