Root-zone CO2 and root-zone temperature effects on photosynthesis and nitrogen metabolism of aeroponically grown lettuce (Lactuca sativa L.) in the tropics

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.