PHOTOSYNTHETICA

Photosynthetica 2013, 51(3):330-340 | DOI: 10.1007/s11099-013-0030-5

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

J. He^1,*, L. Qin¹, S. K. Lee¹

¹ Natural Sciences and Science Education Academic Group, National Institute of Education, Nanyang Technological University, Nanyang, Singapore

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.

Additional key words: elevated root-zone CO₂ concentration; photosynthesis; root-zone temperature; Rubisco; total reduced N.

Received: September 3, 2012; Accepted: January 10, 2013; Published: September 1, 2013  Show citation