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 continuously using thermocouples, Flesh temperatures on the exposed side of fruit in direct sunlight reached as much as 15 degrees C above air temperatures, even when the latter never exceeded 30 degrees C. An increase in expression of heat shock protein (hsp) mRNA and protein synthesis was associated with these high daily flesh temperatures. Elevated levels of hsp transcripts were maintained over the night period, but rapidly dropped after the onset of daytime. There appears to be a diurnal cycle of hsp gene expression which underlies the heat-induced increases. High internal temperatures, even under temperate climatic conditions, are likely to be common in bulky tissues such as fruit, acid the ability to withstand such temperatures is a normal homeostatic response.