Sunburn of apple fruit is a common problem that affects fruit finish and quality. The objective of this project was to determine xanthophyll cycle pool size and composition in the peel of apple fruit and its role in protecting fruit from excessive light damage, with the ultimate goal to reduce sunburn occurrence on apple fruit. We found that the operation of the xanthophyll cycle in apple peel was affected by canopy-light exposure, developmental stages, nitrogen supply, evaporative cooling, and water stress and that there were differences between cultivars in xanthophyll cycle pool size and lutein content associated with their susceptibility to sunburn. Optimizing all the above factors is important for maximizing photoprotection of apple fruit. This study has provided a physiological basis for sunburn protection and will also help apple breeders select sunburn-tolerant apple cultivars.

Sunburn of apple fruit is a common problem in apple orchards in central Washington, resulting in losses averaging 10 percent of the crop each year. It is also a problem in the East, especially in hot years or when fruit is suddenly exposed to full sunlight by summer pruning. Sunburn decreases fruit quality and causes significant economic losses to apple growers. Sunburn is triggered by high light coupled with high temperature. From a physiological standpoint, sunburn only occurs when the photoprotective capacity of apple fruit cannot cope with the damage brought about by excessive light coupled with high temperature. Therefore, there are two overall strategies for sunburn control. One is to reduce the amount of light the fruit surface can absorb, by partial shading or coating the fruit with reflective materials and/or decreasing the temperature of the fruit skin by overhead cooling. The other is to increase the photoprotective capacity of apple fruit. Significant progress has been made in identifying the threshold temperature at which sunburn occurs, determining the effects of reflective materials and overhead cooling, and using computer modeling to predict sunburn occurrence. In contrast, much remains to be learned about the mechanistic aspects of the photoprotective capacity of apple fruit. A better understanding of the photoprotection mechanism has the potential to lead to new ways for controlling sunburn and improve the current sunburn controlling practices.

The objective of this project is to determine xanthophyll cycle size and the composition of apple peel in relation to photoprotection. The following three approaches have been taken. First, we compared the sun-exposed side with the shaded side of apple fruit in terms of xanthophyll cycle size and conversion and the antioxidant system, and their developmental patterns. Second, we determined how environmental factors and cultural practices affect xanthophyll cycle pool size and conversion. Finally, we determined varietal differences in xanthophyll cycle size associated with sunburn susceptibility. We have found that: 1) the sun-exposed side of both Gala and Smoothee fruit has a larger xanthophyll cycle pool size and higher activities of antioxidant enzymes than the shaded side, indicating that the sun-exposed side has a higher photoprotective capacity than the shaded side; 2) under high light at noon, the xanthophyll cycle in the sun-exposed side operates at its full capacity to dissipate excess absorbed light, and the xanthophyll cycle pool size may become limiting; 3) xanthophyll cycle pool size decreases as fruit develops; 4) Gala fruit deficient in nitrogen had a lower xanthophyll cycle pool size; 5) evaporative cooling increases, whereas deficit irrigation and partial root zone drying decrease xanthophyll cycle pool size; and 6) there are differences between cultivars in xanthophyll cycle pool size and lutein content associated with their susceptibility to sunburn.

This study has provided a physiological basis for sunburn protection and will also help apple breeders select sunburn-tolerant apple cultivars.

Federal Formula Funds - Research (e.g., Hatch, McIntire-Stennis, Animal Health)

• Dr. Larry Schrader
• Dr. Jim McFerson
• Dr. Horst Caspari
• Dr. Mike Glenn
• Dr. Lisong Chen
• Dr. Guohai Xia
• David Felicetti
• Richard Raba