Chilling-induced leaf abscission of Ixora coccinea plants. III. Enhancement by high light via increased oxidative processes

The role of increased oxidation induced by successive stresses of chilling and high light in the induction of leaf abscission was studied in Ixora coc cinea plants in relation to auxin metabolism and oxidative processes. Expos ure of plants following dark chilling (7 degreesC for 3 days) to high light (500-700 mu mol m(-2) s(-1) photosynthetically active radiation) for 5 h a t 20-25 degreesC enhanced chilling-induced leaf abscission. This abscission was inhibited by pretreatment with the antioxidant butylated hydroxyanisol e, ae-naphthaleneacetic acid or the ethylene action inhibitor, 1-methylcycl opropene. The oxidative processes initiated during the low light period fol lowing the dark chilling period, such as indoleacetic acid (IAA) decarboxyl ation and lipid peroxidation, were further enhanced by subsequent exposure to high light. Photoinhibition, expressed by the reduction of the chlorophy ll fluorescence parameter Fv/Fm, was evident following exposure to high lig ht, irrespective of the temperature of the pretreatment, but this reduction persisted only in chilled plants. This suggests that oxidative processes g enerated during and after the chilling period might have inhibited the reco very from photoinhibition. The chilling stress under darkness induced a 60% , reduction in superoxide dismutase (SOD) activity and significant increase s (130-600%) in the activities of several other antioxidative enzymes. Thes e data suggest that the chilling-induced reduction in SOD activity may well be responsible for the increase in the oxidative stress induced by the sub sequent light treatment, as expressed by the increased enzymatic activities . Taken together, this study provides further support for the involvement o f oxidative processes in the events occurring in tissues exposed to sequent ial chilling and light stresses, leading to reduction in free IAA content i n the abscission zone and to leaf abscission.