Seasonal Fluctuations and Temperature Dependence of Photosystem II Activity in Branches of Malus × domestica Borkh.

The photosynthetic apparatus in chlorenchyma tissues of lignified tree branches is subjected to fluctuations in environmental temperatures throughout the year. In the present study, the effective quantum yield of photosystem II (Y(II)) in chlorenchyma tissues of apple tree (Malus × domestica Borkh.) branches was monitored on a weekly basis over two years. The activity of photosystem II in these tissues during all seasons has been detected. Microscopic analysis confirmed the presence of chlorenchyma cells mainly localized under the bark during all seasons, even in old branches. Correlation analysis demonstrates that Y(II) is strongly dependent on the air temperature, with an increase in the temperature range of 0 to 10°C contributing most significantly to the Y(II) growth. At negative air temperatures, Y(II) values vary between 0 and 0.2, although in rare instances higher values have been detected. Notably, the photosynthetic potential of winter branches is retained, and incubation of branches at room temperature restores the functionality of photosystem II rapidly. A comparison of the tolerance of the photosynthetic apparatus of M. domestica branches collected at different seasons to elevated temperatures revealed that summer branches demonstrate the highest tolerance to high-temperature stress, while by winter their ability to withstand increased temperatures diminished noticeably. Overall, the results demonstrate that the photosynthetic apparatus in M. domestica branches exhibits persistent activity throughout the year, though its physiological characteristics are subject to seasonal variations.