Abstnrt. The activity of pyruvate kinase was determined in the first leaves of wheat plan,ts grown under K'-deficient conditions. An enhancement of the enzyme activity compared with the normal plants was found to start from eighth day of growth, and about 4-fold increase in the enzyme activity was observed in 14-day wheat leaves. The addition of K.,SO to the nutrient solution given to the K+-deficient plants at tenth day resulted in the restoration of the enzyme activity to the normal level after 3 days. The levels of K4 as well as oarbohydrates and chlorophyll were found to return normal over the same period. These findings are discussed in relation to the metabolic pattern of plants at the early stages of K'-deficiency. germination, the nultrient so-lution was a1pplied daily.Preparationt of Enzymte Solutions. Five grams of first leaves were harvested at eatch growth stage and were macerated with 15 mil of 0.05 M tris-HCI bluffer '(pH 7.5) in a prechilled mortar. The restIting homogenate wa's spun at 10,000 X g for 10 mintu-tes after passing through 'several 4ayers of cheesecloth. Three ml aliquotis of the supernatant flutid were then applied to a column of Sephadex G-25 (1.8 X 20 cm), whic'h was equilibrated with 0.05 \t tris-HGl butffer (ipH 7.5). An aliquot of the ellua'te collected quanltitatively was used as an enzyme 'soturce. Alil the manipulations were carried outt at or near 00, and the enzyme assay' wa's conducted within 30 minutes aiflter collecting the enzyme solution.Antalytical Method. The standard reaction mixtutre for the measurement of pyrtuvate kinase activity c'ontained the following (in pim'oles): tris-HCI buffer (pH 7.4), 50; phosphoenolpyruvate (PEP) (tricyclohexylammonium salt), 1.5; ADP (Na-,salt), 2.5; MgSO4, 10; KCI, 50; and enzyme solution, 0.5 ml (equivalent to about 1.0 mg protein) in a total volume of 1.0 ml. Whenever necessary 1.25 j,moles tris-molybdate was added to the reaction mixture to inhibit phosphatase activity in the crude enzyme preparation (5). In the conitrol assays, ADP was omitte(l fromii the reaction mixture. The assay mixture was inctubated at 370 for 10 minutes.