Glycolate metabolism in young and old tobacco leaves, and effects of α-hydroxy-2-pyridinemethanesulfonic acid

Abstract

Young tobacco leaves photorespire less than older leaves. This difference is reflected by lower activities of photorespiratory enzymes in young leaves, and by an apparent inability to synthesize glycolate, the substrate of photorespiration.It is shown in this paper that young tobacco leaves differ from old ones in the following, additional respects: (1) extracts have a much lower ability to decarboxylate glycine; (2) the transient burst of CO2 after lowering of the light intensity is nearly absent; (3) photosynthesis is much less inhibited when leaves are floated on solutions of the inhibitor of glycolate oxidase α-hydroxy-2-pyridinemethanesulfonic acid (HPMS); (4) the CO2 compensation concentration is higher; and (5) the mesophyll cells are much more densely packed.It is concluded that the small photorespiratory gas exchange of young leaves is due to deficiencies in enzymes of glycolate metabolism and perhaps to a slow synthesis of glycolate. The latter may be explained by a relatively high internal CO2 concentration resulting from large physical resistances to gas exchange in the tissue. However, the rate of glycolate formation possibly is underestimated because of a low permeability of young leaf tissue to HPMS.Other results provide evidence that HPMS alters the biochemical pattern of CO2 photoassimilation in old tobacco leaves under certain conditions. Hence, this poison ought to be used with caution in studies on photorespiration and glycolate metabolism.