Down regulation of photosynthesis after CO_2 enrichment of lettuce; relation to photosynthetic characteristics

The effect of CO_2 enrichment was studied in two sets of lettuce which had been cultivated either in summer conditions (summer plants) or winter conditions (winter plants) during the first two weeks of cultivation, then in the same summer conditions in culture chambers during three weeks at either 340 or 1950ppm CO_2. Summer plants were much heavier, but with smaller and thicker leaves than winter plants, and 6-fold higher specific rates of photosynthesis. Theses differences persisted for more than three weeks after plants were put in the same summer conditions. The stimulation of photosynthesis by elevated CO_2 did not slow off in winter plants, whereas summer plants showed down-regulation of photosynthesis and an inhibition of leaf development at 1950ppm CO_2, resulting in zero gain weight in elevated CO_2. Photorespiration was inhibited by high CO_2, but this effect was fully reversible with no after-effect of enrichment. Down regulation of photosynthesis in summer plants, consisting in a 25% reduction in photosynthetic capacity and 25% reduction in leaf area, appeared to be due to the enhancement by high CO_2 of limiting factors prevalent only at the high photosynthetic rates attained in summer plants