| Abstract
| - Effects of elevated root zone (RZ) CO2 and air temperature on photosynthesis, productivity, nitrate (NO3-), and total reduced nitrogen (N) content in aeroponically grown lettuce plants were studied. Three weeks after transplanting, four different RZ [CO2] concentrations [ambient (360 ppm) and elevated concentrations of 2000, 10 000, and 50 000 ppm] were imposed on plants grown at two air temperature regimes of 28 °C/22 °C (day/night) and 36 °C/30 °C. Photosynthetic CO2 assimilation (A) and stomatal conductance (gs) increased with increasing photosynthetically active radiation (PAR). When grown at 28 °C/22 °C, all plants accumulated more biomass than at 36 °C/30 °C. When measured under a PAR ≥600 μmol m−2 s−1, elevated RZ [CO2] resulted in significantly higher A, lower gs, and higher midday leaf relative water content in all plants. Under elevated RZ [CO2], the increase of biomass was greater in roots than in shoots, causing a lower shoot/root ratio. The percentage increase in growth under elevated RZ [CO2] was greater at 36 °C/30 °C although the total biomass was higher at 28 °C/22 °C. NO3- and total reduced N concentrations of shoot and root were significantly higher in all plants under elevated RZ [CO2] than under ambient RZ [CO2] of 360 ppm at both temperature regimes. At each RZ [CO2], NO3- and total reduced N concentration of shoots were greater at 28 °C/22 °C than at 36 °C/30 °C. At all RZ [CO2], roots of plants at 36 °C/30 °C had significantly higher NO3- and total reduced N concentrations than at 28 °C/22 °C. Since increased RZ [CO2] caused partial stomatal closure, maximal A and maximal gs were negatively correlated, with a unique relationship for each air temperature. However, across all RZ [CO2] and temperature treatments, there was a close correlation between maximal A and total shoot reduced N concentration of plants under different RZ [CO2], indicating that increased A under elevated RZ [CO2] could partially be due to the higher shoot total reduced N.
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