Genetic differences in leaf abscission and the activity of naturally occurring growth regulators in peach

Leaf abscission in peach is correlated with the release of tryptophan into the cytoplasm following a sudden drop in air temperature below the physiological minimun. The tryptophan accrmulates in the variety Elberta and results in increased activity levels of 5-hydrosytryptamine, 5-hydroxyindoleacetic acid, and an nidentihende utral growth promoter. The rapid defoliation of the canker resistant variety Elberta was, however, associated more closely with the endogenous levels of tryptophan and 5-hydrosytryptanine. Susceptible Dixired variety, which defoliates slowly, contains lower activity levels of these compounds at the time of leaf abscission, and shows enhanced activity of kynurenic and xanthurenic acids throughout this same period. This suggests that tryptophan does not accumulate in Dixired to cause rapid abscission because of a greater genetic capacity for degradation by way of the Icyureninhey, droxyanthranilic, nicotinic acid pathway comlnon in the metabolism of animals.