Decrease of Glucose 6-Phosphate and 6-Phosphogluconate Dehydrogenase Activities in the Xylem of Populus gelrica on Budding

The activities of glucose 6-phosphate and 6-phosphogluconate dehydrogenases, transketolase, phosphoglucose isomerase, and fructose 6-phosphate kinase were studied in extracts of wintering poplar (Populus gelrica) xylem. The xylem of wintering poplar showed high levels of transketolase, glucose 6-phosphate, and 6-phosphogluconate dehydrogenases. On recommencement of growth, the two dehydrogenase activities decreased. The three remaining enzymes appeared to be unchanged. In spring and early summer, glucose 6-phosphate dehydrogenase of the xylem was extremely low. On the other hand, 6-phosphogluconate dehydrogenase, which also became lower during the metabolic shift from winter to spring, was readily detected, and was several times higher than glucose 6-phosphate dehydrogenase throughout the year. The low dehydrogenase activities lasted into late October and then appeared to resume their original activity. A shift of metabolism at the beginning of growth was also observed by measuring the amount of sugar phosphates, soluble amino acids and amides, and proteins in the xylem. In contrast to the decrease of the two dehydrogenases and soluble proteins at the time of budding, incorporation of lysine-U-¹⁴C into the xylem protein ramained constant. A method to transfuse radioactive compounds into a section of stem was described.