A relationship between potassium and proline accumulation in salt‐stressed Sorghum bicolor

Abstract

The amount of total monovalent cations in leaves of Sorghum bicolor, L. Moench, RS 610, which were exposed to salinity stress, was a function of both the osmotic potential and the concentration of K+ of growth media. The plants have a Na+ exclusion mechanism that keeps the level of Na+ in leaves low. Thus, most of the osmotic adjustment in leaves was due to K+. Proline did not start to accumulate in leaves until the concentration of total monovalent cations in leaves reached a threshold of approximately 200 μmol/g fresh weight. Above this threshold, the contents of prolioe and monovalent cations in leaves increased with increasing salinity of the medium. The ratio of proline to monovalent cation was 5% of that amount of monovalent cation in excess of the threshold concentration. Therefore, if the cations are located in the vacuoles and proline accumulates in the cytoplasm, then the amount of accumulated proline is sufficient to act as a balancing osmoticum across the tonoplast. Very little proline accumulated in roots because this tissue contained much less total monovalent cations than leaves from the same salt‐stressed plants. The same threshold of 200 μmol/g fresh weight of total monovalent cations was required in roots as in leaves to initiate proline accumulation.