Effects of long-term elevated atmospheric carbon dioxide onLolium perenne andTrifolium repens, using a simple photosynthesis model

Abstract

Changes in gross canopy photo synthetic rate (PGc), produced by long-term exposure to an elevated atmospheric CO2 level (626 ± 50/μmol mol−1), were modelled for Lolium perenne L. cv. Vigor and Trifolium repens L. cv. Blanca, using a simple photosynthesis model, based on biochemical and physiological information (leaf gross CO2 uptake in saturating light, Pmax, and leaf quantum efficiency, α) and structural vegetation parameters (leaf area index, LAI, canopy extinction coefficient, k, leaf transmission, M). Correction of PGc for leaf respiration allowed comparison with previously measured canopy net CO2 exchange rates, with the average divergence from model prediction amounting to about 6%. Sensitivity analysis showed that for a three-week old canopy, the PGc increase in high CO2 could be attributed largely to changes in Pmax and α, while differences in canopy architecture were no longer important for the PGc-stimulation (which they were in the early growth stages). As a consequence of this increasing LAI withcanopy age, the gain of daytime CO2 uptake is progressively eroded by the increasing burden of canopy respiration in high-CO2 grown Lolium perenne. Modelling canopy photosynthesis in different regrowth stages after cutting (one week, two weeks,...), revealed that the difference in a 24-h CO2 balance between the ambient and the high CO2 treatment is reduced with regrowth time and completely disappears after 6 weeks.