Effect of IAA gradient between the peduncle and branch on physiological drop of citrus fruit (Kiyomi tangor)

Summary

Changes in indole acetic acid (IAA) concentrations of the peduncle, branch and abscission zone were examined during the physiological fruit-drop period, to investigate the role of endogenous auxin in the peduncle and branch on the physiological drop of the fruit. The effects of 2,3,5-triiodebenzoic-acid (TIBA), applied to the peduncle or branch, on the IAA concentration in each organ and the drop ratio were also investigated. The fruit-drop ratio increased when TIBA was applied to the peduncle (TIBA-P application), whereas it decreased when the application was to the branch (TIBA-B application). Prior to fruit drop, marked IAA accumulation occurred in the abscission zone in the control and after TIBA-P application. An IAA gradient from the branch to the peduncle was found for TIBA-P applications, but a reverse gradient for TIBA-B applications. These results suggest that the reduction of IAA polar transport above the abscission zone is necessary to promote abscission and that both IAA accumulation in the abscission zone and the IAA gradient between the peduncle and branch are involved in abscission.