Role of Potassium in Carbon Dioxide Assimilation in Medicago sativa L.

Alfalfa was grown hydroponically in 0, 0.6, and 4.8 millimolar K in order to determine the influence of tissue level of K on photosynthesis, dark respiration, photorespiration, stomatal and mesophyll resistance to CO(2), photosystem I and II activity, and synthesis and activity of ribulose 1,5-bisphosphate carboxylase (RuBPc).A severe (0.0 millimolar) and mild (0.6 millimolar) K deficiency, compared to plants grown at 4.8 millimolar K, produced a significant decrease in photosynthesis and photorespiration, but an increase in dark respiration. Both deficient K levels increased hydrophyllic resistance to CO(2), but only the severe deficiency increased stomatal resistance.Photosystem I and II activity of isolated chloroplasts was not affected by K deficiency. The apparent activity of a crude RuBPc preparation was significantly reduced in severely deficient plants. Activity of the enzyme could not be restored to normal rates by the addition of K to the reaction medium.The specific activity of RuBPc isolated from severely K-deficient and K-sufficient leaflets was not significantly different, suggesting that K does not function in RuBPc activity. Incorporation of [(14)C]leucine into RuBPc, as a measure of synthesis, by K-deficient leaflets was reduced to 15% of K-sufficient leaflets. The addition of K to the reaction medium stimulated [(14)C]leucine incorporation into RuBPc and 10 millimolar KNO(3) increased incorporation to 80% of K-sufficient leaflets. Actinomycin D and cycloheximide suppressed the K-stimulated incorporation of [(14)C]leucine into RuBPc, suggesting that the K-stimulated synthesis of RuBPc most likely represents de novo synthesis.