Consequences of growth at two carbon dioxide concentrations and two temperatures for leaf gas exchange in Pascopyrum smithii (C3) and Bouteloua gracilis (C4)*

Continually rising atmospheric CO₂ concentrations and possible climatic change may cause significant changes in plant communities. This study was undertaken to investigate gas exchange in two important grass species of the short-grass steppe, Pascopyrum smithii (western wheat-grass), C₃, and Bouteloua gracilis (blue grama), C4, grown at different CO₂ concentrations and temperatures. Intact soil cores containing each species were extracted from grasslands in north-eastern Colorado, USA, placed in growth chambers, and grown at combinations of two CO₂ concentrations (350 and 700 μmol mol⁻¹) and two temperature regimes (field average and elevated by 4°C). Leaf gas exchange was measured during the second, third and fourth growth seasons. All plants exhibited higher leaf CO₂ assimilation rates (A) with increasing measurement CO₂ concentration, with greater responses being observed in the cool-season C₃ species P. smithii. Changes in the shape of intercellular CO₂ response curves of A for both species indicated photosynthetic acclimation to the different growth environments. The photosynthetic capacity of P. smithii leaves tended to be reduced in plants grown at high CO₂ concentrations, although A for plants grown and measured at 700μmol mol⁻¹ CO₂ was 41% greater than that in plants grown and measured at 350 μmol mol⁻¹ CO₂. Low leaf N concentration may have contributed to photosynthetic acclimation to CO₂. A severe reduction in photosynthetic capacity was exhibited in P. smithii plants grown long-term at elevated temperatures. As a result, the potential response of photosynthesis to CO₂ enrichment was reduced in P. smithii plants grown long-term at the higher temperature.     

Photosynthetic pathway and ontogeny affect water relations and the impact of CO2 on Bouteloua gracilis (C4) and Pascopyrum smithii (C3)

The eastern Colorado shortgrass steppe is dominated by the C₄ grass, Bouteloua gracilis, but contains a mixture of C₃ grasses as well, including Pascopyrum smithii. Although the ecology of this region has been extensively studied, there is little information on how increasing atmospheric CO₂ will affect it. This growth chamber study investigated gas exchange, water relations, growth, and biomass and carbohydrate partitioning in B. gracilis and P. smithii grown under present ambient and elevated CO₂ concentrations of 350 μl l⁻¹and 700 μl l⁻¹, respectively, and two deficit irrigation regimes. The experiment was conducted in soil-packed columns planted to either species over a 2-month period under summer-like conditions and with no fertilizer additions. Our objective was to better understand how these species and the functional groups they represent will respond in future CO₂-enriched environments. Leaf CO₂ assimilation (A _n), transpiration use efficiency (TUE, or A _n/transpiration), plant growth, and whole-plant water use efficiency (WUE, or plant biomass production/water evapotranspired) of both species were greater at elevated CO₂, although responses were more pronounced for P. smithii. Elevated CO₂ enhanced photosynthesis, TUE, and growth in both species through higher soil water content (SWC) and leaf water potentials (Ψ) and stimulation of photosynthesis. Consumptive water use was greater and TUE less for P. smithii than B. gracilis during early growth when soil water was more available. Declining SWC with time was associated with a steadily increased sequestering of total non-structural carbohydrates (TNCs), storage carbohydrates (primarily fructans for P. smithii) and biomass in belowground organs of P. smithii, but not B. gracilis. The root:shoot ratio of P. smithii also increased at elevated CO₂, while the root:shoot ratio of B. gracilis was unresponsive to CO₂. These partitioning responses may be the consequence of different ontogenetic strategies of a cool-season and warm-season grass entering a warm, dry summer period; the cool-season P. smithii responds by sequestering TNCs belowground in preparation for summer dormancy, while resource partitioning of the warm-season B. gracilis remains unaltered. One consequence of greater partitioning of resources into P. smithii belowground organs in the present study was maintenance of higher Ψ and A _n rates. This, along with differences in photosynthetic pathway, may have accounted for the greater responsiveness of P. smithii to CO₂ enrichment compared to B. gracilis. Received: 21 July 1997 / Accepted: 16 December 1997     

Enhanced root system C-sink activity,water relations and aspects of nutrient acquisition in mycotrophic Bouteloua gracilis subjected to CO2 enrichment

In order to better elucidate fixed-C partitioning, nutrient acquisition and water relations of prairie grasses under elevated [CO₂], we grew the C₄ grass Bouteloua gracilis (H.B.K.) lag ex Steud. from seed in soil-packed, column-lysimeters in two growth chambers maintained at current ambient [CO₂] (350 μL L⁻¹) and twice enriched [CO₂] (700 μL L⁻¹). Once established, plants were deficit irrigated; growth chamber conditions were maintained at day/night temperatures of 25/16°C, relative humidities of 35%/90% and a 14-hour photoperiod to simulate summer conditions on the shortgrass steppe in eastern Colorado. After 11 weeks of growth, plants grown under CO₂ enrichment had produced 35% and 65% greater total and root biomass, respectively, and had twice the level of vesicular-arbuscular mycorrhizal (VAM) infection (19.8% versus 10.8%) as plants grown under current ambient [CO₂]. The CO₂-enriched plants also exhibited greater leaf water potentials and higher plant water use efficiencies. Plant N uptake was reduced by CO₂ enrichment, while P uptake appeared little influenced by CO₂ regime. Under the conditions of the experiment, CO₂ enrichment increased root biomass and VAM infection via stimulated growth and adjustments in C partitioning below-ground. The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged. The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Gas Exchange and Carbohydrate and Nitrogen Concentrations in Leaves of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) at Different Carbon Dioxide Concentrations and Temperatures

Pascopyrum smithii (C₃) andBouteloua gracilis (C₄) are importantforage grasses native to the Colorado shortgrass steppe. Thisstudy investigated photosynthetic responses of these grassesto long-term CO₂enrichment and temperature in relation to leafnonstructural carbohydrate (TNC) and [N]. Glasshouse-grown seedlingswere transferred to growth chambers and grown for 49 d at twoCO₂concentrations (380 and 750 µmol mol^-1) at 20 and 35°C, and two additional temperatures (25 and 30 °C) at750 µmol mol^-1CO₂. Leaf CO₂exchange rate (CER) was measuredat a plant's respective growth temperature and at two CO₂concentrationsof approx. 380 and 700 µmol mol^-1. Long-term CO₂enrichmentstimulated CER in both species, although the response was greaterin the C₃,P. smithii . Doubling the [CO₂] from 380 to 750 µmolmol^-1stimulated CER ofP. smithii slightly more in plants grownand measured at 30 °C compared to plants grown at 20, 25or 35 °C. CO₂-enriched plants sometimes exhibited lowerCER when compared to ambient-grown controls measured at thesame [CO₂], indicating photosynthetic acclimation to CO₂growthregime. InP. smithii , such reductions in CER were associatedwith increases in TNC and specific leaf mass, reductions inleaf [N] and, in one instance, a reduction in leaf conductancecompared to controls. InB. gracilis , photosynthetic acclimationwas observed more often, but significant changes in leaf metabolitelevels from growth at different [CO₂] were generally less evident.Temperatures considered optimal for growth (C₃: 20 °C; C₄:35 °C) sometimes led to CO₂-induced accumulations of TNCin both species, with starch accumulating in the leaves of bothspecies, and fructans accumulating only inP. smithii. Photosynthesisof both species is likely to be enhanced in future CO₂-enrichedand warmer environments, although responses will sometimes beattenuated by acclimation. Acclimation; blue grama (Bouteloua gracilis (H.B.K.) Lag ex Steud.); leaf nitrogen concentration; nonstructural carbohydrates; photosynthesis; western wheatgrass (Pascopyrum smithii (Rydb.) Love)     

Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

An experiment was carried out to determine the effects of elevated CO₂, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C₄ perennial grass, or Pascopyrum smithii, a C₃ perennial grass, were placed in growth chambers with 350 or 700 μL L^?1 atmospheric CO₂, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO₂, with no significant difference between the C₃ and C₄ grass. The effect of elevated CO₂ on biomass was greatest at the intermediate water level. The positive effect of elevated CO₂ on shoot biomass was greater at normal temperatures in B. gracilis, and greater at elevated temperatures in P. smithii. Neither root-to-shoot ratio nor production of seed heads was affected by elevated CO₂. Plant tissue N and soil inorganic N concentrations were lower under elevated Co₂, but no more so in the C₃ than the C₄ plant. Elevated CO₂ appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO₂ effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO₂, in spite of greater N limitation.     

Growth and senescence in plant communities exposed to elevated CO2 concentrations on an estuarine marsh

Summary Three high marsh communities on the Chesapeake Bay were exposed to a doubling in ambient CO₂ concentration for one growing season. Open-top chambers were used to raise CO₂ concentrations ca. 340 ppm above ambient over monospecific communities of Scirpus olneyi (C₃) and Spartina patens (C₄), and a mixed community of S. olneyi, S. patens, and Distichlis spicata (C₄). Plant growth and senescence were monitored by serial, nondestructive censuses. Elevated CO₂ resulted in increased shoot densities and delayed sensecence in the C₃ species. This resulted in an increase in primary productivity in S. olneyi growing in both the pure and mixed communities. There was no effect of CO₂ on growth in the C₄ species. These results demonstrate that elevated atmospheric CO₂ can cause increased aboveground production in a mature, unmanaged ecosystem.     

Growth and Partitioning in Pascopyrum smithii (C3) and Bouteloua gracilis(C4) as Influenced by Carbon Dioxide and Temperature

This study investigated how CO₂and temperature affect dry weight(d.wt) accumulation, total nonstructural carbohydrate (TNC)concentration, and partitioning of C and N among organs of twoimportant grasses of the shortgrass steppe,Pascopyrum smithiiRydb. (C₃) andBouteloua gracilis(H.B.K.) Lag. ex Steud. (C₄).Treatment combinations comprised two temperatures (20 and 35°C)at two concentrations of CO₂(380 and 750 µmol mol^-1),and two additional temperatures of 25 and 30°C at 750 µmolmol^-1CO₂. Plants were maintained under favourable nutrient andsoil moisture and harvested following 21, 35, and 49d of treatment.CO₂-induced growth enhancements were greatest at temperaturesconsidered favourable for growth of these grasses. Comparedto growth at 380 µmol mol^-1CO₂, final d.wt of CO₂-enrichedP.smithiiincreased 84% at 20°C, but only 4% at 35°C. Finald.wt ofB. graciliswas unaffected by CO₂at 20°C, but wasenhanced by 28% at 35°C. Root:shoot ratios remained relativelyconstant across CO₂levels, but increased inP. smithiiwith reductionin temperature. These partitioning results were adequately explainedby the theory of balanced root and shoot activity. Favourablegrowth temperatures led to CO₂-induced accumulations of TNCin leaves of both species, and in stems ofP. smithii, whichgenerally reflected responses of above-ground d.wt partitioningto CO₂. However, CO₂-induced decreases in plant tissue N concentrationswere more evident forP. smithii. Roots of CO₂-enrichedP. smithiihadgreater total N content at 20°C, an allocation of N below-groundthat may be an especially important adaptation for C₃plants.Tissue N contents ofB. graciliswere unaffected by CO₂. Resultssuggest CO₂enrichment may lead to reduced N requirements forgrowth in C₃plants and lower shoot N concentration, especiallyat favourable growth temperatures. Acclimation to CO₂; blue grama; Bouteloua gracilis ; carbohydrate; climate change; global change; grass; growth; growth temperature optima; nitrogen; N uptake; Pascopyrum smithii; western wheatgrass     

Long-term photosynthetic response in single leaves of A C3 and C4 salt marsh species grown at elevated atmospheric CO2 in situ

Summary Mono-specific communities of the C₃ sedge, Scirpus olneyi and the C₄ grass, Spartina patens, were exposed to normal ambient or elevated CO₂, (ca. 680 l l^–1) throughout the 1987 and 1988 growing seasons in open-top field chambers located on a tidal marsh. Single stems of C₃ plants grown in ambient or elevated CO₂ showed an increased photosynthetic rate when tested at elevated CO₂ for both seasons. This increase in photosynthetic response in the C₃ species was maintained throughout the 1987 and 1988 growing season. The stimulation of photosynthesis with elevated CO₂ appeared to increase as temperature increased and decreased as photosynthetic photon flux (PPF) increased. Analysis of the photosynthetic response of the C₃ species during the 1988 season indicated that significant differences in light-saturated photosynthetic rate between ambient and elevated CO₂ conditions continued until October. In contrast to the C₃ sedge, the C₄ grass showed no significant photosynthetic increase to elevated CO₂ except at the beginning of the 1988 season.     

Performance of a generalist grasshopper on a C<Subscript>3</Subscript> and a C<Subscript>4</Subscript> grass: compensation for the effects of elevated CO<Subscript>2</Subscript> on plant nutritional quality

The increasing CO₂ concentration in Earths atmosphere is expected to cause a greater decline in the nutritional quality of C₃ than C₄ plants. As a compensatory response, herbivorous insects may increase their feeding disproportionately on C₃ plants. These hypotheses were tested by growing the grasses Lolium multiflorum C₃) and Bouteloua curtipendula C₄) at ambient (370 ppm) and elevated (740 ppm) CO₂ levels in open top chambers in the field, and comparing the growth and digestive efficiencies of the generalist grasshopper Melanoplus sanguinipes on each of the four plant × CO₂ treatment combinations. As expected, the nutritional quality of the C₃ grass declined to a greater extent than did that of the C₄ grass at elevated CO₂; protein levels declined in the C₃ grass, while levels of carbohydrates (sugar, fructan and starch) increased. However, M. sanguinipes did not significantly increase its consumption rate to compensate for the lower nutritional quality of the C₃ grass grown under elevated CO₂. Instead, these grasshoppers appear to use post-ingestive mechanisms to maintain their growth rates on the C₃ grass under elevated CO₂. Consumption rates of the C₃ and C₄ grasses were also similar, demonstrating a lack of compensatory feeding on the C₄ grass. We also examined the relative efficiencies of nutrient utilization from a C₃ and C₄ grass by M. sanguinipes to test the basis for the C₄ plant avoidance hypothesis. Contrary to this hypothesis, neither protein nor sugar was digested with a lower efficiency from the C₄ grass than from the C₃ grass. A novel finding of this study is that fructan, a potentially large carbohydrate source in C₃ grasses, is utilized by grasshoppers. Based on the higher nutrient levels in the C₃ grass and the better growth performance of M. sanguinipes on this grass at both CO₂ levels, we conclude that C₃ grasses are likely to remain better host plants than C₄ grasses in future CO₂ conditions.     

Effect at the ecosystem level of elevated atmospheric CO2in an aquatic microcosm

We studied the responses of an aquatic microcosm in two different eutrophic conditions to elevated atmospheric CO₂concentration. We used microcosms, consisting of Escherichia coli(bacteria), Tetrahymena thermophila(protozoa) and Euglena gracilis(algae), in salt solution with 50 and 500 mg l^–1of proteose peptone (eutrophic and hypereutrophic conditions, respectively) under ambient and elevated CO₂(1550±100 l l^–1) conditions. The density of E. gracilisincreased significantly under elevated CO₂in both eutrophic and hypereutrophic microcosms. In the eutrophic microcosm, the other elements were not affected by elevated CO₂. In the hypereutrophic microcosm, however, the concentrations of ammonium and phosphate decreased significantly under elevated CO₂. Furthermore, the density of T. thermophilawas maintained in higher level than that in the microcosm with ambient CO₂and the density of E. coliwas decreased by CO₂enrichment. Calculating the carbon biomasses of T. thermophilaand E. colifrom their densities, the changes in their biomasses by CO₂enrichment were little as compared with large increase of E. graciliscarbon biomass converted from chlorophyll a. From the responses to elevated CO₂in the subsystems of the hypereutrophic microcosm consisting of either one or two species, the increase of E. graciliswas a direct effect of elevated CO₂, whereas the changes in the density of E. coliand T. thermophilaand the decreases in the concentration of ammonium and phosphate are considered to be indirect effects rather than direct effects of elevated CO₂. The indirect effects of elevated CO₂were prominent in the hypereutrophic microcosm.     

Genetic variability of Bouteloua gracilis populations differing in forage production at the southernmost part of the North American Graminetum

Bouteloua gracilis (blue grama grass) native populations have been shown to be highly variable, however the genetic basis of this variability has not been well established. Determining the extent of genetic variability within and among plant populations have important repercussions for the management and conservation of species, and in particular for those subjected to intensive use such as forage plants. Using RAPD, this study was undertaken to investigate the genetic variability of four B. gracilis native populations developed in three grasslands and one shrubland at the southernmost part of the North American Graminetum in México. Significant differences in grass aboveground production were found among the study sites, while considerable genetic variation within each of the four blue grama populations evaluated was detected. The molecular analysis, based on 55 individuals, revealed a total of 108 scorable repeatable bands, with 99 of them being polymorphic (overall polymorphism= 91.7%). Within every population each individual was genetically distinct and no population-specific bands (fixed marker differences) were identified. Pair-wise Φ _ST comparisons indicated that the four blue grama populations examined were significantly different in their genetic constitution (P<0.001). AMOVA revealed that most of the genetic variation detected in Bouteloua gracilis was explained by intra- (88.53%), rather than by inter-population (11.47%) differences. UPGMA based on the Φ _ST values indicated that the blue grama population collected from the shrubland displayed the RAPD profiles that most differed among the study sites. Possible causes of these results could reside on intensive grazing reducing, and proper management conserving, the forage production and genetic diversity of blue grama native populations. Our results are consistent with previous studies analyzing population genetic variation in outcrossing grasses and, in particular, with ecological and cytological evidence for a high genetic variability in native populations of B. gracilis. The implications of our findings and prospective studies to be undertaken using molecular tools in the study of blue grama biology and ecology are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Field measurements of photosynthesis,water-use efficiency,and growth inAgropyron smithii (C3) andBouteloua gracilis (C4) in the Colorado shortgrass steppe

Summary Field measurements of gas exchange and growth were conducted on a C₃ grass,Agropyron smithii, and a C₄ grass,Bouteloua gracilis, in order to further establish the adaptive significance of the C₄ pathway under natural conditions. Maximum rates of leaf area expansion in tillers and maximum seasonal photosynthesis rates of both species occurred during the cool, early summer month of June. The occurrence of maximum growth and photosynthesis inB. gracilis during this cool period was apparently related to its occupation of warm microenvironments next to the ground surface. As temperatures increased during the midsummer, photosynthesis rates decreased to 47% and 55% of the seasonal maximum inB. gracilis andA. smithii, respectively. Water-use efficiencies in both species were similar or slightly higher forB. gracilis during June, the period of maximum growth. By mid-July, however, leaves of the C₃ grass,A. smithii, exhibited water-use efficiencies approximately half as high asB. gracilis. These differences in water-use efficiency were the result of differences in stomatal conductance, rather than differences in daily CO₂ uptake rates which were similar in both species. The results demonstrate that in certain environments there are no offset periods of growth and maximum photosynthesis during the growing season in these C₃ and C₄ species. The greater amounts of daily water use inA. smithii during the midsummer might contribute to its much greater abundance in lowland sites in the shortgrass steppe. The C₄ grass,B. gracilis, occurs in dry upland sites in addition to the more mesic lowland sites.     

The quantum yield for CO2 uptake in C3 and C4 grasses

The quantum yield for CO₂ uptake was measured in C₃ and C₄ monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O₂ and 340 cm³ m^-3 CO₂, values were very similar in C₃ monocots, C₃ dicots, and C₄ monocots (0.045–0.056 mole CO₂ · mole^-1 quanta absorbed). In the presence of 2% O₂ and 800 cm³ m^-3 CO₂, enhancements of the quantum yield values occurred for the C₃ plants (both monocots and dicots), but not for C₄ monocots. A dependence of the quantum yield on leaf temperature was observed in the C₃ grass, Agropyron smithii, but not in the C₄ grass, Bouteloua gracilis, in 21% O₂ and 340 cm³ m^-3 CO₂. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.     

Nitrogen and carbon dynamics in C3 and C4 estuarine marsh plants grown under elevated CO2 in situ

Summary Carbon dioxide concentrations were elevated in three estuarine communities for an entire growing season. Open top chambers were used to raise CO₂ concentrations ca. 336 ppm above ambient in monospecific communities of Scirpus olneyi (C₃) and Spartina patens (C₄), and a mixed community of S. olneyi, S. patens and Distichlis spicata (C₄). Nitrogen and carbon concentration (% wt) of aboveground tissue was followed throughout growth and senescence. Green shoot %N was reduced and %C was unchanged under elevated CO₂ in S. olneyi. This resulted in a 20%–40% increase in tissue C/N ratio. There was no effect of CO₂ on either C₄ species. Maximum aboveground N (g/m²) was unchanged in S. olneyi, indicating that increased productivity under elevated CO₂ was dependent on reallocation of stored N. There was no change in the N recovery efficiency of S. olneyi in pure stand and a decrease in the mixed community. Litter C/N ratio was not affected by elevated CO₂ suggesting that decomposition and N mineralization rates will also remain unchanged. Continued growth responses to elevated CO₂ could, however, be limited by the ability of S. olneyi to increase the total aboveground N pool.     

Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.

Six open‐top chambers were installed on the shortgrass steppe in north‐eastern Colorado, USA from late March until mid‐October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO₂. Three chambers were maintained at current CO₂ concentration (ambient treatment), three at twice ambient CO₂, or approximately 720 μmol mol^?1 (elevated treatment), and three nonchambered plots served as controls. Above‐ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C₃ and C₄ grasses. Mid‐season and seasonal above‐ground productivity were enhanced from 26 to 47% at elevated CO₂, with no differences in the relative responses of C₃/C₄ grasses or forbs. Annual above‐ground phytomass accrual was greater on plots which were defoliated once in mid‐summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO₂ on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C₃) and Bouteloua gracilis (C₄) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO₂‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO₂, but only Stipa comata (C₃) plants exhibited significant reductions in N under elevated compared to ambient CO₂ chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above‐ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO₂ enriched environments.     

Response of 12 weedy species to elevated CO2 in low-phosphorus-availability soil

We conducted an experiment on responses of weedy species from an orchard ecosystem to elevated CO₂ (700–800 μmol mol⁻¹) under low phosphorus (P) soil in an environment-controlled growth chamber. Twelve local weedy species, Poa annua L., Lolium perenne L., Avena fatua L., Vicia cracca L., Medicago lupulina L., Kummerowia striata (Thunb.) Schindl., Veronica didyma Ten., Plantago virginica L., Gnaphalium affine D.Don., Echinochloa crusgalli var. mitis (L.) Beauv., Eleusine indica (L.) Gaertn. and Setaria glauca (L.) P. Beauv., grouped into four functional groups (C₃ grass, C₃ forb, legume and C₄ grass), were used in the experiment. The total plant biomass, P uptake, and mycorrhizal colonization were measured. The results showed that the total biomass of the 12 weedy species tended to increase under elevated CO₂. But changes in the total biomass under elevated CO₂ significantly differed among functional groups: legumes showed the greatest increase in the total biomass of all functional groups, following the order C₃ forbs > C₄ grasses > C₃ grasses. Elevated CO₂ significantly increased mycorrhizal colonization and P uptake of legumes, C₃ forbs and C₄ grasses but did not change C₃ grasses. Positive correlations between mycorrhizal colonization and shoot P concentration, and between total P uptake and total biomass were found under elevated CO₂. The results suggested that the interspecific difference in CO₂ response at low P availability was caused by the difference in CO₂ response in mycorrhizae and P uptake. These differences among species imply that plant interaction in orchard ecosystems may change under future CO₂ enrichment.     

Soil water dynamics,transpiration, and water losses in a crested wheatgrass and native shortgrass ecosystem

The status of water in soil and vegetation was monitored in a stand of crested wheatgrass (Agropyron cristatum) and a nearby shortgrass steppe during a growing season. This was done to determine if water use and losses were similar among two very different communities in a similar climate. Precipitation was similar throughout the study period for both the crested wheatgrass and native shortgrass communities. However, the native shortgrass community with greater root biomass had consistently greater soil water depletion in the deeper soil horizons than was found in the crested wheatgrass community. Greater depletion of soil water by native shortgrass species suggests that they might be more competitive than crested wheatgrass in a water-limited environment.Crested wheatgrass maintained high leaf water potential early in the season, but lower water potential during the latter part of the growing season as compared with the major species of the shortgrass steppe, blue grama (Bouteloua gracilis) and western wheatgrass (Agropyron smithii). Leaf conductance was lower for crested wheatgrass than for the native grasses during the later part of the growing season. Consequently, seasonal transpiration for crested wheatgrass was lower when compared with blue grama or western wheatgrass. Lower conductance allowed crested wheatgrass to maintain relatively high internal water potential and may have accounted for less soil water use at deeper soil depths during the latter part of the growing season.Water loss through transpiration was less for western wheatgrass than for either blue grama or crested wheatgrass because western wheatgrass had less leaf area. However, western wheatgrass was as efficient as the other species in its use of water. Crested wheatgrass transpired more water than blue grama early in the growing season, but less than either native species for the remainder of the growing season. Estimated seasonal transpiration loss was greater in the shortgrass ecosystem than in the established crested wheatgrass stand.     

Effects of low and elevated CO2 on C3 and C4 annuals

Abutilon theophrasti (C₃) and Amaranthus retroflexus (C₄), were grown from seed at four partial pressures of CO₂: 15 Pa (below Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) in the Duke Phytotron under high light, high nutrient, and wellwatered conditions to evaluate their photosynthetic response to historic and future levels of CO₂. Net photosynthesis at growth CO₂ partial pressures increased with increasing CO₂ for C₃ plants, but not C₄ plants. Net photosynthesis of Abutilon at 15 Pa CO₂ was 70% less than that of plants grown at 35 Pa CO₂, due to greater stomatal and biochemical limitations at 15 Pa CO₂. Relative stomatal limitation (RSL) of Abutilon at 15 Pa CO₂ was nearly 3 times greater than at 35 Pa CO₂. A photosynthesis model was used to estimate ribulose-1,5-bisphosphate carboxylase (rubisco) activity (Vc_max), electron transport mediated RuBP regeneration capacity (J _max), and phosphate regeneration capacity (PiRC) in Abutilon from net photosynthesis versus intercellular CO₂ (A–C _i) curves. All three component processes decreased by approximately 25% in Abutilon grown at 15 Pa compared with 35 Pa CO₂. Abutilon grown at 15 Pa CO₂ had significant reductions in total rubisco activity (25%), rubisco content (30%), activation state (29%), chlorophyll content (39%), N content (32%), and starch content (68%) compared with plants grown at 35 Pa CO₂. Greater allocation to rubisco relative to light reaction components and concomitant decreases in J _max and PiRC suggest co-regulation of biochemical processes occurred in Abutilon grown at 15 Pa CO₂. There were no significant differences in photosynthesis or leaf properties in Abutilon grown at 27 Pa CO₂ compared with 35 Pa CO₂, suggesting that the rise in CO₂ since the beginning of the industrial age has had little effect on the photosynthetic performance of Abutilon. For Amaranthus, limitations of photosynthesis were balanced between stomatal and biochemical factors such that net photosynthesis was similar in all CO₂ treatments. Differences in photosynthetic response to growth over a wide range of CO₂ partial pressures suggest changes in the relative performance of C₃ and C₄ annuals as atmospheric CO₂ has fluctuated over geologic time.     

Genotype-specific response of a lycaenid herbivore to elevated carbon dioxide and phosphorus availability in calcareous grassland

Effects of elevated CO₂ and P availability on plant growth of the legume Lotus corniculatus and consequences for the butterfly larvae of Polyommatus icarus feeding on L. corniculatus were investigated in screen-aided CO₂ control chambers under natural conditions on a calcareous grassland in the Swiss Jura mountains. Elevated CO₂ conditions and P fertilisation increased the biomass production of L. corniculatus plants and affected the plant chemical composition. CO₂ enrichment increased the C/N ratio and sugar concentration and decreased the N and P concentrations. C- and N-based allelochemicals (cyanoglycosides, total polyphenols and condensed tannins) were only marginally affected by CO₂ enrichment. P fertilisation increased the specific leaf area and concentrations of water, N, sugar and P, while the C/N ratio and the concentration of total polyphenols decreased. Furthermore, P availability marginally enhanced the effect of elevated CO₂ on the total dry mass and sugar concentration while the opposite occurred for the total polyphenol concentration. The changes in food-plant chemistry as a result of P fertilisation positively affected larval mass gain and accelerated the development time of P. icarus. Only a marginal negative effect on larval mass gain was found for CO₂ enrichment. However, we found genotype-specific responses in the development time of P. icarus to elevated CO₂ conditions. Larvae originating from different mothers developed better either under elevated CO₂ or under ambient CO₂ but some did not react to CO₂ elevation. As far as we know this is the first finding of a genotype-specific response of an insect herbivore to elevated CO₂ which suggests genetic shifts in insect life history traits in response to elevated CO₂.     

Growth, gas exchange, leaf nitrogen and carbohydrate concentrations in NAD‐ME and NADP‐ME C4 grasses grown in elevated CO2

Plants with the C₄ photosynthetic pathway have predominantly one of three decarboxylation enzymes in their bundle sheath cells. Within the grass family (Poaceae) bundle sheath leakiness to CO₂ is purported to be lowest in the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME, EC 1.1.1.40) group, highest in the NAD-ME (EC 1.1.1.39) group and intermediate in the phosphoenolpyruvate carboxykinase (PCK, EC 4.1.1.32) group. We investigated the hypothesis that growth and photosynthesis of NAD-ME C₄ grasses would respond more to elevated CO₂ treatment than NADP-ME grasses. Plants were grown in 8-1 pots in growth chambers with ample water and fertilizer for 39 days at a continuous CO₂ concentration of either 350 or 700 µl l^?1. NAD-ME species included Bouteloua gracilis Lag. ex Steud (Blue grama), Buchloe dactyloides (Nutt.) Engelm. (Buffalo grass) and Panicum virgatum L. (Switchgrass) and the NADP-ME species were Andropogon gerardii Vittman (Big bluestem), Schizachyrium scoparium (Michx.) Nash (Little bluestem), and Sorghastrum nutans (L.) Nash (Indian grass). Contrary to our hypothesis, growth of the NADP-ME grasses was generally greater under elevated CO₂ (significant for A. gerardii and S. nutans), while none of the NAD-ME grasses had a significant growth response. Increased leaf total non-structural carbohydrate (TNC) was associated with greater growth responses of NADP-ME grasses. Decreased leaf nitrogen in NADP-ME species grown at elevated CO₂ was found to be an artifact of TNC dilution. Assimilation (A) vs intercellular CO₂ (C_i) curves revealed that leaf photosynthesis was not saturated at 350 µl l^?1 CO₂ in any of these C₄ grasses. Assimilation of elevated CO₂-grown A. gerardii was higher than in plants grown in ambient CO₂. In contrast, B. gracilis grown in elevated CO₂ displayed lower A, a trait more commonly reported in C₃ plants. Photosynthetic acclimation in B. gracilis was not related to leaf TNC or nitrogen concentrations, but A:C_i curves suggest a reduction in activity of both phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Some adaptation of stomatal functioning was also seen in B. gracilis and A. gerardii leaves grown in elevated CO₂. Our study shows that C₄ grasses have the capacity for increased growth and photosynthesis under elevated CO₂ even when water and nutrients are non-limiting. While it was the NADP-ME species which had significant responses in the present study, we have previously reported significant growth increases in elevated CO₂ for B. gracilis.