Senescence-Associated Changes during Long-day-induced Leaf Senescence and the Nature of the Graft-transmissible Senescence Substance inKleinia articulata

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Schwabe, W. W. and Kulkarni, V. J. 1987. Senescence-associated changes during long-day-induced leaf senescence and the nature of the graft-transmissible senescence substance in Kleinia articulata. — J. exp. Bot. 38: 1741–1755. The long-day-induced senescence in Kleinia articulata leaves was characterized by a loss in fresh and dry weight, in the rate of leaf expansion and progressive loss of chlorophyll in the detached rooted leaves. Ultrastructural examination of mesophyll cells of leaves from plants grown in continuous light showed that osmiophilic globules accumulating in the chloroplasts were the first visible sign of senescence in the organdies. These first signs of senescence could be detected in very young leaves of plants in continuous light, even before the leaves had expanded. Attempts were made to study the cause of this photoperiodic senescence which, from previous work, appeared to involve a graft-transmissible substance. Leaves in continuous light showed reduced stomatal opening and extracts from them had very much higher activity in the Commelina stomatal closure assay (ABA-like activity ?) compared with non-senescing leaves grown in short days (8 h). However, even if all the activity were due to ABA, this on its own does not appear to be the senescence substance because a much longer exposure to continuous light was required to induce irreversible senescence than to reach maximum stomatal closure promoting activity in the bioassay. Moreover, severe water stress (high ABA?) did not lead to senescence unless combined with continuous light or ethylene treatment. It is postulated that while ABA may play an important role in Kleinia leaf senescence its lethal effect may not be realized unless ethylene-induced membrane changes may synergistically assist.

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