Effects of ionic strength and relative humidity on the efflux of K+ (86Rb) and Ca2+ (45Ca) from roots of intact seedlings of cucumber, oat and wheat

Abstract

AbstractSpring wheat (Triticum aestivum L. cv. Svenno), oat (Avena sativa L. cv. Brighton) and glasshouse cucumber (Cucumis sativus L. cv. Bestseller F1) were cultured for a week after germination on complete nutrient solutions of three different dilutions (1, 25 and 50% of the full strength medium). K+(86Rb) and 45Ca were present during the whole culture period. Relative humidity (RH) was 50% except during the last day, when half the material was transferred to 90% RH. Efflux of labelled ions was then followed during eight hours on unlabelled solutions of the same composition as before, and at both 50% and 90% RH in the atmosphere. – Uptake of K+(86Rb) during growth tended to be saturated in the 25% medium. Contrariwise, the level of Ca2+ in the roots increased continuously with strength of the medium. At low concentrations cucumber roots were higher in Ca2+ than roots of oat or wheat, whereas all three species showed similar levels of Ca2+ in 50% medium. – At the lowest ionic strength, smooth efflux curves were obtained that could be resolved according to the three‐compartment theory. At higher ionic strength, irregularities were observed, and more for Ca2+ than for K+; but for practical purposes compartment analysis with the same time constants could be applied as for the lowest concentration. – Discrimination between K+ and Rb+ differed between the roots, but not much between the shoots of different species. The roots of oat and wheat took up Rb+ preferentially over K+ in the 25% and 50% media; whereas K+ was preferred over Rb+ or little discrimination made in 1% medium and for cucumber. The shoots generally showed less discrimination than the roots. The main variability in discrimination between K+ and Rb+ thus appears to be localized in the tonoplasts of the roots cells. – Low RH around the shoots increased efflux of K+(86Rb) from the cytoplasm and vacuoles of the root cells as compared to the efflux at high RH. DNP (2,4‐dinitrophenol) in the medium had the same effect as high RH around the shoots. The signal system that must exist between shoots and roots is discussed as a response to “drought” conditions. In relation to investigations of others, it is assumed that the effect of DNP may indicate that part of the chain between roots and shoots consists of metabolically influenced sites, whose output is influenced by the rate of water transport.