Acclimation of photosynthesis to elevated CO2 concentration and increased soil N
Photosynthetic rates are well known to increase in C3 plants in response to short-term increases in CO2 concentrations (Bazzaz, 1990; Drake et al., 1997). In this study, photosynthetic rates increased an average of 55% for the C3 species and 13% for the C4 species in response to short-term CO2 enrichment, which is comparable in magnitude with the 60% average increase found in other studies of C3 species (Strain & Cure, 1994; Curtis, 1996; Drake et al., 1997). Theory based on simple CO2 diffusion into C3 leaves suggests that a photosynthetic enhancement of 55% is expected with +200 µmol mol−1 enrichment in CO2 (as in this study) and no photosynthetic acclimation (Katul et al., 2000).
The degree of relative photosynthetic responsiveness to elevated CO2 over longer-terms varies substantially. Recent reviews summarize long-term photosynthetic enhancements in response to growth under elevated CO2 on the average of 30% and 24% in C3 and C4 Poaceae species, respectively (Wand et al., 1999), and 50% and 51% across 41 woody C3 species (Curtis, 1996), and 15 field-based studies on forest tree species (Medlyn et al., 1999), respectively. The long-term enhancement of photosynthesis by elevated CO2 we observed was modest and not statistically significant in either year. C3 species on average increased photosynthesis when grown under elevated CO2 by 13% and 8%, in 1998 and 1999, respectively, and C4 species demonstrated negligible responses in both years. This is in contrast to many field-based studies that have found strong and persistent stimulation of photosynthetic rates in C3 species grown under elevated CO2 over one to three growing seasons (Ellsworth et al., 1995; Jackson et al., 1995; Drake et al., 1996; Stirling et al., 1997; Bryant et al., 1998; Curtis et al. 2000) as well as positive enhancements for C4 species (Read et al., 1997; Wand et al., 1999). However, other studies demonstrate limited stimulation of photosynthesis of plants grown for weeks to months under elevated CO2 (Tissue & Oechel, 1987; Li et al., 1999; Roumet et al., 2000) or a loss of the initial stimulation in photosynthesis over time (Oechel et al., 1994; Körner et al., 1997).
Species variation in responsiveness of photosynthesis to elevated CO2 can be explained by differences in the extent of photosynthetic acclimation in plants grown under elevated CO2. In our study on nutrient poor soil, species on average demonstrated 80% acclimation of photosynthesis to elevated atmospheric CO2 concentrations as judged by comparing the magnitude of long-term photosynthetic enhancements with elevated CO2 (A560 vs A368) to the photosynthetic response to short-term exposure to elevated CO2 (A′560 vs A368). This is much larger than shown in most previous studies (Curtis, 1996; Medlyn et al., 1999; Wand et al., 1999).
Is photosynthetic acclimation to elevated CO2 modulated by soil N supply? The magnitude of the CO2 effect on gas exchange and other leaf traits in these 13 grassland species was apparently independent of soil N supply in these two years. In only three specific cases (Achillea 98 and 99, and Lespedeza 98, Fig. 1) was there an increase in magnitude of enhancement of photosynthesis due to elevated CO2 under high N, though not statistically significant (Table 2). Several other studies have found plants to be comparably responsive to CO2 at both high and low nutrient concentrations (Hättenschwiler & Körner, 1996; Lloyd & Farquhar, 1996; Körner et al., 1997; Cotrufo et al., 1998). However, many studies have found greater photosynthetic responsiveness to elevated CO2 at higher N availability (Curtis, 1996; Miglietta et al., 1996; Rogers et al., 1998; Sims et al., 1998; Wolfe et al., 1998; Weerakoon et al., 1999; Curtis et al., 2000). Moreover, in some cases, there is evidence of acclimation of photosynthesis under CO2 enrichment only under certain conditions such as low nutrient supply (Jones et al., 1996; Miglietta et al., 1996; Rogers et al., 1998, Sims et al., 1998, Weerakoon et al., 1999). In our study, the positive effects of high N on traits such as net photosynthetic rates, A@growth/gs, and leaf N concentrations were found only in the first growing season and diminished by the second year. While this difference in response between two years does not necessarily constitute a trend, it was contrary to expectations that the effects of adding N into the system each year would result in stronger N effects over time. This minimal N effect may have contributed to the lack of a CO2 × N interaction.
One of the most consistent responses across species and growing seasons in our study was a decline in stomatal conductance to water vapor (gs). All species decreased gs by an average 24% when grown under elevated CO2. While some studies report little or inconsistent effects of elevated CO2 on gs (Gunderson & Wullschleger, 1994; Ellsworth et al., 1995; Curtis, 1996; Stirling et al., 1997) the more common response is a decline in gs comparable in magnitude to this study (Roumet et al., 2000). Reviews cite average declines in gs of 24% and 29% in C3 and C4 Poaceae species, respectively (Wand et al., 1999), 23% across 23 tree species (Field et al., 1995), and 34% across crop species (Kimball & Idso, 1983) in elevated compared with ambient CO2 grown plants.
As leaf N concentration declined an average of 13%, carbon assimilation expressed per unit leaf N (i.e. PNUE) increased an average of 11% across the species in this study. However, this response was not statistically significant and not consistent across all species. Cotrufo et al. (1998) found a comparable reduction in tissue N concentrations (average of 14%) of elevated CO2 grown C3 and C4 plants across 75 published studies and other studies have also found that PNUE increases in response to growth under elevated CO2 (Bryant et al., 1998; Tjoelker et al., 1998; Peterson et al., 1999b; Curtis et al., 2000), however, in some cases this did not occur consistently (Roumet et al., 2000).
Several possible explanations for the acclimation of photosynthesis in plants grown under elevated CO2 over longer terms have been proposed. Hypotheses include possible stomatal limitations of photosynthesis due to reduced gs (Drake et al., 1997), or nonstomatal limitations such as reduced tissue N concentrations potentially leading to a reduced photosynthetic capacity in plants grown under elevated CO2 (Peterson et al., 1999), or a potential feedback inhibition of photosynthesis induced by an accumulation of excess carbohydrates (Farrar & Williams, 1991; Stitt, 1991).
We examined the relationships between A@560-gs and Am,@560-%N to test the first two hypotheses. Results varied among species, but did provide evidence for both possible mechanisms. Some species grown under elevated CO2 responded with decreases in photosynthesis that were proportional to decreases in gs (Fig. 5a). In these cases, there was a concomitant decrease in Ci : Ca (data not shown) further suggesting that the decline in gs of elevated CO2 grown plants was associated with a lower intercellular CO2 supply and reduced photosynthetic rates. In other species, photosynthesis was lower at a given gs, with similar or slightly higher Ci : Ca, in elevated compared with ambient CO2 grown plants (Fig. 5b), suggesting that nonstomatal limitations are also involved.
A potential nonstomatal limitation involves the commonly strong relationship between tissue N and photosynthesis. Regarding the N hypothesis to explain photosynthetic acclimation, we did find a decline in photosynthesis in elevated compared with ambient CO2 grown plants in proportion to the change in leaf N concentration (Fig. 5c) in roughly half the species. Thus CO2-induced decreases in leaf N concentration are associated with reduced photosynthetic potential probably via changes in N-rich photosynthetic enzymes that are reflected in total leaf N. However, in other cases, photosynthesis was lower at a given leaf N in elevated compared with ambient CO2 grown plants (Fig. 5d) suggesting that stomatal or other nonstomatal limitations explain the observed acclimation in such cases.
Studies have found that an increase in total nonstructural carbohydrates (TNC) correlates with decreased photosynthesis in elevated CO2 grown plants (Tjoelker et al., 1998; Roumet et al., 2000), however, others report increased TNC without a decrease in photosynthetic enhancement (Wullschleger et al., 1992; Will & Ceulemans, 1997). Total nonstructural carbohydrates determined from leaves collected from the same plots used for gas exchange in this study in 1999, indicate an overall 24% greater TNC concentration in elevated CO2 grown foliage, but this varies substantially across species (M. G. Tjoelker et al., unpublished). Because acclimation of photosynthesis occurs across all the species in the study, but not all species increased TNC when grown under elevated compared with ambient CO2, these data are not sufficient to support or refute this as a possible explanation of the photosynthetic acclimation seen in this study.
Most species showed intermediate responses with respect to the examples shown in Fig. 5. Considering this evidence of stomatal and nonstomatal limitations on photosynthesis, no single mechanism explains the magnitude of photosynthetic acclimation seen across all the species in this study, and the data suggests that even within a single species a combination of these mechanisms are possible. Thus, we can ascribe the low or negligible photosynthetic enhancement observed at our site to the combined effects of decreased gs, decreased leaf N concentration, and possibly increased TNC in response to growth under elevated CO2.
In our study, the grouping of species into the functional classifications: C3 grasses, C4 grasses, legumes, and nonleguminous forbs, are based on discrete physiological and growth form traits commonly used to group species. Our objective was to evaluate whether these categories are helpful in explaining variation in species response to elevated CO2 and increased N supply across the 13 perennial prairie species in this study. The C4 grasses are grouped owing to their photosynthetic pathway that concentrates CO2 in bundle sheath cells, which effectively increases the concentration of CO2 at the site of carboxylation and therefore, presumably results in near saturation at current CO2 levels. As predicted, the C4 species were less responsive than the C3 species to elevated CO2 over the long-term with negligible effects on photosynthesis compared with a 10% average increase for all the C3 species combined. The legumes, unique in the ability to symbiotically fix N2, responded to the high N treatment oppositely in terms of net photosynthesis, A/gs, and tissue N relative to the other functional groups. However, species pooled by functional groups responded differently to elevated CO2 or high soil N treatments in relatively few cases, only for the C4 grasses and the legumes, and predominantly in the first year of growth under CO2 and N treatments. In a companion study by Reich et al. (2001b), which looked at plot level traits such as total biomass, total plant N, soil solution N and soil water on these same monoculture plots, it was also found that in general, C4 grasses were less responsive to elevated CO2 than all C3 species as a group and legumes were less responsive to the N treatment. Overall, the variation in species responsiveness to elevated CO2 and soil N supply was generally unrelated to their functional groupings.