Salt excretion from the salt glands in Rhodes grass (Chloris gayana Kunth) as evidenced by low-vacuum scanning electron microscopy

Abstract

Some salt-tolerant plants belonging to the Poaceae excrete salts to the leaf surfaces under salinity conditions, and the bicellular glands on their leaf surfaces have been postulated to excrete salt. However, clear evidence of the salt excretion from these bicellular salt glands has not been shown at the electron-microscope level because soluble attachments on the leaf surface are completely removed during specimen preparation for conventional electron microscopy. To determine whether the bicellular salt glands actually excrete salt, we examined the leaves of Rhodes grass (Chloris gayana Kunth), Poaceae, by scanning electron microscopy in a low-vacuum mode, which allows to observe specimens without preparation procedures. Unwashed and washed fresh leaf surfaces were examined, and excreted materials on the leaf surface were analyzed by energy dispersive X-ray spectrometry. On the unwashed leaf surfaces, globular materials were observed arranged along the same lines as the macrohairs of the leaf surface, but the salt glands were hardly observed. After leaf surfaces were washed, the globular materials disappeared, and the salt glands appeared localized at the same lines as the macrohairs. Density of the globular materials observed under unwashed conditions and the salt glands under washed conditions was equal. These findings indicate that the glands indeed excrete globular materials just above their cap cells. The excreted materials contained sodium, chlorine, and potassium, and the counts of sodium and chlorine was increased greatly with NaCl treatment of the plants. After removing the excreted materials, most of the cap cells of the salt glands were smooth globular, without ruptures in their cuticle. We conclude that the leaves of Rhodes grass indeed excrete salt from the bicellular salt glands, but without rupturing the cuticle on the cap cell.