Allelopathic potential of citrus fruit peel and abscisic acid-glucose ester

Aqueous methanol extracts of Citrus junos, C. unshiu and C. hassaku fruit peel inhibited the growth of the roots and hypocotyls of lettuce (Lactuca sativa L.) seedlings. Significant reductions in the root and hypocotyl growth were observed as the extract concentration increased in all bioassays. The inhibitory activity of C. junos extract on the growth of lettuce roots and hypocotyls was about 13- and 24-fold greater than that of C. unshiu and C. hassaku extracts, respectively. The concentration of abscisic acid-β-D-glucopyranosyl ester (ABA-GE) in fruit peel of C. junos, C. unshiu and C. hassaku was determined since ABA-GE had been found to be a possible cause of the growth inhibitory effect of C. junos. The concentration was 132, 10.6 and 5.0 µg g−1 dry weight in C. junos, C. unshiu and C. hassaku fruit peel, respectively. Thus, there was a good correlation between ABA-GE concentrations in C. junos, C. unshiu and C. hassaku fruit peel and the inhibitory activities of their aqueous extracts, which suggests that ABA-GE may be involved in the growth inhibitory effect of C. junos, C. unshiu and C. hassaku fruit peel.