Possible Role of Pectic Enzymes in Abscission

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Abstract
Factors known to affect abscission such as additions of indoleacetic acid (IAA) and methionine were studied as to their effect upon the activity of various pectic enzymes. Pectin-methylesterase (PME) activity was found to be high in the abscission region of tobacco pedicels. By the addition of 1000 ppm IAA this activity was increased from 12.2 to 16.7 units which was significant at the 0.1% probability level. Additions of 5 ppm IAA and methionine decreased PME activity. Addition of 1000 ppm IAA overcame the inhibition of activity caused by methionine. Additions of several pectic enzyme preparations to tissue sections taken from the pedicel and abscission zone of tobacco flowers identified 2 which could cause the dissolution of the middle lamella. The activities of these 2 preparations (pectinase and enzyme B) were affected also by additions of IAA and methionine. However, concentrations of 100 ppm and 1000 ppm IAA decreased the separation activity of these enzymes. A concentration of 5 ppm IAA and additions of methionine increased separation activity as they do when applied to the intact plant. Two possible roles for pectic enzymes in abscission are proposed. First, conditions which prevent PME activity may cause abscission by allowing an accumulation of water soluble pectin which would be ineffective as a cementing layer. Or secondly, other enzymes may be involved either in the production of pectin or in other chemical changes which ultimately cause the dissolution of the middle lamella as observed when pectinase and enzyme B were added to tissue sections.