Light-Induced H+ Secretion and the Relation to Senescence of Oat Leaves

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Abstract
When abraded oat (Avena sativa L. cv Victory) leaf segments are floated on KCl solution, white light causes acidification of the solution external to leaf tissue. The presence of mannitol amplifies the light-induced proton secretion. Mature leaves as well as young ones acidify the medium in light, while senescing leaves (after 3 to 4 days incubated in water in the dark) lose the ability to produce this response to light. The decrease in H+ secretion is already measureable after as little as 30 minutes in darkness, while the increase in proteolysis rate was detected only after 6 hours in dark. The decrease in capacity to secrete protons is one of the symptoms of leaf senescence. Moreover, fusicoccin mimics light in stimulating H+ pumping and delaying the senescence in the dark. On the other hand vanadate, an apparent inhibitor of plasma membrane H+ ATPase, blocks the acidification and promotes the chlorophyll and protein degradation in leaf segments during the 2-day period of incubation. These results, which show a parallel between cessation of H+ secretion and acceleration of senescence, may suggest a regulatory role for H+ secretion in leaf senescence.