REGULATION OF K+ UPTAKE AND TRANSPORT TO THE XYLEM IN BARLEY ROOTS - K+ DISTRIBUTION DETERMINED BY ELECTRON-PROBE X-RAY-MICROANALYSIS OF FROZEN-HYDRATED CELLS

Intact roots of young barley plants (Hordeum vulgare L. cv. Proctor) were induced to transport K+ to the xylem at rapid or slow rates. Roots were then rapidly frozen in liquid nitrogen and fractured in the zone 70 mm behind the root tip to give transverse faces for electron probe microanalysis. With SEM visualization, analyses were made over the cytoplasm and vacuole (or lumen) of 14 cells types along the root radius between the outer cortex and stele, with particular emphasis on the xylem, xylem parenchyma, and phloem. Data were recorded in the form of colour-coded maps and also quantitatively. For both types of roots, K+ concentration was lower over the xylem and phloem than in the remainder of the root. The concentration of K+ was greater in the vacuole than in the cytoplasm, while for P it was the reverse. Significantly, in roots induced to transport K+ rapidly the concentration of K+ was low in the early maturing metaxylem and protoxylem, and in the sieve tubes of the metaphloem and protophloem. The concentrations of K+ in various cell types are discussed in relation to regulation of K+ loading of the xylem in long-distance ion transport. © 1990 Oxford University Press.