Changes in the Distribution of Pectin Methylesterase across Leaf Abscission Zones ofPhaseolus vulgaris

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Abstract
Determinations of pectin methylesterase in abscission zones and surrounding parts of Phaseolus leaves have shown a relatively high activity in young tissues, which decreases as the leaf grows older. The greatest fall occurs in the pulvinus. In non-senescent leaves, the enzyme activity is higher in pulvini than elsewhere and there is a gradient of activity across the line of abscission from the pulvinus to the petiole or stem. During senescence this gradient falls, and is generally reversed at abscission. Data suggest that the advent of abscission may be linked to the steepness of this gradient. If abscission of leaves or isolated abscission zones is accelerated by appropriate treatments there is a more rapid decrease in pectin methylesterase activity than in the controls, and a more rapid fall in the gradient across the line of abscission. If abscission is retarded the enzyme activity is sustained and the gradient is maintained or increased. The results are discussed in relation to the changes that may occur in the pectic constituents of cell walls during senescence and abscission, and in relation to some of the known effects of auxins on pectin metabolism.