Alleviating sunburn injury in apple fruit using natural and fertilizer forms of S-abscisic acid and its underlying mechanism

Twelve-year-old apple (Malus pumila Mill.) trees cultivar Tsugaru and Sensyu grown at the University of Tsukuba, Japan, were subjected to high temperatures and high light intensity during summer, 2007 after treatment with 200 ppm natural S-abscisic acid or S-ABA [5-(1-hydroxy-2,6,6-trimethyl-4- oxo-2-cyclohexen- 1-yl)-3-methyl-2,4-pentadienoic acid] or with 400 and 800 ppm ABA fertilizer [MIYOBI, a new fertilizer formulation containing 8.0,5.0,0.9,0.5 and 0.3 % a.i.(w/w) of K, P, Mg, B and Mn, respectively, and 10.0 % a.i. (w/w) S-ABA] 35 days before harvesting fruit. All ABA treatments effectively suppressed the development of sunburn injury manifested as necrosis in 'Tsugaru' and browning in 'Sensyu' as compared to untreated fruit (control). Total antioxidant capacity as well as ascorbic acid, total phenols, chlorophyll a and b and total anthocyanin contents were higher while malondialdehyde content, a product of lipid peroxidation, was lower in ABA-treated fruit than in the control. These results suggest that ABA increased antioxidant levels to inhibit sunburn injury, a disorder resulting from oxidative cell damage caused by high temperature and solar radiation.