Effects of elevated root-zone (RZ) CO2 concentration (RZ [CO2]) and RZ temperature (RZT) on photosynthesis, productivity, nitrate (NO3-), total reduced nitrogen (TRN), total leaf soluble and Rubisco proteins were studied in aeroponically grown lettuce plants in a tropical greenhouse. Three weeks after transplanting, four different RZ [CO2] concentrations (ambient, 360 ppm, and elevated concentrations of 2,000; 10,000; and 50,000 ppm) were imposed on plants at 20°C-RZT or ambient(A)-RZT (24-38°C). Elevated RZ [CO2] resulted in significantly higher light-saturated net photosynthetic rate, but lower light-saturated stomatal conductance. Higher elevated RZ [CO2] also protected plants from both chronic and dynamic photoinhibition (measured by chlorophyll fluorescence Fv/Fm ratio) and reduced leaf water loss. Under each RZ [CO2], all these variables were significantly higher in 20°C-RZT plants than in A-RZT plants. All plants accumulated more biomass at elevated RZ [CO2] than at ambient RZ [CO2]. Greater increases of biomass in roots than in shoots were manifested by lower shoot/root ratios at elevated RZ [CO2]. Although the total biomass was higher at 20°C-RZT, the increase in biomass under elevated RZ [CO2] was greater at A-RZT. Shoot NO3- and TRN concentrations, total leaf soluble and Rubisco protein concentrations were higher in all elevated RZ [CO2] plants than in plants under ambient RZ [CO2] at both RZTs. Under each RZ [CO2], total leaf soluble and Rubisco protein concentrations were significantly higher at 20°C-RZT than at A-RZT. Our results demonstrated that increased P Nmax and productivity under elevated [CO2] was partially due to the alleviation of midday water loss, both dynamic and chronic photoinhibition as well as higher turnover of Calvin cycle with higher Rubisco proteins. and J. He, L. Qin, S. K. Lee.