Seasonal acclimation to light and temperature in an evergreen understory shrub

Abstract

Evergreens in temperate regions often maintain leaves longer than one year. These leaves are exposed to large changes in temperature with the seasons. If plants are growing in the understory of a deciduous forest the light intensity also changes strongly over the year. In summer the plants in the understory grow in the shade of the canopy trees in autumn, when the canopy trees drop their leaves, the understory receives more light. In spring the opposite happens. Under natural conditions it is the question if plants in the understory acclimate to changes in light and/or temperature over the year. We investigated the acclimation of evergreen leaves of a common shrub in Japan, Aucuba japonica, by biochemical, gas-exchange, fluorescence and leaf anatomical analysis. The photosynthetic apparatus was divided into a light-harvesting- and light-energy-using-part to discuss seasonal acclimation to light and temperature. The balance between both parts changed mainly with changes in temperature during the year. We also calculated the maximum daily carbon gain per unit nitrogen that was strongly correlated with the actual leaf nitrogen content with changes in light and temperature over the year. The leaf thickness of the evergreen leaves of A. japonica did not differ largely between different growth light environments but the total volume of chloroplasts did change largely over the year. The leaves developed at the end of spring had a relatively large amount of open space in summer. This open space enabled a functional increase in chloroplast volume in winter. With more detailed knowledge about acclimation to changes in light and temperature under natural conditions we can explain the distribution of evergreen species better. With a simple calculation we showed the effect of rise in temperature within a period of 20 years on A. japonica in a deciduous forest. A rise in temperature will increase the photosynthetic rate, leading to a higher carbon gain, but at the same time canopy trees will maintain their leaves longer so less light over the year reaches the understory, leading to a lower carbon gain. Considering both factors I calculated that the net daily carbon gain will be 5% higher 20 years later.