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 concent ration of 560+/-40 mu mol mol(-1) (EC), one ambient CO2 concentration chamb er (AC), and one unenclosed plot (U). In the second stage, plants were grow n in five EC chambers (CO2 concentration = 680+/-30 mu mol mol(-1)), five A C chambers, and five unenclosed plots. During the first weeks under EC in t he first stage, plants of all the species had a very marked increase in the ir maximal net photosynthetic rates (P-max) of 3.5 times on average; this s timulatory effect was maintained for 11-15 weeks, rates dampening afterward to values still higher than controls for 37 weeks. After a suspension of C O2 enrichment for 6 weeks, an increase in P-max of EC plants over the contr ols was found in plants of all the species until week 82 of the experiment. Stomatal conductance (g) showed no response to EC. Carboxylation efficienc y decreased in all the species under EC and this was correlated with a decr ease in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) content in all the species except for T. triangulare. During drought P-max was higher in all species, except for T. triangulare, grown under EC relative to cont rols. Ecosystem photosynthetic rates at EC were higher than in the controls during the second stage under irrigation as well as after 30 d of drought.