Sustained enhancement of photosynthesis in coffee trees grown under free-air CO₂ enrichment conditions: disentangling the contributions of stomatal, mesophyll, and biochemical limitations

Coffee (Coffea spp.), a globally traded commodity, is a slow-growing tropical tree species that displays an improved photosynthetic performance when grown under elevated atmospheric CO₂ concentrations ([CO₂]). To investigate the mechanisms underlying this response, two commercial coffee cultivars (Catuaí and Obatã) were grown using the first free-air CO₂ enrichment (FACE) facility in Latin America. Measurements were conducted in two contrasting growth seasons, which were characterized by the high (February) and low (August) sink demand. Elevated [CO₂] led to increases in net photosynthetic rates (A) in parallel with decreased photorespiration rates, with no photochemical limitations to A. The stimulation of A by elevated CO₂ supply was more prominent in August (56% on average) than in February (40% on average). Overall, the stomatal and mesophyll conductances, as well as the leaf nitrogen and phosphorus concentrations, were unresponsive to the treatments. Photosynthesis was strongly limited by diffusional constraints, particularly at the stomata level, and this pattern was little, if at all, affected by elevated [CO₂]. Relative to February, starch pools (but not soluble sugars) increased remarkably (>500%) in August, with no detectable alteration in the maximum carboxylation capacity estimated on a chloroplast [CO₂] basis. Upregulation of A by elevated [CO₂] took place with no signs of photosynthetic downregulation, even during the period of low sink demand, when acclimation would be expected to be greatest.