Photosynthesis and photoprotection in overwintering plants

Seasonal differences in the capacity of photosynthetic electron transport, leaf pigment composition, xanthophyll cycle characteristics and chlorophyll fluorescence emission were investigated in two biennial mesophytes (Malva neglecta and Verbascum thapsus) that grow in full sunlight, and in leaves/needles of sun and shade populations of several broad-leafed evergreens and conifers (Vinca minor, Euonymus kiautschovicus, Mahonia repens, Pseudotsuga menziesii [Douglas fir], and Pinus ponderosa). Both mesophytic species maintained or upregulated photosynthetic capacity in the winter and exhibited no upregulation of photoprotection. In contrast, photosynthetic capacity was downregulated in sun leaves/needles of V minor, Douglas fir, and Ponderosa pine, and even in shade needles of Douglas fir. Interestingly, photosynthetic capacity was upregulated during the winter in shade leaves/needles of V. minor, Ponderosa pine and Euonymus kiautschovicus. Nocturnal retention of zeaxanthin and antheraxanthin, and their sustained engagement in a state primed for energy dissipation, were observed largely in the leaves/needles of sun-exposed evergreen species during winter. Factors that may contribute to these differing responses to winter stress, including chloroplast redox state, the relative levels of source and sink activity at the whole plant level, and apoplastic versus symplastic phloem loading, are discussed.