The uptake of na and its redistribution between leaves during the development and alleviation of K deficiency

Abstract

AbstractChanges in the concentration of Na in the shoot dry matter and in the petiole sap of individual leaves of lettuce were measured in order to determine the extent to which Na was able to replace K within the plants during K deficiency. The results were used to estimate changes in the distribution of the total ionic concentration between leaves and to make deductions about likely variations in osmotic potential within the shoots and their effect on plant growth. Measurements of Na concentration revealed both a pronounced delay in the uptake of additional amounts of Na and in its translocation within the K‐deficient plants. Sodium concentrations increased most rapidly in the young expanding leaves during the onset of K deficiency but the changes in its concentration were ultimately larger in the middle to older ones. Elevated Na concentrations in the younger leaves of K‐deficient plants also declined more rapidly than those in any of the other leaves following reintroduction of the K supply. Although the pattern of these changes broadly mirrored the corresponding changes in K concentration within the shoots there were significant reductions in the combined K+Na concentrations during the onset and development of K deficiency. The deficit in K+Na was initially greatest in the young leaves but subsequently disappeared more quickly when either the uptake of Na was increased or the K supply was resumed. Since the combined concentrations of K+Na were directly related to the changes in total ionic concentrations within the shoots, the results suggest that during K deficiency, plants adopt a strategy which reduces the effects of a decline in ionic strength at the most actively growing sites in order to minimise the damage to normal growth processes from changes in the internal osmotic pressure.