Morphology and excreting-function of microhairs in salt-tolerant Zoysia japonica, comparing adaxial and abaxial leaf surfaces

Bicellular trichomes called ‘microhairs’ are known as ‘salt glands’ in some Chloridoideae in the Poaceae. The adaxial leaf surface of Zoysia japonica, a salt-tolerant grass, excretes salts, whereas the abaxial leaf surface does not. In this study, we investigated the morphology and excreting-function of the microhairs of Z. japonica, comparing the adaxial and abaxial leaf surfaces. Scanning electron microscopy of the leaf surfaces showed the distribution of microhairs on both sides and revealed significant excretion on the adaxial side but a small excretion on the abaxial side. Energy-dispersive X-ray spectrometry revealed that microhairs on the adaxial side excrete sodium, magnesium, and chlorine, whereas those on the abaxial side excrete or secrete insoluble substances, which sometimes contain silicon. Transmission electron microscopy was used to clarify the ultrastructure of microhairs, as in the first detailed report on Zoysia spp.; however, the microhairs on both leaf sides showed typical structures of bicellular salt glands reported in Chloridoideae. The cap cells of the microhairs had smooth surfaces without epicuticular wax structures. These results indicate that the ultrastructural features of Z. japonica microhairs are important for understanding physiological mechanisms in common with salt-excretion and the excretion or secretion of other substances.