Epicuticular wax of leaf epidermis: a functional structure for salt excretion in a halophyte Puccinellia tenuiflora
盐生植物星星草叶表皮具有泌盐功能的蜡质层

As an adaptive consequence, plants growing in saline habitats have developed various salt tolerant mechanisms such as ion selectivity through the membrane system and intracellular ion compartmentation, anatomical modifications such as salt gland on their leaves and special transfer cells for ion transport. Puccinellia. tenuiflora is prevalent in the saline swamps of northern China where soil is essentially composed of sodium carbonate, sodium sulphate or sodium chloride. P. tenuiflora grows well in these areas because of its high salt tolerance. Based on reported work, it was thought that the salt tolerant mechanism of P. tenuiflora could be attributed to their high ion selectivity. From data obtained through leaf-washing experiments, it appeared that the salt tolerance of P. tenuiflora was due to salt excretion through the leaves. However, the salt gland was not observed. This study investigated epicuticular wax from the leaf epidermis of P. tenuiflora as a mode of salt excretion using environmental scanning electron microscopy and X-ray electron probe microanalysis through different preparation methods for structure and elemental analysis. The results showed that the epidermis consisted of epidermis cells and stoma complexes. On the lower surface of leaves, epidermal hairs were observed along with a mass of sunken stomata grouped in parallel rows between the veins. In addition, wax ornamentation and wax grains were present on the surface of the leaves. These structures are characteristic for plants resisting the saline-alkaline and the drought environment. Wax ornamentation and wax grains contained significant amounts of salt ions in response to salt stress. This indicated that epicuticular wax possessed salt excretion capability. Therefore, it is reasonable to assume that the epicuticular waxes of P. tenuiflora may undergo structural and physiological changes to excrete salt ions and adapt to halomorphic conditions.