Elevated CO2 enhances plant growth in droughted N2‐fixing alfalfa without improving water status

The long‐term interaction between elevated CO 2 and soil water deficit was analysed in N 2 ‐fixing alfalfa plants in order to assess the possible drought tolerance effect of CO 2 . Elevated CO 2 could delay the onset of drought stress by decreasing transpiration rates, but this effect was avoided by subjecting plants to the same soil water content. Nodulated alfalfa plants subjected to ambient (400 μmol mol −1 ) or elevated (700 μmol mol −1 ) CO 2 were either well watered or partially watered by restricting water to obtain 30% of the water content at field capacity (ampproximately 0.55 g water cm −3 ). The negative effects of soil water deficit on plant growth were counterbalanced by elevated CO 2 . In droughted plants, elevated CO 2 stimulated carbon fixation and, as a result, biomass production was even greater than in well‐watered plants grown in ambient CO 2 . Below‐ground production was preferentially stimulated by elevated CO 2 in droughted plants, increasing nodule biomass production and the availability of photosynthates to the nodules. As a result, total nitrogen content in droughted plants was higher than in well‐watered plants grown in ambient CO 2 . The beneficial effect of elevated CO 2 was not correlated with a better plant water status. It is concluded that elevated CO 2 enhances growth of droughted plants by stimulating carbon fixation, preferentially increasing the availability of photosynthates to below‐ground production (roots and nodules) without improving water status. This means that elevated CO 2 enhances the ability to produce more biomass in N 2 ‐fixing alfalfa under given soil water stress, improving drought tolerance.