EXPRESSION OF HEAT-SHOCK-PROTEIN GENES IN APPLE FRUIT IN THE FIELD

Temperatures in the flesh of apple fruit (Malus domestica Borkh., cv. Braeburn), during diurnal cycles in summer conditions, were measured c ontinuously using thermocouples, Flesh temperatures on the exposed sid e of fruit in direct sunlight reached as much as 15 degrees C above ai r temperatures, even when the latter never exceeded 30 degrees C. An i ncrease in expression of heat shock protein (hsp) mRNA and protein syn thesis was associated with these high daily flesh temperatures. Elevat ed levels of hsp transcripts were maintained over the night period, bu t rapidly dropped after the onset of daytime. There appears to be a di urnal cycle of hsp gene expression which underlies the heat-induced in creases. High internal temperatures, even under temperate climatic con ditions, are likely to be common in bulky tissues such as fruit, acid the ability to withstand such temperatures is a normal homeostatic res ponse.