EFFECT OF AUTUMN FOLIAR SENESCENCE ON CHLOROPHYLL A-B RATIO AND RESPIRATORY ENZYMES OF POPULUS-TREMULOIDES

A standard leaf color classification system was used to describe five progressive stages of autumn senescence for leaves of the deciduous quaking aspen Populus tremuloides Michx. For five years, during the period 1981 to 1989, leaves of each senescence stage, from full green to full yellow, were assayed quantitatively for total chlorophyll, for chlorophylls a and b as well as for the specific activities of malate dehydrogenase, succinate dehydrogenase and fumarate hydratase. As senescence progressed the total chlorophyll decreased by 99%, from a mean of 24.2 mg/g leaf to 0.226 mg/g leaf, whereas the chlorophyll a:b ratio increased during each progressive stage, a condition not reported previously for deciduous species. Senescence was accompanied by large decreases in specific activities of malate dehydrogenase and fumarate hydratase, and by a large increase in K(m) of malate dehydrogenase with oxaloacetate as substrate. Succinate dehydrogenase was of low specific activity and showed only a slight decrease during senescence.