Nutrient supply has greater influence than sink strength on photosynthetic adaptation to CO2 elevation in white birch seedlings

To study the effects of source–sink ratio and nutrient supply on photosynthetic acclimation to CO2 elevation, we subjected white birch seedlings to two levels of nutrient supply (high vs. low) and CO2 concentrations (ambient vs. doubled [CO2]) for two months and then shaded the lower canopy on half of the seedlings to reduce source/sink ratio for an additional month. The CO2 elevation significantly increased Pn and IWUE at both nutrient levels but the increase was greater in the high than low nutrient treatment. The CO2 elevation resulted in a down-regulation of Vcmax in the low nutrient treatment but up-regulation of Jmax, TPU, (F′m−F)/F′m(F′m−F)/F′m and Jc in the high nutrient after 3 months of treatment. Both the CO2 elevation and high nutrient supply increased the partition of total electron transport to carboxylation at the expense of oxidation. The seedlings responded to the shading of the lower canopy by reducing biomass allocation to roots rather than making physiological adjustments to unshaded leaves in the upper canopy. Our results suggest that the direction of photosynthetic acclimation to CO2 elevation in white birch was nutrient-dependent and an increase in sink strength could reduce the feedback inhibition of photosynthesis.

► Photosynthetic acclimation to CO2 elevation was measured in white birch. ► The photosynthetic up-regulation occurred at CO2 elevation and high nutrient. ► Direction of photosynthetic acclimation to CO2 elevation was nutrient-dependent. ► Increased sink strength could reduce the feedback inhibition of photosynthesis.