Abstract

Sun-damage (=sunburn, SB) pre-harvest and sunscald (SS) symptoms post-harvest on apples (Malus domestica Borkh.) can significantly reduce fruit quality. Fruit evaluations in apple orchards cv. Fuji have shown that as much as 40% of total fruit yield can show SB symptoms. Both, SB as well as SS, are physiological disorders derived from photooxidative stress (PS) on fruit grown under high solar radiation environments, which is the prevalent condition in most parts of the world where temperate fruit is grown. Research has demonstrated that both high light and elevated temperature are responsible for SB symptoms (Rabinowitch et al., 1974, 1983; Torres et al., 2006). Studies from Schrader et al. (2003) have shown that when apple skin temperature exceeds 46°C to 49°C (cultivar-dependent), SB symptoms (browning) appear. This occurs only in the presence of high light, and the consequence is PS. Chlorophyll fluorescence (CF), which is an indicator of photoinhibition of photosystem II, has been used as an effective tool to determine PS in fruit. Apple tissue exposed to direct sunlight shows lower CF values than shaded or non stressed sections of the fruit. In addition, fruit exposed to direct sunlight and already showing SB symptoms has higher level of various antioxidants (AO) and phenolic compounds (Torres et al.,2006; Yuri et al., 2001; Wünsche et al., 2001) probably due to the excess of reactive oxygen species (ROS) generated by this PS. Unfortunately, most of the times this increase in AO levels does not prevent the development of SB, especially in fruit that tries to acclimate when suddenly exposed. Currently, there is a lack of information on how exposed fruit acclimates during the growing season to increasingly higher solar radiation, but it can end up in SB symptoms regardless. Furthermore, there is no information regarding the differences between a cultivar perceived as “susceptible”, such as Fuji and one perceived as “tolerant” such as Gala. In addition, most of the work available in this topic has been carried out in fruit detached from the plant (Cheng et al., 2008; Torres et al., 2006), which may be only partially representative of a fruit attached to the plant. Research on fruit attached to the tree is a must to understand the metabolic orchestra involved in SB development. This will allow us to change the approach to prevent this physiological disorder pre-harvest and thus postharvest, expressed as SS or fruit staining. Fruit with SS or staining in most cases show some degree of SB, but not all sunburned fruit stored at low temperature develops sunscald symptoms. There is no information regarding the biochemistry involved in these two postharvest disorders.The working hypothesis for this project is that when fruit is under PS in the presence of elevated temperature, that causes SB and sunscald symptoms on apples, not only makes adjustments to AO components to cope with ROS, but also to water stress-coping mechanisms to withstand higher transpiration rates. Genes associated with defense mechanisms against these stresses will progressively be expressed throughout fruit development. Patterns of expression will be different in acclimated versus non-acclimated fruit that is suddenly exposed. These responses will be cultivar-dependent. The objectives are: - To study PS in apples attached to the tree by characterizing gene expression, enzymes activities and metabolites synthesis during SB development in two apple cultivars with different susceptibilities. - To correlate CF parameters with PS during sunburn development in fruit acclimated and nonacclimated attached to the tree. - And, to correlate stress load with appearance of SS and other physiological disorders associated with oxidative stress after harvest and during cold storage. Apples cvs. Fuji, Gala and Granny Smith grown under commercial conditions in orchards located in VII Region, Chile will be used to carry out all the experiments. Three experiments will be conducted:1.Biochemical and molecular characterization of fruit peel during SB development on fruit attached to the tree and exposed to direct solar radiation in two cultivars with different susceptibility. 2.Biochemical and molecular characterization of fruit peel with 3 levels of acclimation to sunlight upon sudden exposure to direct sunlight. 3.Biochemical and molecular characterization of SS and other PS-related physiological disorders post-harvest during cold storage. Apple fruitlets (8 weeks after full bloom) from Gala and Fuji tagged early in the season will be separated according to their acclimation condition. Field evaluations such as fruit chlorophyll fluorescence (light and dark-adapted), tissue-water potential, and SB incidence, among others will be done periodically. Laboratory determinations will include: protein and gene expression, non-enzymatic and enzymatic AO, metabolites contents (chlorophylls, carotenoids, etc.), and compatible solutes content (K+, Na+, sorbitol, etc). For post-harvest studies Granny Smith and Fuji apples with different levels of PS will be characterized as mentioned previously and correlated with sunscald/peel staining throughout storage.