Synergistic interaction between coumarin 1,2-benzopyrone and indole-3-butyric acid in stimulating adventitious root formation in Vigna radiata (L.) Wilczek cuttings: I. Endogenous free and conjugated IAA and basic isoperoxidases

Cuttings from 7-day-old Vigna radiata seedlings were treated for 24 h with various concentrations of coumarin and/or indole-3-butyric acid (IBA), applied either alone or in combination, in order to stimulate adventitious root formation (ARF). The effects of treatment on endogenous free and conjugated indole-3-acetic acid (IAA), basic peroxidase (basic PER) activity and its isoperoxidases analysis and their relation to ARF were then investigated at the potential rooting sites during the first 96 h after application. Simultaneously, combined treatments acted synergistically in inducing more adventitious roots in treated cuttings than in those treated with coumarin or IBA individually, as compared with the control. Endogenous free IAA increased transiently in treated cuttings as compared with the control and the maximum increase occurred with the combined treatment. This suggests that coumarin and IBA may act synergistically in increasing the endogenous free IAA level during the induction phase of rooting to initiate more roots. Likewise, higher level of conjugated IAA was also found in treated cuttings than in untreated ones, during the primary events of ARF, with the maximum level occurring in the combined treatment. Comparison of the dynamics of conjugated IAA and activity of basic PERs led to conclusion that the former but not the latter is responsible for downregulation of endogenous IAA levels significantly during the primary events of ARF. A sharp increases in basic PERs occurred during the secondary events of ARF, suggesting their role in root initiation and development rather than root induction.