Photosynthetic & respiratory activities of growing pea leaves

Abstract

It is well known that leaves of different ages show differences in their rates of photosynthesis and respiration and indeed a number of detailed studies have been conducted to follow the changes in photosynthetic and respiratory rates during leaf ontogeny. The cellular mechanisms controlling these changes are not understood, although it is known that they can be influenced by altering either the internal or external environment. Decreased photosynthetic or respiratory rates in leaves of certain ages may result from the inability of the leaf cells to fully utilize their potential enzymic capacity. Alternatively, such decreases could be the direct result of a decrease in the enzymic capacity of the leaf. Accordingly, a study was made of the relationship between the changes in photosynthetic and respiratory rates accompanying leaf maturation and the activities of a number of photosynthetic and respiratory enzymes. The pea plant was chosen for these studies since several compositional changes occurring during the growth of pea leaves have been described already (32). The present study established a correlation between changes in the photosynthetic rate of growing pea leaves and the activities of photosynthetic enzymes. A similar correlation was noted for the respiratory rate and several respiratory enzymes. Transketolase, an enzyme which functions in both photosynthesis and respiration, showed a variation with leaf growth which was intermediate between those obtained for photosynthesis and respiration. These results indicated that the observed changes in the rates of photosynthesis and respiration reflected changes in the cellular levels or activities of leaf enzymes. Leaves contain a number of enzymes whose participation in either photosynthetic or respiratory processes has not been satisfactorily established. Since the changes in the activities of photosynthetic enzymes during leaf growth were easily distinguished from those given by respiratory enzymes, measurements of this type may be useful in determining whether enzymes are functional in either photosynthesis, respiration, or both processes. Materials & Methods