Seasonal changes in light and temperature affect the balance between light harvesting and light utilisation components of photosynthesis in an evergreen understory shrub

In a temperate climate, evergreen species in the understory are exposed to large changes in photosynthetic photon flux density (PPFD) and temperature over the year. We determined the photosynthetic traits of leaves of an evergreen understory shrub Aucuba japonica at three sites at monthly intervals: understorys of a deciduous forest; an evergreen forest; and a gap in a mixed forest. This set up enabled us to separate the effects of seasonal change in PPFD and temperature on photosynthetic acclimation under natural conditions. The effects of PPFD and temperature were analysed by simple and multiple regression analyses. The amounts of light utilisation components (LU), represented by nitrogen and rubisco contents per area, were higher in winter, when temperature was low and PPFD was high. The LU relative to the amount of light harvesting components (LH),. represented by chlorophyll a/b and rubisco/chlorophyll ratios, and the inverse of chlorophyll/nitrogen ratio were also higher in winter. We quantified the effects of PPFD and temperature on the LU and LH components. Across sites PPFD had stronger effects than air temperature, while within a site temperature had stronger effects on photosynthetic acclimation. We concluded that the photosynthetic apparatus is strongly affected by the prevailing PPFD at the time of leaf development. Within a given light regime, however, plants acclimated by increasing LU relative to LH primarily in response to temperature and to a lesser extent to PPFD.