Elevated root-zone CO2 protects lettuce plants from midday depression of photosynthesis

Effects of elevated root-zone (RZ) CO2 on photosynthesis and productivity of lettuce plants grown in a greenhouse were investigated over a period of 2 weeks. Four levels of RZ CO2 were imposed on lettuce roots 3 weeks after transplanting by supplying air with CO2 concentrations in the range between ambient (360 ppm) and 50,000 ppm. Elevated RZ CO2 produced no significant increase of photosynthetic CO2 assimilation (A) and stomatal conductance (gs) during the first 2–3 h following sun rise. Compared to control plants grown at ambient RZ CO2, elevated RZ CO2 treatment resulted in significant increases and decreases in A and gs, respectively, during the late morning. By midday, however, there was a substantial decline in A, which could attribute to both increase in stomatal limitation of A and dynamic photoinhibition of PSII as gs and Fv/Fm ratios were lowest in all plants during midday between 13:00 and 14:00 h. Midday depressions of A and Fv/Fm ratios were alleviated in elevated RZ CO2 treated plants. However, elevated RZ CO2 significantly decreased gs accompanying higher midday RWC. During the onset of different RZ CO2 treatments, leaf expansions were measured to examine the response of leaf growth to elevated RZ CO2 over a period of 5 days. Productivities of shoot and root were also determined by the end of 2 weeks treated with different RZ CO2. The results indicate that elevated RZ CO2 enhanced leaf area, shoot and root productivities. It was predicted that increasing RZ CO2 could alleviate midday depression of photosynthesis and hence increase the productivity of lettuce plants.