Effect of Freezing Temperatures on the Photosynthetic Apparatus in the Branch Chlorenchyma of the Invasive Tree Acer negundo

Photosynthetic apparatus in the chlorenchyma tissues of branches of perennial land plants has developed strategies that allow it to withstand significant temperature fluctuations throughout all seasons. In this study, we investigated the effect of freezing temperature treatment on the functional activity of the photosynthetic apparatus in the surface chlorenchyma of branches of the invasive maple Acer negundo L. We found that the maximum quantum yield of photosystem II in branches changed only slightly after freezing treatment, whereas the leaves inevitably died after this treatment. Analysis of fast chlorophyll fluorescence kinetics with high temporal resolution revealed that the operation of the water-oxidizing complex and the photosystem II reaction center remained unaffected by low-temperature treatment, whereas electron transfer beyond the primary electron acceptor QA was noticeably affected. Plastoquinone, which mediates electron transfer between photosystem II and I, was present at several-fold lower levels in branch chlorenchyma than in leaves, and its level was not affected by freezing. The levels of malondialdehyde, an indicator of oxidative membrane damage, significantly increased in leaves after freezing but remained low in the branch chlorenchyma. The total antioxidant capacity, assessed by ferric reducing/antioxidant power (FRAP) assay, was several times lower in the branch chlorenchyma compared to leaves. The obtained data demonstrate a marked difference between A. negundo L. leaves and branch chlorenchyma in the tolerance of the photosynthetic apparatus to low temperatures and contribute to our understanding of the physiological basis for the high freeze tolerance of the photosynthetic machinery.