Phenylalanine ammonia-lyase and ethylene in relation to chilling injury as affected by fruit age in citrus

Abstract

Abstract Fruit of many citrus cultivars become injured when exposed to low, non-freezing temperatures. In this study we have determined changes in ethylene production and phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) in fruit of three citrus cultivars, ‘Fortune’ mandarins, and ‘Navelina’ and ‘Valencia’ late oranges, with different tolerance to chilling injury (CI) and demonstrated the influence of fruit physiological stage on those stress responses. We have shown that the increase in ethylene production and PAL are cold-induced responses which are only stimulated in fruit of citrus cultivars showing chilling damage and that both responses may occur concomitantly with the development of chilling symptoms. However, the magnitude of these responses was not indicative of the degree of tolerance of a specific cultivar to chilling. The influence of fruit age on both responses was evaluated in the most (‘Navelina’) and the least (‘Fortune’) chilling tolerant cultivars. Chilling damage was not developed in ‘Navelina’ fruit at any physiological stage, but our results in ‘Fortune’ mandarins, which always developed chilling symptoms, indicated that the induction of PAL in response to chilling was dependent on the fruit physiological stage. Interestingly, increases in both PAL mRNA and activity were barely affected by cold stress in the youngest ‘Fortune’ fruit harvested in December in spite of its noticeable CI. For a similar CI index, the older the fruit, the higher was the shift in the levels of PAL transcript and in PAL activity in response to cold. In contrast, the cold-induced ethylene production was little affected by the physiological stage of the fruit.