Potassium-transport rate from root to shoot unrelated to potassium-use efficiency in tomato grown under low-potassium stress

Four tomato (Lycopersicon esculentum Mill.) strains (203, 474, 546, and 576) that are equally efficient in potassium (K) absorption, but different in efficiency of K use were studied using a continuously flowing solution culture to determine the importance of K-transport rate from root to shoot in relation to K-use efficiency. Strains 203 and 546 are inefficient in K use, ratios of K in shoots to roots were found to be three times higher than those of the two K-use efficient strains 474 and 576 when grown under low-K stress (0.1 mM). In addition, both the K concentrations and rates of K flow in xylem exudates of decapitated strains 203 and 546 were significantly higher than those of strains 474 and 576. These results suggest that high rates of K transport from roots to shoots are unrelated to K-use efficiency in the tomato strains grown under low-K stress. Further studies of K distributions In leaves and stems found that K concentrations in matured leaves and stems of the two efficient strains 474 and 576 were markedly lower than those of the two inefficient strains 203 and 546, while K in young and expanding leaves of the efficient strains were significantly higher than those of the inefficient strains, indicating that the ability to mobilize K from matured leaves to young and expanding leaves is an important factor that contributes to K-use efficiency in tomato plants grown under low-K stress.