Phenological, Physiological, and Ultrastructural Analyses of 'Green Islands' on Senescent Leaves of Norway Maple (Acer platanoides L.)

'Green island' symptoms in the form of vivid green, round spots visible on the senescent leaves of many plants and trees are mostly the results of pathogenic colonization by fungi, and the greenish tissue is often dead. Therefore, this study investigates whether green spots observed on senescent Norway maple (Acer platanoides L.) leaves were still alive and photosynthetically active. The appearance of 'green islands' on the leaves of young Norway maple trees was observed from the autumn of 2019 to 2022 in an urban forest (Bialystok, eastern Poland). However, in the late summer (September) of 2023 and 2024, mostly tar spots caused by the fungus Rhytisma spp. on maple leaves could be observed, with only a few leaves having 'green island' symptoms. The percentage of 'green island' areas on senescent leaves observed during the 4 years (2019-2022) was influenced by a year of sampling (p < 0.001). A non-destructive physiological analysis of chlorophyll, flavonoids, and nitrogen balance index (NBI) in leaves revealed that these parameters were significantly lower in 'green islands' than in the summer leaves, but higher than in the senescent yellow area of the autumn leaves. In the case of anthocyanins, their level was significantly higher in 'green islands' than in yellow areas, although, in the summer leaves, anthocyanins were undetectable. The amount of chlorophyll and most photosynthetic parameters were significantly (p < 0.05) reduced in the 'green islands' of the senescent leaves compared to the mature green leaves. However, these parameters were significantly higher in the 'green islands' than in senescent yellow leaves. Carotenoid content in the 'green island' and yellow areas of senescent leaves were at the same level, twice as higher than in summer leaves. Green mature leaves and the 'green islands' on senescent leaves had the same structure and anatomy. The main differences concerned the chloroplasts, which were smaller and had less grana and starch grains, but had more plastoglobuli in 'green island' cells. The cells building the mesophyll in the yellow area of the leaf deteriorated and their chloroplasts collapsed. Epiphytes were present on the adaxial epidermis surface in all types of samples.