Epidermal silicon in sugarcane: Cultivar differences and role in resistance to sugarcane borer Eldana saccharina

Silicon (Si) application can significantly increase resistance of plants to insect herbivory. In sugarcane, Si-mediated resistance to the lepidopteran stem borer Eldana saccharina involves reduced survival, feeding efficiency and stalk penetration. In a pot trial, this study examined: (1) the effect of calcium silicate treatment on the accumulation of amorphous epidermal Si at three sites on the sugarcane stalk where the borer may penetrate, and (2) whether the accumulation of epidermal Si at these sites in Si-treated and control cane plants varied between a borer-resistant (N33) and borer-susceptible (N11) cultivar. Sections of mature stalk were subjected to Energy Dispersive X-ray (EDX) microanalysis to locate and quantify Si accumulation in the stem epidermis. In both cultivars, Si-treated plants had increased silica in each epidermal tissue zone (internode, root band, leaf bud). X-ray mapping confirmed that Si accumulation was restricted mainly to the epidermis of the internode and root band, but was sparse in the underlying tissues. By contrast, there was no evident concentration of Si in the bud scale epidermis compared with the underlying bud tissue. We contend that these patterns of Si deposition, especially at the internode and root band, may explain the previously reported enhanced resistance of Si+ sugarcane to penetration and feeding by E. saccharina at these sites. This is consistent with an hypothesis of increased mechanical hindrance to feeding in Si-treated plants. At all sites, epidermal Wt% of Si was higher in N33 plants (both Si+ and Si−) than in N11 plants, indicating that the higher total stalk Si recorded for N33 compared with N11 was expressed to an appreciable degree at the epidermal level. If amorphous Si increases mechanical resistance to stalk penetration, then the low Si content of the bud scale epidermis compared with the internode and root band epidermis may in part explain the observation that the leaf bud is a preferred entry point on the sugarcane stalk for E. saccharina larvae.