Alternation of respiratory pathways during the development of wheat leaf

During the development of the wheat leaf a decrease is observed in the degree of inhibition of respiration by fluoride, iodoacetate and malonate and in the ratio of the radioactivities of14CO2 liberated from glucose-6-14C and glucose-l-14C (C6/C1), this pointing to an increase in the role of the pentose cycle in total respiration. This change is not due to an absolute increase in the activity of the pentose cycle in older leaves but mostly to a decrease in the activity of the glycolytic system. On the other hand, in leaves separated from the seed the ratio between the respiratory pathways changes due to the activation of the pentose cycle when atmospheric humidity decreases. On the basis of the disproportion between the percentage of the glycolytic fraction of respiration as calculated from the decrease of the C6/C1 ratio during inhibition by fluoride and the percentage of inhibition of respiration by fluoride, the possible causes of the high C6/C1 ratios in young leaves are discussed. The values of these ratios occasionally exceed unity.