Perioxidases play important roles in abscisic acid (ABA)-simulating photosystem II (PSII) thermostabilty of apple tree rootstock leaves
Leaf photosynthetic activity limited by summer heat stress represents large constraint to production process of fruit trees. To cope with this problem, we tested photosystem II (PS II) thermostability in clonal apple tree rootstocks with different growth intensity - semi-vigorous-MM106 and dwarfing-J-TE-F experiencing summer heat stress, and related antioxidative activity and phytohormonal balance. For this purpose, expanded leaves were collected and utilized for determination of 3-indoleacetic acid (IAA), cytokinins (CKs), abscisic acid (ABA) concentrations, and superoxide dismutase (SOD), total peroxidases (POX), catalase (CAT), total antioxidative activity (TAA) and PS II thermostability before and after 30 min dark exposition to temperature of 42°C (because of relatively-stable PS II). Leaves from dwarfing-J-TE-F showed higher PS II thermostability than that from semi-vigorous-MM106 as indicated by higher maximal quantum yield of primary photochemistry (ΦPo), excitation transfer efficiency to electron transport chain (ψo) , electron transport yield (ΦEo) and lower thermal dissipation yield (ΦDo). From antioxidant parameters, genotypic specifity can be seen in total peroxidases activity and only JTE- F leaves predominated. Despite higher CKs concentration in leaves of MM106, active CKs did not show any difference between two rootstocks. IAA also exhibited balanced level, but ABA concentration was four times higher in J-TE-F leaves than MM106 ones. We can conclude that in PS II heat-hardening of apple tree rootstock leaves, which is at least partly provided by enhanced peroxidases activity, ABA plays the central role and J-TE-F rootstock can be recomended to heat stress in the prone regions.
Key words: Photosystem II thermostability, antioxidant activity, phytohormones, heat stress, oxidative stress, apple tree rootstocks, global climate change.