Plant pigment cycles in the high-Arctic Spitsbergen

Plants in the high Arctic are exposed to a 24-h photoperiod for several months. These conditions can be damaging for plants at lower latitudes. When common crops are artificially maintained under continuous light (CL), photosynthetic processes maintain endogenous circadian rhythms, but it is unclear whether plants naturally acclimated to CL also maintain such rhythmicity. Alternatively, Arctic plants have to utilise the favourable conditions during the short Arctic summers. In this study, we evaluate the existence of daily cycles in pigment composition in three Arctic plants in a tundra ecosystem at 78 degrees N that display examples of different growth forms: the bryophyte Polytrichum hyperboreum, the herb Bistorta vivipara and the dwarf shrub Salix polaris. Changes in pigment composition are excellent indicators of the restructuring of the light-harvesting apparatus. Most pigment parameters analysed did not show any consistent pattern of variation between subjective noon and midnight. P. hyperboreum had the highest level of rhythmicity, while S. polaris was the most stable. Despite these subtle changes, the primary effects observed were induced by light and its effect on the de-epoxidation state of the xanthophyll cycle pigments (AZ/VAZ). Both short- and long-term adjustments of the AZ/VAZ correlated with changes in photochemical efficiency (phi(PSII)). When the plants were artificially darkened during the night, it became evident that the midnight sun prevents the complete relaxation of the xanthophyll cycle. These results indicate that light is the primary driver of photochemical efficiency in Arctic plants, and consequently, photosynthesis is not completely interrupted at night.