Chilling-induced leaf abscission of Ixora coccinea plants. II. Alteration of auxin economy by oxidative stress

Chilling-induced leaf abscission of ixora (Ixora coccinea) plants was almos t completely inhibited by alpha-naphthaleneacetic acid (NAA), even in the p resence of exogenous ethylene, which enhanced the chilling effect on leaf a bscission. Chilling reduced free indoleacetic acid (IAA) content, quantifie d immediately after chilling, in the abscission zone (AZ) and leaf blade, F ree IAA content in chilling-treated plants continued to decrease gradually with time after chilling, Application of the antioxidant butylated hydroxya nisole (BHA) before or after chilling not only prevented the post-chilling decline in free IAA content, but also restored free IAA level during 6-48 h of the post-chilling period almost to the control level, No significant ef fect of chilling on the endogenous content of ester- and amide-conjugates o f IAA or the metabolism of exogenous labeled IAA were observed. Chilling en hanced the decarboxylation of IAA, particularly in the AZ tissue, Auxin tra nsport capacity was significantly inhibited by chilling, and this effect wa s counteracted by BHA applied before chilling, The data indicate that chill ing reduces free IAA content in the AZ, an effect that may lead to increase d sensitivity to ethylene, The chilling-induced reduction in IAA content in the AZ seems to result, at least in part, from increased IAA decarboxylati on and reduced auxin transport capacity. These processes seem to be trigger ed by the oxidative stress imposed on the tissues by chilling.