Light and hormonal control of leaf abscission

The mechanism of light control of leaf abscission was studied in coleus (Coleus blumei Benth.) and mung bean (Vigna radiata (L.) Wilczek) cuttings by evaluating the roles of IAA, carbohydrate, and cellulase in the process of abscission. Treatment of the cuttings with red light maintained high break strength in the abscission zone in both tested plants, whereas treatment with far-red light initiated a dramatic increase in abscission. Leaf diffusate collected from red light treated leaves contained 3 times more IAA than that collected from leaves maintained in the dark or treated with far-red light. Treatment of the cuttings with 2,3,5-triiodobenzoic acid (TIBA), an auxin transport inhibitor, eliminated the inhibitory effect of red light on abscission. No significant difference in IAA oxidase activity was detected among the light treatments. The rate of decarboxylation of IAA in leaf tissue treated with red light, however, was higher than in the leaves receiving dark or far-red light. Conversion of tryptophan to IAA was higher in red light-treated leaf blades than in leaves maintained in dark or treated with far-red light. Treatment of mung bean leaf cuttings with red or far-red light, produced no significant differences in the content of fructose 6-phosphate, glucose plus fructose, sucrose, total soluble sugars, or starch. A higher level of glucose 6-phosphate was detected in red light-treated leaves. Loading the tissue with sucrose, a sugar reported to affect organ abscission, did not change the effect of light on abscission. The differences in the rate of IAA synthesis observed in leaf tissue under various light treatments is proposed as the controlling mechanism for light regulation of abscission. Total sugar or starch content in the leaf blade was not related to leaf abscission, indicating that photoassimilate in leaf tissue was not involved in light regulation of abscission. While cellulase activity in the abscission zone accounts for the decrease in break strength of abscission zone, the enzyme per se was not affected by light