Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to Free Air CO₂ Enrichment (FACE)

The initial stimulation of photosynthesis observed on elevation of [CO ₂] in grasslands has been predicted to be a transient phenomenon constrained by the loss of photosynthetic capacity due to other limitations, notably nutrients and sinks for carbohydrates. Legumes might be expected partially to escape these feedbacks through symbiotic N₂ fixation. The Free-Air Carbon dioxide Enrichment (FACE) experiment at Eschikon, Switzerland, has been the longest running investigation of the effects of open-air elevation of [CO₂] on vegetation. The prediction of a long-term loss of photosynthetic capacity was tested by analysing photosynthesis in Trifolium repens L. (cv. Milkanova) in the spring and autumn of the eighth, ninth and tenth years of treatment. A high and low N treatment also allowed a test of the significance of exogenous N-supply in maintaining a stimulation of photosynthetic capacity in the long-term. Prior work in this Free Air CO₂ Enrichment (FACE) experiment has revealed that elevated [CO₂] increased both vegetative and reproductive growth of T. repens independent of N treatment. It is shown here that the photosynthetic response of T. repens was also independent of N fertilization under both current ambient and elevated (600 μmol mol^-1) [CO₂]. There was a strong effect of season on photosynthesis, with light-saturated rates (A _sat) 37% higher in spring than in autumn. Higher A_sat in the spring was supported by higher maximum Rubisco carboxylation rates (V _c,max) and maximum rates of electron transport (J_max) contributing to RuBP regeneration. Elevated [CO₂] increased A _sat by 37% when averaged across all measurement periods and both N fertilization levels, and decreased stomatal conductance by 25%. In spring, there was no effect of elevated [CO₂] on photosynthetic capacity of leaves, but in autumn both V_c,max and J_max were reduced by approximately 20% in elevated [CO₂]. The results show that acclimation of photosynthetic capacity can occur in a nitrogen-fixing species, in the field where there are no artificial restrictions on sink capacity. However, even with acclimation there was a highly significant increase in photosynthesis at elevated [CO₂].