Photosynthetically active radiation (PAR) × ultraviolet radiation (UV) interact to initiate solar injury in apple

- We exposed 5 apple cultivars to UV and visible radiation at 25 °C and 40 °C.
- We measured the UV and PAR radiation effect on fruit peel chlorophyll fluorescence.
- 'Granny Smith' fruit peel was more tolerant to radiation stress than 'Gala'.
- Moderate UV, PAR and peel temperatures cause irreversible damage on a diurnal basis.
- Moderate UV, PAR and peel temperatures cause irreversible damage on a diurnal basis.

Solar injury (SI) in apple is associated with high temperature, high visible light and ultraviolet radiation (UV). Fruit surface temperature (FST) thresholds for SI related disorders have been developed but there are no thresholds established for solar radiation. The objective of the study was to measure the effect of different levels of FST, PAR and UV on quantum efficiency (Fv/Fm) alone and in combination to identify interactions affecting Fv/Fm recovery to serve as a guideline in evaluating SI-prevention technology. The variable transmission of UVb, UVa, PAR, and NIR through films of Surround™ and Raynox™ provided a wide range of radiation levels at the fruit surface to model Fv/Fm of photosystem II (PSII) in growth chamber and greenhouse studies. Increasing UV and PAR reduced Fv/Fm the greatest amount in 'Gala' while 'Granny Smith' had the least reduction suggesting that PSII in 'Granny Smith' has higher UV and PAR tolerance. All cultivars except 'Fuji' at harvest had some interaction component of PAR and UV that decreased Fv/Fm compared to untreated control fruit as both PAR and UV increased. When re-exposure was conducted on the 5 cultivars, Fv/Fm further decreased with the cumulative PAR and UV despite having a period of dark recovery between each 24 h exposure. Cultivar differences have a strong influence on the sensitivity of the peel to FST and radiation levels. Cultivars differ in their response to FST, UV and PAR as they mature and also exhibit year-to-year variation. This inconsistency suggests that acclimation to the environment also varies between years. PAR × UV interactions are complex and include the acclimation of the cultivar to the stress but in the prevention of SI, reducing FST, PAR and UV simultaneously is likely the most effective strategy.