Evidence of Adaptation to Recent Changes in Atmospheric CO(2) in Four Weedy Species

Seeds of three C(3) and one C(4) annual weedy species were collected from agricultural fields in Beltsville, Maryland in 1966 and 2006, when atmospheric CO(2) concentrations averaged about 320 and 380 mol mol(−1), respectively. Plants from each collection year were grown over a range of CO(2) concentrations to test for adaptation of these weedy species to recent changes in atmospheric CO(2). In all three of the C(3) species, the increase in CO(2) concentration from 320 mol mol(−1) to 380 mol mol(−1) increased total dry mass at 24 days in plants from seeds collected in 2006, but not in plants from seeds collected in 1966. Shoot and seed dry mass at maturity was greater at the higher growth CO(2) in plants collected in 2006 than in 1966 in two of the species. Down-regulation of photosynthetic carboxylation capacity during growth at high CO(2) was less in the newer seed lots than in the older in two of the species. Overall, the results indicate that adaptation to recent changes in atmospheric CO(2) has occurred in some of these weedy species.