When the sun never sets: daily changes in pigment composition in three subarctic woody plants during the summer solstice

Subarctic plants in summer (subjected to continuous light) showed photosynthetic pigment contents mainly driven by PPFD (unrelated to day/night cycles) and a xanthophyll cycle responsiveness to PPFD exacerbated during night-times. Composition and content of photosynthetic pigments is finely tuned by plants according to a subtle equilibrium between the absorbed and used energy by the photosynthetic apparatus. Subarctic and Arctic plants are subjected to extended periods of continuous light during summer. This condition represents a unique natural scenario to study the influence of light on pigment regulation and the presence of diurnal patterns potentially governed by circadian rhythms. Here, we examined the modulation of the photosynthetic apparatus in three naturally co-occurring woody species: mountain birch (Betula pubescens ssp. czerepanovii), alpine bearberry (Arctostaphylos alpina) and Scots pine (Pinus sylvestris) around the summer solstice, at 67 A degrees N latitude. Plants were continuously exposed to solar radiation during the 3-day study period, although PPFD fluctuated, being lower during night-times. Photochemical efficiencies for a given PPFD were similar during daytime and night-time for the three species. In Scots pine, for a given PPFD, net assimilation was slightly higher during daytime than during night-time. Overall, the dynamism in pigment content was mainly driven by PPFD, and was generally unrelated to day/night cycles. Weak indications of potential circadian regulation were found over a few pigments only. Interestingly, the xanthophyll cycle was active at any time of the day in the three species but its responsiveness to PPFD was exacerbated during night-times. This was particularly evident for bearberry, which maintained a highly de-epoxidised state even at night-times. The results could indicate an incomplete acclimation to a 24-h photoperiod for these species, which have colonised subarctic latitudes only recently.