Accumulation of PAL transcript and PAL activity as affected by heat-conditioning and low-temperature storage and its relation to chilling sensitivity in mandarin fruits

The effects of different periods of heating at 37 degrees C on phenylalanine ammonia-lyase (PAL) and how this relates to chilling tolerance was investigated in fruits of the chilling-sensitive Fortune mandarin. All effective heat-conditioning treatments caused an early and transient increase in PAL mRNA and PAL activity. Conditioning fruits at 37 degrees C for 1 or 2 days prevented the manifestation of chilling symptoms but not the accumulation of PAL mRNA and PAL activity observed in untreated fruits. In fruits conditioned for 3 days, cold-induced damage and PAL activity were also suppressed but not the accumulation of PAL transcript upon subsequent storage at 2 degrees C. Storage of 3-day-heated fruits at a nonchilling temperature (12 degrees C) induced an early and transient increase in both PAL mRNA and PAL activity. High levels of PAL transcript and PAL activity were detected in freshly harvested fruits of a chilling-resistant mandarin (Hernandina) that decreased upon cold storage at 2 degrees C in heat-treated and nontreated fruits. These results indicate that sensitivity of mandarins to chilling correlates with low constitutive levels of PAL mRNA and PAL activity and with the inducibility of both upon exposure to low temperatures.