Effect of Potassium Nutrition on Some Enzymes of the Tomato Plant

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Abstract
The effects of monovalent cations on the activities of tomato leaf succinyl CoA synthetase (SCS) and malate dehydrogenase (MDH) were examined. SCS was purified 35-fold and a 2-fold stimulation in activity was observed in the presence of KCl, NaCl having little effect and LiCl producing an inhibition. MDH was inhibited non-selectively by monovalent cations, 250 mM salt inhibiting the reaction by about 50 per cent. The concentrations of pyruvate kinase (PK), SCS and MDH were determined in leaves of tomato plants grown for 8 weeks in sand culture and supplied with various levels of potassium in the nutrient solution. There were only small changes in the concentrations of SCS in the expanding leaves with variation in K nutrition, while the concentrations of MDH and PK were significantly affected. Maximum concentrations of MDH occurred in leaves of plants receiving 0.53 mequiv. K+ l−1. The PK concentration began to increase in the shoot apex, in the expanding and fully-expanded leaves when the nutrient K was reduced below 2.03, 0.53 and 0.28 mequiv. K+; 1–1, respectively. The critical K content below which the PK concentration increased in all three tissues was about 1 g K per 100 g dry matter. It is suggested that the increase in concentration of PK in K-deficient tissue could be a mechanism to compensate for the reduced enzyme activity in situ, due to the presence of the low concentration of the principal activator, K+. A simplified calculation shows that a tissue K content of 1 g K per 100 g dry matter is equivalent to a mean concentration, on a fresh weight basis, of about 45 mM K+. The mean K concentration in laminae of vigorously growing tomato plants appears to correspond closely with that needed for maximum stimulation of SCS and PK.