Comparison of thermotolerance of sun-exposed peel and shaded peel of ‘Fuji’ apple

The thermotolerance of the sun-exposed peel and the shaded peel of ‘Fuji’ apple (Malus domestica Borkh.) fruit was evaluated by measuring pigments, chlorophyll a fluorescence transients and O2 evolution or uptake after exposure to 25, 35, 40, 42, 44, 46 or 48°C for 30min in the dark. A major effect of heat stress at 46–48°C on the chlorophyll a fluorescence transients was the appearance of a very clear K step at 200–300μs for both peel types. The K step was slightly more pronounced in the sun-exposed peel than in the shaded peel, suggesting that the resistance of oxygen-evolving complex to heat stress is slightly lower in the sun-exposed peel than in the shaded peel. Minimal fluorescence (F O), relative to the value at 25°C, increased to a greater extent in the shaded peel than in the sun-exposed peel after exposure to 46–48°C, but the temperature dependencies of F O changes were similar for both peel types. Maximum quantum yield of PSII (F V/F M) decreased to a similar extent in the sun-exposed peel and the shaded peel as temperature rose from 25 to 44°C, but the sun-exposed peel reached slightly lower values at 46–48°C. Correspondingly, gross O2 evolution rate, relative to that at 25°C, was also slightly lower in the sun-exposed peel than in the shaded peel at 46–48°C. In response to heat stress, the ratio of QA-reducing reaction centers (RCs) to total RCs and the ratio of QB-reducing RCs to QA-reducing RCs decreased, but both of them decreased to lower values in the sun-exposed peel than in the shaded peel at 46–48°C, indicating that the capacity of electron transfer between P680 + and QB via QA was damaged to a greater extent in the sun-exposed peel than in the shaded peel. At each given temperature, dark respiration was... [Copyright &y& Elsevier]