SYNTHESIS OF RADIOACTIVE SUCROSE BY TOBACCO LEAVES FROM C14 UNIFORMLY LABELLED GLUCOSE AND GLUCOSE-1-PHOSPHATE

Detached tobacco leaves were placed for six hours on solutions of C14 labelled glucose and glucose-1-phosphate. At the end of this time leaves were extracted with boiling 80% ethanol and the activities of their various carbohydrate fractions were determined. The specific activities of both glucose and fructose moieties of sucrose were found to be high and very close one to another, while that of the free fructose was low. From this it has been concluded that the fructose component of sucrose is derived from the glucose introduced and not from the free fructose present in leaves. Low activity in starch observed in photosynthesizing leaves indicates preferential transformation of the glucose introduced into sucrose and not into starch. Moreover, respiration of labelled glucose ought to produce labelled carbon dioxide, and after a period of photosynthesis, labelled starch. The isolated starch contained only a small amount of activity indicating that, in light, sugars were not utilized in respiration to any great extent. Moreover, they did in darkness, as the carbon dioxide evolved was very radioactive.