This study has laid a solid foundation for:
1) Using chlorophyll fluorescence to monitor and predict damage to apple peel caused by high temperature coupled with high light.
2) Screening for tolerant apple genotypes for breeding tolerant cultivars for high temperature and high light stress.
3) Understanding the molecular mechanism of high temperature/high light stress to apple peel.
impact statement issue
Sunburn of apple fruit is a common problem in apple orchards in central Washington, resulting in losses averaging 10% 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. Three types of sunburn have been identified, of which sunburn browning is most common.
However, the mechanism of sunburn browning was not clearly understood. Ultraviolet radiation has been shown to be involved in induction of sunburn browning and peel temperatures in the range of 45 to 49 degrees centigrade (dependent on cultivar) are required. These temperatures do not appear to affect the integrity of cell membranes. Sunburn browning occurs only on the sun-exposed side of well-exposed fruit, which has a high photoprotective capacity in terms of xanthophyll cycle and the antioxidant system.
We reasoned that, for sunburn browning to occur, high temperature coupled with high light must either increase the photo-oxidation potential or decrease the photo-protective capacity of fruit peel. From a practical standpoint, how to detect high peel temperature stress to fruit peel during the growing season and high temperature-induced disorders at harvest or early during storage before the appearance of the symptoms is important to both growers and packers. The objective of this project is to understand the underlying physiology of high temperature stress to apple fruit peel with an ultimate goal of detecting and reducing high peel temperature-induced fruit disorders both preharvest and postharvest.
impact statement response
By combining environmental monitoring under Washington conditions, controlled temperature and light treatments, and peel photosynthetic oxygen evolution and chlorophyll fluorescence measurements, we found:
1) Maximum photosystem II (PSII) quantum efficiency (Fv/Fm) of the sun-exposed peel of well-exposed fruit in the southwest canopy decreased during the day in response to high peel temperatures, and very little recovery was made during overnight dark relaxation, indicating that the high peel temperature has damaged the PSII centers of the peel.
2) After a couple days of high temperature exposure, more fruit in the west side of the canopy had very low Fv/Fm value than those in the east side. This difference corresponds to the different profiles of peel temperatures and sunburn occurrence between the two sides of the canopy. This along with the diurnal Fv/Fm data indicates that Fv/Fm is a very sensitive indicator of high temperature stress in apple peel.
3) Compared with the non-sunburned peel, the sunburned peel had lower chlorophyll content, lower Fv/Fm, lower net oxygen evolution rate, and lower activities of key photosynthetic enzymes, but higher activities of antioxidant enzymes and higher content of antioxidant metabolites and higher xanthophyll cycle activity on a chlorophyll basis, and higher hydrogen peroxide and malondialdehyde content. This indicates that high peel temperature most likely has increased the photo-oxidation potential, rather than decreased the photo-protective capacity of fruit peel.
4) Controlled temperature treatments of fruit peel samples in the dark showed that high peel temperature led to decreases in Fv/Fm and net O2 evolution, and appearance of “K” step in chlorophyll a fluorescence transient. This indicates that high temperature has damaged the oxygen evolution complex of the PSII, leading to oxidative stress.
5) Simultaneous high temperature and high light treatment decreased Fv/Fm and O2 evolution of Gala peel more than high temperature or high light alone. The clear “K” step in chlorophyll fluorescence, which appeared in the high temperature treatment, disappeared under simultaneous high temperature and high light treatment. This indicates that high temperature mainly affects the oxygen evolution complex of the PSII (the donor side) whereas high light mainly affects the acceptor side of the PSII. The photo-oxidative damage occurring in the sun-exposed peel of apple fruit under high temperature coupled with high light is initiated by high temperature on the oxygen evolution complex of the PSII, and exacerbated by high light.
6) Apple cultivars differ in their responses to high temperature and high light stress. Of the cultivars tested, Red Delicious is most tolerant of high temperature and high light stress whereas Cameo is the least tolerant. Our data show that chlorophyll fluorescence is an effective tool for testing varietal difference in tolerance to high temperature and high light stress.
impact statement summary
Sunburn of apple fruit is a common problem that affects fruit finish and quality in both the Pacific Northwest and the East Coast. The objective of this project was to understand the underlying physiology of high temperature stress to apple fruit peel with an ultimate goal of detecting and reducing high peel temperature-induced fruit disorders both preharvest and postharvest.
We found: 1) high temperature coupled with high light most likely has increased the photo-oxidation potential, rather than decreased the photo-protective capacity of fruit peel; 2) the photo-oxidative damage of high temperature coupled with high light on the sun-exposed peel of apple was initiated by high temperature on the oxygen evolution complex of PSII,, and then exacerbated by high light; 3) chlorophyll fluorescence was a very sensitive and effective indicator of high temperature stress on apple peel; and 4) apple cultivars differ in their responses to high temperature and high light stress.
The study has laid a solid foundation for: 1) using chlorophyll fluorescence to monitor and predict damage to apple peel caused by high temperature coupled with high light; 2) screening for tolerant apple genotypes for breeding tolerant cultivars for high temperature and high light stress; and 3) understanding the molecular mechanism of high temperature/high light stress to apple peel.
