Performance of the legume-feeding herbivore, Colias philodice (Lepidoptera: Pieridae) is not Affected by Elevated CO2

Presumably due to their association with nitrogen-fixing bacteria, the nutritional quality of legumes decreases less than that of non-legume C₃ plants when grown under elevated atmospheric CO₂. Therefore, it seems likely that legume-feeding herbivores will be less adversely affected than herbivores of non-legume C₃ plants by anthropogenic increases in atmospheric CO₂. When the legumes Medicago sativa (alfalfa), Trifolium repens (white clover), and Lotus corniculatus (birdsfoot trefoil) were grown under elevated (756 ppm) CO₂, leaf nitrogen remained the same or increased, and C:N ratio did not change. Unlike most insects fed non-legume C₃ plants, Colias philodice (sulfur butterfly) larvae fed elevated-grown M. sativa and T. repens did not exhibit reduced relative growth rate (RGR), and larvae fed elevated-grown L. corniculatus exhibited a nearly significant 37% increase in RGR. Pupal weight was unaffected by growth of host plants under elevated CO₂. Relative nitrogen growth rate (RGRN) did not change for larvae fed elevated-grown M. sativa or T. repens, but increased by 34% for larvae fed elevated-grown L. corniculatus. These results suggest that legume-feeding herbivores will be relatively buffered against the adverse effects of elevated CO₂ typically experienced by herbivores of non-legume C₃ plants.     

Photosynthetic and growth response to fumigation with SO2 at elevated CO2 for C3 and C4 plants

Summary Six early successional plant species with differing photosynthetic pathways (3 C₃ species and 3 C₄ species) were grown at either 300, 600, or 1,200 ppm CO₂ and at either 0.0 or 0.25 ppm SO₂. Total plant growth increased with CO₂ concentration for the C₃ species and varied only slightly with CO₂ for the C₄ species. Fumigation with SO₂ caused reduced growth of the C₃ species at 300 ppm CO₂ but not at the higher concentrations of CO₂. Fumigation with SO₂ reduced growth of the C₄ species at high CO₂ and increased growth at 300 ppm CO₂. Leaf area increased with increasing CO₂ for all plant species. Fumigation with SO₂ reduced leaf area of C₃ plants more at low CO₂ than at high CO₂ while leaf area of C₄ plants was reduced more at high CO₂ than at low CO₂. These results support the notion that C₃ species are more sensitive to SO₂ fumigation than are C₄ species at concentrations of CO₂ equal to that found in normal ambient air. However, the difference in sensitivity to SO₂ between C₃ and C₄ species was found to be reversed at higher concentrations of CO₂. A possible explanation for this reversal based upon differences in stomatal response to elevated CO₂ between C₃ and C₄ species is discussed.     

Biomass responses to intraspecific competition differ between wild species and CO2

Aims: How growth of wild and crop species responds to global environmentalperturbations has both ecological and agricultural significancein a changing world. The primary aim of this synthesis was toquantitatively assess the interactive effects of intraspecificcompetition and elevated CO₂ on biomass production in herbaceousspecies. Methods: Using meta-analytical techniques, we synthesized data from publicationsbefore 2006 that reported biomass responses to elevated CO₂in 321 herbaceous species grown in isolation or in competitionwith con-specific individuals. Important findings: Intraspecific competition differentially modified biomass responsesto elevated CO₂ in wild and crop species. For example, competitionreduced CO₂ stimulation of total biomass (W_T) from 27 to 23%in wild species, but by a much greater magnitude, i.e., from43 to 32% in crops. Competition had no effect on responses ofeither above- (W_AG) or below-ground (W_BG) biomass to elevatedCO₂ in wild species, but significantly diminished CO₂ enhancementof W_AG, although not of W_BG, in crops. Considerable variationswere found among functional groups in the modification of growthresponses to elevated CO₂ by intraspecific competition, whichexerted greater depression on CO₂ enhancement in C₃ than inC₄ species and in non-legumes than in legumes. Elevated CO₂affected leaf and stem growth of individually grown C₃ graminoidsand forbs similarly, but increased leaf growth only in C₄ graminoidsand stem growth only in C₄ forbs. Results from this synthesisdemonstrated that intraspecific competition differentially affectedgrowth responses to elevated CO₂ in wild and crop species. Thewild–crop species differences will have important implicationsfor understanding primary production by herbaceous species inboth natural and agricultural ecosystems in the future whenatmospheric CO₂ is significantly higher than the current level.     

PHOTOSYNTHETIC CHARACTERISTICS OF C3-C4 INTERMEDIATE FLAVERIA FLORIDANA (ASTERACEAE) IN NATURAL HABITATS: EVIDENCE OF ADVANTAGES TO C3-C4 PHOTOSYNTHESIS AT HIGH LEAF TEMPERATURES

Measurements of leaf gas exchange were conducted in situ for the C₃-C₄ intermediate plant Flaveria floridana. Leaves exhibited measurable CO₂ assimilation at atmospheric CO₂ concentrations as low as 20 μmol/mol. This result demonstrates that the low CO₂ compensation points observed in past studies of greenhouse-grown C₃-C₄ intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO₂ concentrations as low as 112 μmol/mol. The existence of near-maximum photosynthesis rates at such low intercellular CO₂ concentrations is interpreted as evidence for the existence of a CO₂-concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO₂ concentration as the leaf-to-air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C₃ plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C₃ species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C₃-C₄ intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C₃ plants.     

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C₄ species can be stimulated by increasing CO₂ concentration, [CO₂], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO₂ leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO₂]. Three C₄ species that differed in the reported degree of bundle sheath leakiness to CO₂, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO₂] of 350 μl l^?1 (ambient) or 700 μl l^?1 (elevated). Assimilation as a function of increasing [CO₂] at high photosynthetic photon flux density (PPFD, 1 600 μmol m^?2 s^?1) indicated that leaf photosynthesis was not saturated under current ambient [CO₂] for any of the three C₄ species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO₂] was light dependent for all three C₄ species. The stimulation of leaf photosynthesis at elevated [CO₂] was not associated with previously published values of CO₂ leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO₂] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C₄ species can respond directly to increased [CO₂] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.     

Interactive effects of atmospheric CO2 enrichment,salinity and flooding on growth of C3 (Elymus athericus) and C4 (Spartina anglica) salt marsh species

The growth response of Dutch salt marsh species (C₃ and C₄) to atmospheric CO₂ enrichment was investigated. Tillers of the C₃ speciesElymus athericus were grown in combinations of 380 and 720 11^-1 CO₂ and low (O) and high (300 mM NaCl) soil salinity. CO₂ enrichment increased dry matter production and leaf area development while both parameters were reduced at high salinity. The relative growth response to CO₂ enrichment was higher under saline conditions. Growth increase at elevated CO₂ was higher after 34 than 71 days. A lower response to CO₂ enrichment after 71 days was associated with a decreased specific leaf area (SLA). In two other experiments the effect of CO₂ (380 and 720 11^-1) on growth of the C₄ speciesSpartina anglica was studied. In the first experiment total plant dry weight was reduced by 20% at elevated CO₂. SLA also decreased at high CO₂. The effect of elevated CO₂ was also studied in combination with soil salinity (50 and 400 mM NaCl) and flooding. Again plant weight was reduced (10%) at elevated CO₂, except under the combined treatment high salinity/non-flooded. But these effects were not significant. High salinity reduced total plant weight while flooding had no effect. Causes of the salinity-dependent effect of CO₂ enrichment on growth and consequences of elevated CO₂ for competition between C₃ and C₄ species are discussed.     

Morphological and anatomical determinants of mesophyll conductance in wild relatives of tomato (Solanum sect. Lycopersicon,sect. Lycopersicoides; Solanaceae)

Natural selection on photosynthetic performance is a primary factor determining leaf phenotypes. The complex CO₂ diffusion path from substomatal cavities to the chloroplasts – the mesophyll conductance (g_m) – limits photosynthetic rate in many species and hence shapes variation in leaf morphology and anatomy. Among sclerophyllous and succulent taxa, structural investment in leaves, measured as the leaf dry mass per area (LMA), has been implicated in decreased g_m. However, in herbaceous taxa with high g_m, it is less certain how LMA impacts CO₂ diffusion and whether it significantly affects photosynthetic performance. We addressed these questions in the context of understanding the ecophysiological significance of leaf trait variation in wild tomatoes, a closely related group of herbaceous perennials. Although g_m was high in wild tomatoes, variation in g_m significantly affected photosynthesis. Even in these tender‐leaved herbaceous species, greater LMA led to reduced g_m. This relationship between g_m and LMA is partially mediated by cell packing and leaf thickness, although amphistomy (equal distribution of stomata on both sides of the leaf) mitigates the effect of leaf thickness. Understanding the costs of increased LMA will inform future work on the adaptive significance of leaf trait variation across ecological gradients in wild tomatoes and other systems.     

Host-specific aphid population responses to elevated CO2 and increased N availability

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO₂. Recent models predict that increased nitrogen will increase aphid population size under elevated CO₂, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO₂, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C₃ plant (Solanum dulcamara), and a C₄ plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO₂, with the degree of increase depending on the N availability. We found a significant CO₂× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO₂, low N conditions increased in response to either high N availability or elevated CO₂. No population size responses were observed for aphids infesting A. viridis. Elevated CO₂ increased plant biomass, specific leaf weight, and C : N ratios of the C₃ plant, S. dulcamara but did not affect the C₄ plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO₂ decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO₂ or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity.     

An examination of the advantages of C3-C4 intermediate photosynthesis in warm environments

Abstract. The photosynthetic responses to temperature in C₃, C₃-C₄ intermediate, and C₄ species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C₃-C₄ intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO₂ compensation points at all temperatures examined in the C₃-C₄ intermediate, Flaveria floridana, compared to the C₃ species, F. cronquistii. The C₃-C₄ intermediate, F. floridana, exhibited a C₃-like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C₃ species, F. cronquistii. Using models of C₃ and C₃-C₄ intermediate photosynthesis, it was predicted that by recycling photorespired CO₂ in bundle-sheath cells, as occurs in many C₃-C₄ intermediates, photosynthesis rates at 35°C could be increased by 28%, compared to a C₃ plant. Without recycling photorespired CO₂, it was calculated that in order to improve photosynthesis rates at 35°C by this amount in C₃ plants, (1) intercellular CO₂ partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water-use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen-use efficiency. In addition to the recycling of photorespired CO₂, leaves of F. floridana appear to effectively concentrate CO₂ at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO₂-concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35°C. The C₃-C₄ intermediate, F. ramosissima, exhibited a photosynthetic temperature temperature response curve that was more similar to the C₄ species, F. trinervia, than the C₃ species, F. cronquistii. The C₄-like pattern is probably related to the advanced nature of C₄-like biochemical traits in F. ramosissima The results demonstrate that reductions in photorespiration rates in C₃-C₄ intermediate plants create photosynthetic advantages at warm leaf temperatures that in C₃ plants could only be achieved through substantial costs to water-use efficiency and/or nitrogen-use efficiency.     

Gas exchange responses of Chesapeake Bay tidal marsh species under field and laboratory conditions

Summary Laboratory and field gas exchange measurements were made on C₃ (Scirpus olneyi Gray) and C₄ (Spartina patens (Ait.) Mahl., Distichlis spicata (L.) Green) species from an irregularly flooded tidal marsh on the Chesapeake Bay. Laboratory measurements were made on plants grown from root stocks that were transplanted to a greenhouse and grown under high light and high nutrient conditions. The two C₄ species were similar in their laboratory gas exchange characteristics: both had higher net carbon exchange rates, higher mesophyll conductances, higher photosynthetic temperature optima and lower leaf conductances than the C₃ species. The laboratory photosynthetic water use efficiency of the C₄ species was approximately three times that of the C₃ species.Field gas exchange responses of the above species were measured in situ a Chesapeake Bay tidal marsh. Despite differences in biological potential measured in the laboratory, all three species had similar in situ carbon exchange rates on a leaf area basis. On a dry weight basis, leaves of the two C₄ species had about 1.4 times higher light saturated CO₂ assimilation rates than the C₃ species. Light saturation of CO₂ exchange occurred at photosynthetic photon flux densities of 80 n Einstein cm^-2s^-1, compared with 160 n Einstein cm ^-2s^-1 in the laboratory grown plants. Spartina patens and Scirpus olneyi had similar daily CO₂ assimilation rates, but the daily transpiration rate of the C₃ species was almost twice that of the C₄ species. Spartina patens showed greater seasonal decrease in photosynthesis than Distichlis spicata and Scirpus olneyi. The two C₄ grass species maintained higher mesophyll conductances and photosynthetic water use efficiencies than the C₄ sedge.     

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.     

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.     

Assimilation,respiration and allocation of carbon inPlantago major as affected by atmospheric CO2 levels

The response ofPlantago major ssp,pleiosperma plants, grown on nutrient solution in a climate chamber, to a doubling of the ambient atmospheric CO₂ concentration was investigated. Total dry matter production was increased by 30% after 3 weeks of exposure, due to a transient stimulation of the relative growth rate (RGR) during the first 10 days. Thereafter RGR returned to the level of control plants. Photosynthesis, expressed per unit leaf area, was stimulated during the first two weeks of the experiment, thereafter it dropped and nearly reached the level of the control plants. Root respiration was not affected by increased atmospheric CO₂ levels, whereas shoot, dark respiration was stimulated throughout the experimental period. Dry matter allocation over leaves stems and roots was not affected by the CO₂ level. SLA was reduced by 10%, which can partly be explained by an increased dry matter content of the leaves. Both in the early and later stages of the experiment, shoot respiration accounted for a larger part of the carbon budget in plants grown at elevated atmospheric CO₂. Shifts in the total carbon budget were mainly due to the effects on shoot respiration. Leaf growth accounted for nearly 50% of the C budget at all stages of the experiment and in both treatments.Abbreviations LAR leaf area ratio - LWR leaf weight ratio - RGR relative growth rate - R/S root to shoot ratio - RWR root weight ratio - SLA specific leaf area - SWR stem weight ratio     

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.     

Physiological and growth responses of two African species, Acacia karroo and Themeda triandra, to combined increases in CO2 and UV-B radiation

The interactive effects of increased carbon dioxide (CO₂) concentration and ultraviolet-B (UV-B, 280–320 nm) radiation on Acacia karroo Hayne, a C₃ tree, and Themeda triandra Forsk., a C₄ grass, were investigated. We tested the hypothesis that A. karroo would show greater CO₂-induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C₄ grasslands, but that increased UV-B could mitigate this advantage. Seedlings were grown in open-top chambers in a greenhouse in ambient (360 μmol mol^-1) and elevated (650 μmol mol^-1) CO₂, combined with ambient (1.56 to 8.66 kJ m^-2 day^-1) or increased (2.22 to 11.93 kJ m^-2 day^-1) biologically effective (weighted) UV-B irradiances. After 30 weeks, elevated CO₂ had no effect on biomass of A. karroo, despite increased net CO₂ assimilation rates. Interaction between UV-B and CO₂ on stomatal conductance was found, with conductances decreasing only where elevated CO₂ and UV-B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO₂. Elevated UV-B caused only an increase in foliar carbon concentrations. In T. triandra, net CO₂ assimilation rates were unaffected in elevated CO₂, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO₂. and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO₂. Significantly moderating effects of elevated UV-B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO₂-induced growth responses of the species therefore do not support the hypothesis of CO₂-driven woody encroachment of C₄ grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV-B radiation is unlikely to substantially alter the CO₂ response of these species.     

Effect of soil drying on growth,biomass allocation and leaf gas exchange of two annual grass species

Influence of short-term water stress on plant growth and leaf gas exchange was studied simultaneously in a growth chamber experiment using two annual grass species differing in photosynthetic pathway type, plant architecture and phenology:Triticum aestivum L. cv. Katya-A-1 (C₃, a drought resistant wheat cultivar of erect growth) andTragus racemosus (L.) All. (C₄, a prostrate weed of warm semiarid areas). At the leaf level, gas exchange rates declined with decreasing soil water potential for both species in such a way that instantaneous photosynthetic water use efficiency (PWUE, mmol CO₂ assimilated per mol H₂O transpired) increased. At adequate water supply, the C₄ grass showed much lower stomatal conductance and higher PWUE than the C₃ species, but this difference disappeared at severe water stress when leaf gas exchange rates were similarly reduced for both species. However, by using soil water more sparingly, the C₄ species was able to assimilate under non-stressful conditions for a longer time than the C₃ wheat did. At the whole-plant level, decreasing water availability substantially reduced the relative growth rate (RGR) ofT. aestivum, while biomass partitioning changed in favour of root growth, so that the plant could exploit the limiting water resource more efficiently. The change in partitioning preceded the overall reduction of RGR and it was associated with increased biomass allocation to roots and less to leaves, as well as with a decrease in specific leaf area. Water saving byT. racemosus sufficiently postponed water stress effects on plant growth occurring only as a moderate reduction in leaf area enlargement. For unstressed vegetative plants, relative growth rate of the C₄ T. racemosus was only slightly higher than that of the C₃ T. aestivum, though it was achieved at a much lower water cost. The lack of difference in RGR was probably due to growth conditions being relatively suboptimal for the C₄ plant and also to a relatively large investment in stem tissues by the C₄ T. racemosus. Only 10% of the plant biomass was allocated to roots in the C₄ species while this was more than 30% for the C₃ wheat cultivar. These results emphasize the importance of water saving and high WUE of C₄ plants in maintaining growth under moderate water stress in comparison with C₃ species.     

The impact of elevated CO2 on growth and photosynthesis in Agrostis canina L. ssp. monteluccii adapted to contrasting atmospheric CO2 concentrations

 The aim of this study was to characterise growth and photosynthetic capacity in plants adapted to long-term contrasting atmospheric CO₂ concentrations (C _a). Seeds of Agrostis canina L. ssp. monteluccii were collected from a natural CO₂ transect in central-western Italy and plants grown in controlled environment chambers at both ambient and elevated CO₂ (350 and 700 μmol mol⁻¹) in nutrient-rich soil. Seasonal mean C _a at the source of the plant material ranged from 610 to 451 μmol CO₂ mol⁻¹, derived from C₄ leaf stable carbon isotope discrimination (δ¹³C). Under chamber conditions, CO₂ enrichment stimulated the growth of all populations. However, plants originating from elevated C _a exhibited higher initial relative growth rates (RGRs) irrespective of chamber CO₂ concentrations and a positive relationship was found between RGR and C _a at the seed source. Seed weight was positively correlated with C _a, but differences in seed weight were found to explain no more than 34% of the variation in RGRs at elevated CO₂. Longer-term experiments (over 98 days) on two populations originating from the extremes of the transect (451 and 610 μmol CO₂ mol⁻¹) indicated that differences in growth between populations were maintained when plants were grown at both 350 and 700 μmol CO₂ mol⁻¹. Analysis of leaf material revealed an increase in the cell wall fraction (CWF) in plants grown at elevated CO₂, with plants originating from high C _a exhibiting constitutively lower levels but a variable response in terms of the degree of lignification. In vivo gas exchange measurements revealed no significant differences in light and CO₂ saturated rates of photosynthesis and carboxylation efficiency between populations or with CO₂ treatment. Moreover, SDS-PAGE/ LISA quantification of leaf ribulose bisphosphate carboxylase/oxygenase (Rubisco) showed no difference in Rubisco content between populations or CO₂ treatments. These findings suggest that long-term adaptation to growth at elevated CO₂ may be associated with a potential for increased growth, but this does not appear to be linked with differences in the intrinsic capacity for photosynthesis. Received: 16 August 1996 / Accepted: 19 October 1996     

Comparative responses of model C3 and C4 plants to drought in low and elevated CO2

Interactive effects of CO₂ and water availability have been predicted to alter the competitive relationships between C3 and C4 species over geological and contemporary time scales. We tested the effects of drought and CO₂ partial pressures (pCO₂) ranging from values of the Pleistocene to those predicted for the future on the physiology and growth of model C3 and C4 species. We grew co-occurring Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) in monoculture at 18 (Pleistocene), 27 (preindustrial), 35 (current), and 70 (future) Pa CO₂ under conditions of high light and nutrient availability. After 27 days of growth, water was withheld from randomly chosen plants of each species until visible wilting occurred. Under well-watered conditions, low pCO₂ that occurred during the Pleistocene was highly limiting to C3 photosynthesis and growth, and C3 plants showed increased photosynthesis and growth with increasing pCO₂ between the Pleistocene and future CO₂ values. Well-watered C4 plants exhibited increased photosynthesis in response to increasing pCO₂, but total mass and leaf area were unaffected by pCO₂. In response to drought, C3 plants dropped a large amount of leaf area and maintained relatively high leaf water potential in remaining leaves, whereas C4 plants retained greater leaf area, but at a lower leaf water potential. Furthermore, drought-treated C3 plants grown at 18 Pa CO₂ retained relatively greater leaf area than C3 plants grown at higher pCO₂ and exhibited a delay in the reduction of stomatal conductance that may have occurred in response to severe carbon limitations. The C4 plants grown at 70 Pa CO₂ showed lower relative reductions in net photosynthesis by the end of the drought compared to plants at lower pCO₂, indicating that CO₂ enrichment may alleviate drought effects in C4 plants. At the Pleistocene pCO₂, C3 and C4 plants showed similar relative recovery from drought for leaf area and biomass production, whereas C4 plants showed higher recovery than C3 plants at current and elevated pCO₂. Based on these model systems, we conclude that C3 species may not have been at a disadvantage relative to C4 species in response to low CO₂ and severe drought during the Pleistocene. Furthermore, C4 species may have an advantage over C3 species in response to increasing atmospheric CO₂ and more frequent and severe droughts.     

Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra

We investigated how light and CO₂ levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO₂. At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO₂ than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO₂ Elevated CO₂ prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO₂. Elevated CO₂ did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO₂ and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A/C_i curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO₂. Absolute levels of chlorophyll were affected by the interaction of CO₂ and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A/C_i curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO₂ accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.     

Meta‐analysis reveals profound responses of plant traits to glacial CO2 levels

A general understanding of the links between atmospheric CO₂ concentration and the functioning of the terrestrial biosphere requires not only an understanding of plant trait responses to the ongoing transition to higher CO₂ but also the legacy effects of past low CO₂. An interesting question is whether the transition from current to higher CO₂ can be thought of as a continuation of the past trajectory of low to current CO₂ levels. Determining this trajectory requires quantifying the effect sizes of plant response to low CO₂. We performed a meta‐analysis of low CO₂ growth experiments on 34 studies with 54 species. We quantified how plant traits vary at reduced CO₂ levels and whether C₃ versus C₄ and woody versus herbaceous plant species respond differently. At low CO₂, plant functioning changed drastically: on average across all species, a 50% reduction in current atmospheric CO₂ reduced net photosynthesis by 38%; increased stomatal conductance by 60% and decreased intrinsic water use efficiency by 48%. Total plant dry biomass decreased by 47%, while specific leaf area increased by 17%. Plant types responded similarly: the only significant differences being no increase in SLA for C₄ species and a 16% smaller decrease in biomass for woody C₃ species at glacial CO₂. Quantitative comparison of low CO₂ effect sizes to those from high CO₂ studies showed that the magnitude of response of stomatal conductance, water use efficiency and SLA to increased CO₂ can be thought of as continued shifts along the same line. However, net photosynthesis and dry weight responses to low CO₂ were greater in magnitude than to high CO₂. Understanding the causes for this discrepancy can lead to a general understanding of the links between atmospheric CO₂ and plant responses with relevance for both the past and the future.