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

The effects of different periods of heating at 37 degrees C on phenylalanin e ammonia-lyase (PAL) and how this relates to chilling tolerance was invest igated 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 o f PAL mRNA and PAL activity observed in untreated fruits. In fruits conditi oned for 3 days, cold-induced damage and PAL activity were also suppressed but not the accumulation of PAL transcript upon subsequent storage at 2 deg rees C. Storage of 3-day-heated fruits at a nonchilling temperature (12 deg rees C) induced an early and transient increase in both PAL mRNA and PAL ac tivity. High levels of PAL transcript and PAL activity were detected in fre shly harvested fruits of a chilling-resistant mandarin (Hernandina) that de creased upon cold storage at 2 degrees C in heat-treated and nontreated fru its. These results indicate that sensitivity of mandarins to chilling corre lates with low constitutive levels of PAL mRNA and PAL activity and with th e inducibility of both upon exposure to low temperatures.