What was done
The authors grew four tropical plants common to semiarid zones of northwestern Venezuela in open-top chambers receiving average atmospheric CO2 concentrations of 415 and 620 ppm for about 1.5 years.  In addition, water was withheld from the plants for one month beginning at week 82 of the study to investigate the interactive effects of elevated CO2 and soil moisture on gas exchange in these tropical species that differ greatly in growth form and photosynthetic physiology (Ipomoea carnea Jacq., a perennial deciduous C3 shrub; Jatropha gossypifolia L., a C3 shrub; Alternanthera cruces Moq., a C3/C4 herb; Talinum triangulare Jacq., an inducuble CAM herb).

What was learned
Elevated CO2 consistently enhanced rates of net photosynthesis in these tropical species throughout the study.  At the onset of drought (week 82), for example, photosynthetic rates in CO2-enriched plants of Ipomoea carnea, Jatropha gossypifolia, Alternanthera cruces and Talinum triangulare were 44, 51, 63, and 250% greater, respectively, than those of ambiently-grown control plants.  Moreover, after four weeks of drought, the CO2-induced photosynthetic stimulation increased by 450% for Jatropha gossypifolia, 238% for Alternanthera cruces, 350% for Talinum triangulare and by an essentially infinite amount for Ipomoea carnea (which exhibited carbon loss and gains under ambient and elevated CO2 concentrations, respectively).

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that these tropical species common to northwestern Venezuela will exhibit enhanced rates of photosynthesis during both wet and dry seasons, which should ultimately increase their biomass production.  Indeed, the authors concluded that "in a future atmosphere with a higher CO2 concentration, these species should be able to show a higher productivity and ... lengthen the period of carbon acquisition during drought."