Changes of photorespiratory activity with leaf age.

We have discovered that younger leaves do not exhibit the same large increase in photosynthesis as do older ones when the oxygen tension is lowered. This phenomenon was observed in various tobacco and citrus species and indicated less photo-respiration in young leaves. Activities of glycolate oxidase and of glyoxylate reductase were found to be significantly lower in younger leaves than in more mature ones from the same plant. Assays of glycolate content revealed the same trend. Levels of phosphoenolpyruvate carboxylase were only slightly higher in the younger leaves. This finding lessens the possibility of differing enzymatic pathways for CO(2) fixation. It is concluded that the small photorespiratory activity of young leaves can be attributed to a lower activity of the photorespiratory enzymes and perhaps also to a slightly higher rate of internal recycling of CO(2).