Fig. 5. Changes in the contents of carbon and nitrogen and the C/N ratio during sun- flower primary leaf development. Plants were grown under different atmospheric CO2 concentrations: 400 L L−1 (closed circles) and 800 L L−1 (open circles). Data are means ± SD of duplicate determinations from three separate experiments. Aster- isks indicate statistically significant differences among the CO2 treatments at the indicated times according to Student’s t-test (P < 0.05)