Effects of elevated root zone CO₂ and air temperature on photosynthetic gas exchange, nitrate uptake, and total reduced nitrogen content in aeroponically grown lettuce plants

摘要

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 (dayight) and 36 °C/30 °C. Photosynthetic CO₂ assimilation (A) and stomatal conductance (g_s) 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 g_s, 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 g_s 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.