Résumé
Four Mediterranean herbaceous species, the two grasses
Bromus madritensis (annual) and
B.
erectus (perennial), and the two legumes
Medicago minima (annual) and
M.
glomerata (perennial) were grown in glasshouses at two levels of atmospheric CO
2 (350 and 700 μmol mol
−1), under non-limiting nutrient conditions. After 6 months of growth, short and long-term responses of whole plant photosynthesis and stomatal conductance to elevated CO
2 were measured, together with changes in leaf total non-structural carbohydrate concentration, leaf nitrogen concentration and specific leaf area. Short-term exposure to elevated CO
2 increased whole plant photosynthesis by 30% on average. However, this stimulation did not persist in the long term, indicating a down-regulation of photosynthesis in plants grown at elevated CO
2. By contrast, stomatal conductance was similarly or more decreased after long-term than after short-term exposure to elevated CO
2. As a result, the short-term effect of CO
2 on instantaneous water use efficiency was conserved in the long-term and the
c
i/
c
a ratio remained nearly constant after both short and long-term exposure to elevated CO
2. Analysis of the main leaf components revealed that when grown at elevated CO
2, leaves of the two grass species showed a large accumulation of total non-structural carbohydrates and a decrease in their nitrogen concentration, while leaf total non-structural carbohydrate and nitrogen concentrations of the two legume species were unaffected by elevated CO
2. Species-specific differences in down-regulation of photosynthesis were positively correlated with the long-term response of stomatal conductance and negatively correlated with changes in total non-structural carbohydrate concentration. This suggests that source–sink relationship may play a role in the control of photosynthetic response to high CO
2 concentration.