Photochemical efficiency of photosystem II in two apple cultivars affected by elevated temperature and excess light in vivo

In orchards, high temperature and high light stress are the most common abiotic stresses, which negatively affect fruit yield and quality. In this study, the physiological response of apple (Malus × domestica Borkh) cultivars to combination of increased temperature and excess light, which causes disorder of photosynthesis, was investigated. The PSII functionality and regulatory mechanisms in leaves of two apple cultivars in the field, under elevated temperature and excess light at midday, were evaluated. Chlorophyll a (Chl a) fluorescence transient was recorded using Plant efficiency analyzer (PEA), analyzed according to JIP test (analysis of O-J-I-P fluorescence transient) and several parameters were derived to explain the PSII behavior and photosynthetic efficiency. Tested cultivars showed different responses to investigated conditions and PCA analysis identified the cultivar Gold Rush as more tolerant with better thermostability of PSII. The lower efficiency of PSII antenna complexes in the Gala Galaxy cultivar caused higher increase in the heat dissipation value of excess absorbed light per active reaction center (DI0/RC) and a decrease in the captured photon flow rate and energy dissipation (TR0/DI0), which significantly contributes to its lower photosynthetic efficiency of performance index (PIABS) and total photosynthetic performance (PItotal). Cultivar Gold Rush had higher connectivity of light harvesting complexes to its reaction centers, less damage to OEC activity and better stability of the PS II system. The results obtained in this study may be useful in apple breeding programs, particularly for the selection of apple cultivars more tolerant and adjustable to higher temperature and irradiation.