PREHARVEST FIELD HEAT AND POSTHARVEST FRUIT RESPONSE

Abstract

Postharvest heat treatments of fruit are being used for disinfestation of quarantine pests, and for reducing postharvest chilling injury in cold-sensitive fruits. Preharvest high temperatures of the fruit may influence tolerance to high and low postharvest temperatures. To assess the effect of field temperature history of fruit, we monitored temperatures in the flesh of apple and avocado fruit in the field over diurnal cycles, and followed the postharvest response of these fruit. In both fruits, flesh temperatures exceeded 35°C, and sometimes reached 40°C and above, for 2 h or more in direct sunlight, even though air temperatures did not exceed 30°C. With night temperatures dropping to 10°C or lower, the diurnal range sometimes exceeded 25°C. A ubiquitous response to high temperatures in all organisms is the induction of heat-shock proteins (hsp) genes and associated protein synthesis. Hsp play a role in thermotolerance. In both fruits, mRNA for hsp were upregulated at temperatures above about 33°C, and the transcripts remained at an elevated level over the cooler night period. With warming up of fruit, hsp's mRNA levels rapidly declined until temperatures reached inducible levels again. This induction of hsp in fruit in the field indicates that fruit, even under temperate conditions, respond daily to temperatures commonly regarded as being in the stress range. Postharvest responses associated with prior high temperature or exposure history of the fruit included exposed sides of apple fruit having higher flesh firmness, and previously exposed avocado fruit having greater resistance to low temperature injury, slower ripening rates, and higher thermotolerance. Both long-term exposure of fruit and immediate temperature history close to harvest, are likely to influence fruit responses to high temperature postharvest treatments and low temperature storage.