Elevated root-zone CO 2 protects lettuce plants from midday depression of photosynthesis

Abstract

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