Response of apple (Malus domestica Borkh.) fruit peel photosystems to heat stress coupled with moderate photosynthetic active radiation at different fruit developmental stages

• Fruit peel photosystem heat stress sensitivity is constant during fruit growth.
• The critical temperature for apple fruit peel photodamge is approximately 45 °C.
• Fruit peel biochemistry and anatomy does not correlate with heat induced ΔFv/Fm.

Damage to the photosystems of fruit peel of Granny Smith, Fuji and Cripps’ Pink apple (Malus × domestica Borkh.) cultivars by heat stress of: 30, 35, 40, 45 and 50 °C, coupled with a 500 μmol m−2 s−1 photosynthetic active radiation (PAR) was analysed under laboratory conditions at these fruit development stages: 50, 95 and 150 days after full bloom. Photodamage was assessed by measuring the maximum light use efficiency of photosystem II (Fv/Fm) at each fruit development stage. The critical temperature for photodamage was approximately 45 °C. The initial Fv/Fm before stress of apple fruit peels did not change during fruit development. The thickness of the epicuticular wax and ratio of chlorophyll a/b of the peel increased while stomata density, concentrations of total phenolic, carotenoid and chlorophyll decreased during fruit development. There was no significant correlation between both the biochemical and anatomical features of fruit peel and high temperature stress induced change in Fv/Fm. There was no difference in the susceptibility of photosystems of fruit peel to high temperature stress among all the fruit development stages. These results show that photosystems of apple fruit peel remain equally susceptible to heat stress, and heat stress related damage, throughout fruit development.