J. Riov

Exposing intact ixora (Ixora coccinea) plants or petiole expiants to chilling (3 days/3, 7 or 9°C) resulted in 20-80% abscission of mature, non-senescent leaves, manifested only 2 days after transfer to 20°C. The degree of leaf abscission increased with reduction of the chilling temperature. Chilling exposure induced also a significant increase in ethylene production rates in petiole expiants during the initial 4 h after transfer to 20°C. A similar pattern of increased ethylene production was obtained in petiole expiants treated with β-naphthaleneacetic acid (NAA) or with the antioxidant butylated hydroxyanisole (BHA), although these compounds significantly reduced the chilling-induced leaf abscission. On the other hand, application of the ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), which reduced ethylene production of petiole expiants by 60%, inhibited leaf abscission by 70%. However, treating intact plants with the ethylene action inhibitor, 1-methylcyclopropene (1-MCP) prior to chilling, completely prevented their chilling-induced leaf abscission. These results suggest that endogenous ethylene is essential for the chilling-induced leaf abscission. Exposure of intact plants to exogenous ethylene (3–10 u.1/1) for 1–3 days or treating petiole expiants with 1-aminocyclopropane-1-carboxylic acid (ACC), significantly enhanced their leaf abscission only when they had been pre-exposed to chilling. These results indicate that abscission induced by chilling is closely correlated with increased sensitivity of the abscission zone (AZ) to ethylene rather than with chilling-induced ethylene production. NAA and BHA inhibited both the chilling-induced and the ACC-enhanced leaf abscission of petiole expiants. This indicates the possible involvement of oxidative processes, probably of IAA, in the chilling-induced leaf abscission that is mediated by sensitivity to ethylene. It is therefore proposed, that chilling initially induces oxidative processes in the AZ, which then reduces IAA levels resulting in increased sensitivity of the AZ to ethylene, which finally causes leaf abscission.