Root-zone CO<sub>2</sub> and root-zone temperature effects on photosynthesis and nitrogen metabolism of aeroponically grown lettuce (Lactuca sativa L.) in the tropics

Abstract

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.