ABSCISIC-ACID IN THE RESPONSE OF FORTUNE MANDARINS TO CHILLING - EFFECT OF MATURITY AND HIGH-TEMPERATURE CONDITIONING

The possible role of abscisic acid (ABA) and peel colour in chilling t olerance of 'Fortune' mandarins has been examined. Fruit stored at 2.5 degrees C, but not at 12 degrees C, showed pitting damage. The degree of damage changed considerably during development and maturity from N ovember until May. Green fruit were more tolerant to chilling injury ( CI). After colour break, chilling susceptibility increased from the be ginning of December until January and February (the coolest season dur ing the growing period) and declined towards May. This trend did not p arallel changes in fruit colour index or free and bound ABA levels, wh ich increased from November until May. At the time of chloroplast tran sformation to chromoplast a noticeable shift in free but not in bound ABA occurred. During storage, free ABA levels increased in fruits stor ed at 12 degrees C, but not at 2.5 degrees C. Changes in ABA with stor age temperature depended on the stage of maturity of the fruits, but n ot on their susceptibility to CI. It has also been shown that there wa s no relationship between ABA content following exposure to 37 degrees C and chilling tolerance. At all stages of maturity, the susceptibili ty of the fruits to CI and the free ABA levels of the flavedo decrease d after conditioning the fruits for 3 days at 37 degrees C. After stor age at 2.5 degrees C, the ABA levels of the conditioned fruit were sim ilar or even lower than those of non-conditioned fruit. Treatment of f ruit for 3 days with 10 mu l litre(-1) ethylene increased free ABA abo ut four times, whereas bound ABA remained nearly constant. This treatm ent enhanced cold-induced peel damage after prolonged storage. Thus, t he ethylene-induced changes in free ABA appear not to be related to th e induced changes in chilling susceptibility. The results obtained ind icated, therefore, that free and bound ABA appear not to be involved o n the adaptative mechanism developed in 'Fortune' mandarins to cope wi th chilling stress.