Short and long-term responses of whole-plant gas exchange to elevated CO2 in four herbaceous species

Four Mediterranean herbaceous species, the two grasses Bromus madritensis ( annual) and B. erectus (perennial), and the two legumes Medicago minima (an nual) and M. glomerata (perennial) were grown in glasshouses at two levels of atmospheric CO2 (350 and 700 mu mol mol(-1)), under non-limiting nutrien t conditions. After 6 months of growth, short and long-term responses of wh ole plant photosynthesis and stomatal conductance to elevated CO2 were meas ured, together with changes in leaf total non-structural carbohydrate conce ntration, leaf nitrogen concentration and specific leaf area. Short-term ex posure to elevated CO2 increased whole plant photosynthesis by 30% on avera ge. However, this stimulation did not persist in the long term, indicating a down-regulation of photosynthesis in plants grown at elevated CO2 By cont rast, stomatal conductance was similarly or more decreased after long-term than after short-term exposure to elevated CO2. As a result, the short-term effect of CO2 on instantaneous water use efficiency was conserved in the l ong-term and the c(i)/c(a) ratio remained nearly constant after both short and long-term exposure to elevated CO2. Analysis of the main leaf component s revealed that when grown at elevated CO2, leaves of the two grass species showed a large accumulation of total non-structural carbohydrates and a de crease in their nitrogen concentration, while leaf total non-structural car bohydrate and nitrogen concentrations of the two legume species were unaffe cted by elevated CO2. Species-specific differences in down-regulation of ph otosynthesis were positively correlated with the long-term response of stom atal conductance and negatively correlated with changes in total non-struct ural carbohydrate concentration. This suggests that source-sink relationshi p may play a role in the control of photosynthetic response to high CO2 con centration. (C) 2000 Elsevier Science B.V. All rights reserved.