Photosynthesis in Plants of Four Tropical Species Growing under Elevated CO2

We studied the responses of leaf gas exchange and growth to an increase in atmospheric CO₂ concentration in four tropical deciduous species differing in carbon fixation metabolism: Alternanthera crucis, C3-C4; Ipomoea carnea, C3; Jatropha gossypifolia, C3; and Talinum triangulare, inducible-CAM. In the first stage, plants were grown in one open-top chamber at a CO₂ concentration of 560±40 mol mol^-1 (EC), one ambient CO₂ concentration chamber (AC), and one unenclosed plot (U). In the second stage, plants were grown in five EC chambers (CO₂ concentration = 680±30 mol mol^-1), five AC chambers, and five unenclosed plots. During the first weeks under EC in the first stage, plants of all the species had a very marked increase in their maximal net photosynthetic rates (P _max) of 3.5 times on average; this stimulatory effect was maintained for 11-15 weeks, rates dampening afterward to values still higher than controls for 37 weeks. After a suspension of CO₂ enrichment for 6 weeks, an increase in P _max of EC plants over the controls was found in plants of all the species until week 82 of the experiment. Stomatal conductance (g) showed no response to EC. Carboxylation efficiency decreased in all the species under EC