The Presence of Heat-shock Proteins in Sun-exposed and Heat-treated Apple Fruit

Abstract

Sunburn of apples is frequently a significant cause of cullage in Washington State. Some cultivars are more susceptible to sunburn than others. Further, apples that have acclimated to full sunlight exposure in the field (“sun”) have been reported to be less likely to sunburn when exposed to full sunlight than apples growing within the canopy (“shade”). We hypothesize that heat-shock proteins (HSPs) might be involved in these differences in sunburn susceptibility. Western blots of total protein extracts from apple peel identified the presence of protein recognized by antibodies raised against small HSPs (smHSPs) from different plant species. SmHSPs in apple were very low or non-detectable in “shade” fruit, but were relatively high in “sun” or heat-treated (4 h at 40 °C) “shade” fruit. SmHSP accumulation in `Fuji' apples could be detected after the first hour of a 4-h heat treatment at 40 °C, and continued to increase over the next 8 to 10 h. The smHSPs did not begin to decline even after the apples had been held for 48 h at room temperature. Initial studies indicate that cultivars differ in smHSP content. Results from biweekly samplings of several cultivars will be reported.