It has been known for a long time that the photosynthesis of single leaves of C3 species generally increases under elevated [CO2]. This increase depends not only on [CO2], but also on light, temperature, nutrients, water, leaf age, etc. The daily average photosynthesis of young leaves increased by 30-80% at 60 Pa (600 ppm) compared with ambient (35 Pa) CO2 partial pressure in perennial ryegrass swards (Rogers et al., 1998). Drake et al. (1997) determined an average increase of 58% in photosynthesis from 60 experiments with elevated [CO2].
Acclimation of photosynthesis occurs during the long-term exposure of plants to elevated [CO2], when the initial strong increase of photosynthesis declines. Leaves grown under elevated [CO2] show reduced photosynthesis compared with leaves grown at ambient [CO2], when both are measured at ambient [CO2]. Processes that affect the acclimation of photosynthesis have been related to situations in which the sink limits growth under elevated [CO2] (Drake et al., 1997). During acclimation, concentrations of carbohydrates are increased and concentrations of soluble proteins and Rubisco are decreased. The lower concentrations of protein, and especially of Rubisco, are due to reduced gene expression mediated by the increase in carbohydrate concentrations. Thus, acclimation has been interpreted as a mechanism by which plants reduce leaf N content in order to increase sink growth. Rogers et al. (1998) studied acclimation of leaf photosynthesis before and after a cut in perennial ryegrass swards fertilized with low or high amounts of N. Before the cut, acclimation of photosynthesis occurred at elevated [CO2] and low N supply. After the cut, when the small leaf area and the high demand of growing leaves for carbohydrates drastically reduced the source/sink ratio, acclimation disappeared. No significant acclimation was found in ryegrass plants fertilized with high amounts of N. Acclimation of photosynthesis was also found for the canopy (Casella and Soussana, 1997) but it was relatively small (-8 to -13%) and independent of N.
Enhanced leaf photosynthesis under elevated [CO2] should be reflected by an increase in canopy photosynthesis, provided that neither the CO2 assimilation of the leaves in the lower canopy (where photosynthesis is not saturated) nor the leaf area index (LAI) are reduced, and they can compensate for the increased CO2 uptake of the upper canopy. In perennial ryegrass swards grown in large containers in highly ventilated tunnels with low or high N supply, there was an average increase of 29% and 36%, respectively, in net canopy photosynthesis at double ambient [CO2] (Casella and Sousanna, 1997). Schapendonk et al. (1997) reported an annual increase in net canopy assimilation of 29 and 43% in 2 subsequent years in perennial ryegrass swards grown at 70 Pa CO2 and with ample supplies of water and nutrients. The annual harvested biomass increased by 20 and 25%. Thus, canopy photosynthesis increased much more than biomass in the second year compared with the first year. This indicates that canopy C assimilation and harvestable biomass are not closely correlated and that partitioning of carbohydrates to shoots and roots changed.
Drake et al. (1997) found that dark respiration of leaves is reduced by about 20% at double ambient [CO2]. This reduction is due to an inhibition of the activity of two key enzymes of the mitochondrial electron transport chain. Due to the fundamental nature of this mechanism, it is assumed to occur in all respiring tissues (Drake et al., 1997). Schapendonk et al. (1997) also reported reduced specific dark respiration in perennial ryegrass swards. They found no CO2-related change in canopy dark respiration even though harvested shoot biomass (+20%) and LAI increased considerably in 1 year. In the second year, however, canopy dark respiration increased by 28% and harvested shoot biomass by 25%. Below-ground respiration (soil and roots) decreased by 10%, though root biomass increased by 29 and 86% under double ambient [CO2] by the end of the first and second growing seasons, respectively. Casella and Soussana (1997) measured an increase of canopy dark respiration proportional to shoot biomass. They found an increased below-ground respiration of 33 and 36% at low and high levels of N fertilizer, respectively.
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