Changes in the concentrations of malic acid, citric acid, calcium and potassium in the leaves during the growth of normal and iron-deficient mustard plants (Sinapis alba)

Abstract

Mustard plants (S. alba) were grown in soil and on various nutrient solutions. Measurements were made of the major leaf anions, malate and citrate (and occasionally also of ascorbate, nitrate and phosphate), and of certain cations (Na, K, Ca and Mg.) While green the leaves accumulated malic acid and Ca in equivalent amounts under most of the conditions studied. The content of citric acid, and of K, Na, and Mg changed relatively little. The content of malic acid was lower than that of Ca in Fe-deficiency and in conditions of poor illumination. It was depressed by a high K/Ca ratio in the nutrient solution, but not to the same extent as the Ca content. The content of citric acid was increased by Fe.-deficiency, but not much affected by other variations in conditions. Usually the sum of the content of malic acid and citric acid balanced the sum of the principal cations, Ca and K, but when the malic acid content was depressed a discrepancy existed that could be only partly explained by accumulation of nitrate. When the leaves turned yellow the content of malic acid fell and that of citric acid rose correspondingly until it became the major component. There were no consistent changes in cation content. The results are discussed with special reference to the significance of the K/Ca and citric acid/malic acid ratios in normal and deficient leaves.