CO2 enrichment reduces reproductive dominance in competing stands of Ambrosia artemisiifolia (common ragweed)

Plants growing in dense stands may not equally acquire or utilize extra carbon gained in elevated CO₂. As a result, reproductive differences between dominant and subordinate plants may be altered under rising CO₂ conditions. We hypothesized that elevated CO₂ would enhance the reproductive allocation of shaded, subordinate Ambrosia artemisiifolia L. (Asteraceae) individuals more than that of light-saturated dominants. We grew stands of A. artemisiifolia at either 360 or 720 μL L⁻¹ CO₂ levels and measured the growth and reproductive responses of competing individuals. To test whether elevated CO₂ altered size and reproductive inequalities within stands, we compared stand-level coefficients of variation (CV) in height growth and final shoot, root, and reproductive organ biomasses. Elevated CO₂ enhanced biomass and reduced the CV for all aspects of plant growth, especially reproductive biomass. Allocation to reproduction was higher in the elevated CO₂ than in the ambient treatment, and this difference was more pronounced in small, rather than large plant positive relationships between the CV and total stand productivity declined under elevated CO₂, indicating that growth enhancements to smaller plants diminished the relative biomass advantages of larger plants in increasingly crowded conditions. We conclude that elevated CO₂ stimulates stand-level reproduction while CO₂-induced growth gains of subordinate A. artemisiifolia plants minimize differences in the reproductive output of small and large plants. Thus, more individuals are likely to produce greater amounts of seeds and pollen in future populations of this allergenic weed.     

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.     

Elevated CO2 and leaf shape: Are dandelions getting toothier?

Heteroblastic leaf development in Taraxacum officinale is compared between plants grown under ambient (350 ppm) vs. elevated (700 ppm) CO₂ levels. Leaves of elevated CO₂ plants exhibited more deeply incised leaf margins and relatively more slender leaf laminae than leaves of ambient CO₂ plants. These differences were found to be significant in allometric analyses that controlled for differences in leaf size, as well as analyses that controlled for leaf developmental order. The effects of elevated CO₂ on leaf shape were most pronounced when plants were grown individually, but detectable differences were also found in plants grown at high density. Although less dramatic than in Taraxacum, significant effects of elevated CO₂ on leaf shape were also found in two other weedy rosette species, Plantago major and Rumex crispus. These observations support the long-standing hypothesis that leaf carbohydrate level plays an important role in regulating heteroblastic leaf development, though elevated C0₂ may also affect leaf development through direct hormonal interactions or increased leaf water potential. In Taraxacum, pronounced modifications of leaf shape were found at CO₂ levels predicted to occur within the next century.     

CO2-induced growth enhancements of co-occurring tree species decline at different rates

To elucidate how enriched CO₂ atmospheres, soil fertility, and light availability interact to influence the long-term growth of tree seedlings, six co-occurring members of temperate forest communities including ash (Fraxinus americana L.), gray birch (Betula populifolia), red maple (Acer rubrum), yellow birch (Betula alleghaniensis), striped maple (Acer pensylvanicum), and red oak (Quercus rubra L.) were raised in a glasshouse for three years in a complete factorial design. After three years of growth, plants growing in elevated CO₂ atmospheres were generally larger than those in ambient CO₂ atmospheres, however, magnitudes of CO₂-induced growth enhancements were contingent on the availability of nitrogen and light, as well as species identity. For all species, magnitudes of CO₂-induced growth enhancements after one year of growth were greater than after three years of growth, though species' growth enhancements over the three years declined at different rates. These results suggest that CO₂-induced enhancements in forest productivity may not be sustained for long periods of time. Additionally, species' differential growth responses to elevated CO₂ may indirectly influence forest productivity via long-term species compositional changes in forests.     

Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure

Increased levels of atmospheric CO₂ may alter the structure and composition of plant communities by affecting how species respond to their physical and biological environment. We investigated how elevated CO₂ influenced the response of paper birch ( Betula papyrifera Marsh.) and yellow birch (Betula alleghaniensis Britt.) seedlings to variation in soil moisture. Seedlings were grown for four months on a soil moisture gradient, individually and in mixed species stands, in controlled environment facilities at ambient (375 μL L^–1) and elevated (700 μL L^–1) atmospheric CO₂. For both individually and competitively grown paper birch seedlings, there was a greater CO₂ growth enhancement for seedlings watered less frequently than for well-watered seedlings. This differential change in CO₂ responsiveness across the moisture gradient reduced the difference in seedling growth between high and low water levels and effectively broadened the regeneration niche of paper birch. In contrast, for yellow birch seedlings, elevated CO₂ only produced a significant growth enhancement at the wet end of the soil moisture gradient, and increased the size difference between seedlings at the two ends of the gradient. Gas exchange measurements showed that paper birch seedlings were more sensitive than yellow birch seedlings to declines in soil moisture, and that elevated CO₂ reduced this sensitivity. Additionally, elevated CO₂ improved survival of yellow birch seedlings growing in competition with paper birch in dry stands. Thus, elevated CO₂ may influence regeneration patterns of paper birch and yellow birch on sites of differing soil moisture. In the future, as atmospheric CO₂ levels rise, growth of paper birch seedlings and survival of yellow birch seedlings may be enhanced on xeric sites, while yellow birch may show improved growth on mesic sites.     

Influence of elevated CO2 on canopy development and red:far-red ratios in two-storied stands ofRicinus communis

Vertical structure of plant stands and canopies may change under conditions of elevated CO₂ due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO₂ on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO₂-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands ofRicinus communis exposed to ambient (340 μl l⁻¹) and elevated (610 μl l⁻¹) CO₂. Plants in each stand were grown in a common soil as closed “artificial ecosystems” with a ground area of 6.7 m². LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO₂ treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO₂ the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO₂. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO₂. CO₂ enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO₂ at the stand-level. We suggest that, under elevated CO₂, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction.     

System-level adjustments to elevated CO2 in model spruce ecosystems

Atmospheric carbon dioxide enrichment and increasing nitrogen deposition are often predicted to increase forest productivity based on currently available data for isolated forest tree seedlings or their leaves. However, it is highly uncertain whether such seedling responses will scale to the stand level. Therefore, we studied the effects of increasing CO₂ (280, 420 and 560 μL L^-1) and increasing rates of wet N deposition (0, 30 and 90 kg ha^-1 y^-1) on whole stands of 4-year-old spruce trees (Picea abies). One tree from each of six clones, together with two herbaceous understory species, were established in each of nine 0.7 m² model ecosystems in nutrient poor forest soil and grown in a simulated montane climate for two years. Shoot level light-saturated net photosynthesis measured at growth CO₂ concentrations increased with increasing CO₂, as well as with increasing N deposition. However, predawn shoot respiration was unaffected by treatments. When measured at a common CO₂ concentration of 420 μL L^-1 37% down-regulation of photosynthesis was observed in plants grown at 560 μL CO₂ L^-1. Length growth of shoots and stem diameter were not affected by CO₂ or N deposition. Bud burst was delayed, leaf area index (LAI) was lower, needle litter fall increased and soil CO₂ efflux increased with increasing CO₂. N deposition had no effect on these traits. At the ecosystem level the rate of net CO₂ exchange was not significantly different between CO₂ and N treatments. Most of the responses to CO₂ studied here were nonlinear with the most significant differences between 280 and 420 μL CO₂ L^-1 and relatively small changes between 420 and 560 μL CO₂ L^-1. Our results suggest that the lack of above-ground growth responses to elevated CO₂ is due to the combined effects of physiological down-regulation of photosynthesis at the leaf level, allometric adjustment at the canopy level (reduced LAI), and increasing strength of below-ground carbon sinks. The non-linearity of treatment effects further suggests that major responses of coniferous forests to atmospheric CO₂ enrichment might already be under way and that future responses may be comparatively smaller.     

Plant growth and reproduction along CO2 gradients: non-linear responses and implications for community change

The effects of rising atmospheric CO₂ concentrations on natural plant communities will depend upon the cumulative responses of plant growth and reproduction to gradual, incremental changes in climatic conditions. We analysed published studies of plant responses to elevated CO₂ to address whether reproductive and total biomass exhibit similar enhancement to elevated vs. ambient CO₂ concentrations, and to assess the patterns of plant response along gradients of CO₂ concentrations. In six annual plant species, mean enhancement at double ambient vs. ambient CO₂ was 1.13 for total biomass and 1.30 for reproductive biomass. The two measures were significantly correlated, but there was considerable scatter in the relationship, indicating that reproductive responses cannot be consistently predicted from enhancement of total biomass. Along experimental CO₂ gradients utilizing three concentrations, there was a great diversity of response patterns, including positive, negative, non-monotonic and non-significant (flat) responses. The distribution of response patterns differed for plants grown in stands compared to those grown individually. Positive responses were less frequent in competitive environments, and non-monotonic responses were more frequent. These results emphasize that interpolation of plant response based on enhancement ratios measured at elevated vs. ambient CO₂ concentrations is not sufficient to predict community responses to incremental changes in atmospheric conditions. The consequences of differential response patterns were assessed in a simulation of community dynamics for four species of annual plants. The model illustrates that the final community composition at a future point in time depends critically on both the magnitude and the rate of increase of atmospheric CO₂.     

Thirty years of in situ tree growth under elevated CO2: a model for future forest responses?

Rising concentrations of atmospheric carbon dioxide have been predicted to stimulate the growth of forest trees. However, long-term effects on trees growing to maturity and to canopy closure while exposed to elevated CO₂ have never been examined. We compared tree ring chronologies of Mediterranean Quercus ilex which have been continuously exposed to elevated CO₂ (around 650 μmol mol^–1) since they were seedlings, near two separate natural CO₂ springs with those from trees at nearby ambient-CO₂‘control’ sites. Trees grown under high CO₂ for 30 years (1964–93) showed a 12% greater final radial stem width than those growing at the ambient-CO₂ control sites. However, this stimulation was largely due to responses when trees were young. By the time trees were 25–30 y old the annual difference in tree ring width between low and high CO₂ grown trees had disappeared. At any given tree age, elevated CO₂ had a relatively greater positive effect on tree ring width in years with a dry spring compared to years with more rainfall between April and May. This indicates a beneficial effect of elevated CO₂ on tree water relations under drought stress. Our data suggest that the early regeneration phase of forest stands can be accelerated in CO₂-enriched atmospheres and that maximum biomass per land area may be reached sooner than under lower CO₂ concentrations. In our study, high CO₂ grown Q. ilex trees reached the same stem basal area at the age of 26 y as control trees at 29 y, i.e. three years earlier (faster turnover of carbon?). Reliable predictions of the future development of forests need to account for the variable responses of trees over their entire lifetime. Such responses to elevated CO₂ can presently only be assessed at such unique field sites.     

Photosynthetic downregulation in leaves of the Japanese white birch grown under elevated CO2 concentration does not change their temperature‐dependent susceptibility to photoinhibition

To determine the effects of elevated CO₂ concentration ([CO₂]) on the temperature‐dependent photosynthetic properties, we measured gas exchange and chlorophyll fluorescence at various leaf temperatures (15, 20, 25, 30, 35 and 40°C) in 1‐year‐old seedlings of the Japanese white birch (Betula platyphylla var. japonica), grown in a phytotron under natural daylight at two [CO₂] levels (ambient: 400 µmol mol^?1 and elevated: 800 µmol mol^?1) and limited N availability (90 mg N plant^?1). Plants grown under elevated [CO₂] exhibited photosynthetic downregulation, indicated by a decrease in the carboxylation capacity of Rubisco. At temperatures above 30°C, the net photosynthetic rates of elevated‐CO₂‐grown plants exceeded those grown under ambient [CO₂] when compared at their growth [CO₂]. Electron transport rates were significantly lower in elevated‐CO₂‐grown plants than ambient‐CO₂‐grown ones at temperatures below 25°C. However, no significant difference was observed in the fraction of excess light energy [(1 ? q_P)× F_v′/F_m′] between CO₂ treatments across the temperature range. The quantum yield of regulated non‐photochemical energy loss was significantly higher in elevated‐CO₂‐grown plants than ambient, when compared at their respective growth [CO₂] below 25°C. These results suggest that elevated‐CO₂‐induced downregulation might not exacerbate the temperature‐dependent susceptibility to photoinhibition, because reduced energy consumption by electron transport was compensated for by increased thermal energy dissipation at low temperatures.     

Responses to elevated CO2 of Flaveria linearis plants having a reduced activity of cytosolic fructose-1,6-bisphosphatase

Wide variation exists in the growth responses of C₃ plants to elevated CO₂ levels. To investigate the role of photosynthetic feedback in this phenomenon, photosynthetic parameters and growth were measured for lines of Flaveria linearis with low, intermediate or high cytosolic fructose-1,6-bisphosphatase (cytFBPase) activity when grown at either 35 or 65 Pa CO₂. The effects of pot size on the responses of these lines to elevated CO₂ were also examined. Photosynthesis and growth of plants with low cytFBPase activity were less responsive to elevated CO₂, and these plants had a reduced maximum potential for photosynthesis and growth. Plants with intermediate cytFBPase activity also showed a lower relative growth enhancement when grown at 65 Pa CO₂. There was a significant pot size effect on photosynthesis and growth for line 85-1 (high cytFBPase). This effect was greatest for line 85-1 when grown at 35 Pa CO₂, since these plants showed the greatest downward acclimation of photosynthesis when grown in small pots. There was a minimal pot size effect for line 84-9 (low cytFBPase), and this could be partly attributed to the reduced CO₂ sensitivity of this line. It is proposed that the capacity for sucrose synthesis in C₃, plants is partly responsible for their wide variation in CO₂ responsiveness.     

Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

In order to separate the net effect of growth at elevated [CO₂] on stomatal conductance (g_s) into direct and acclimatory responses, mid‐day values of g_s were measured for plants grown in field plots in open‐topped chambers at the current ambient [CO₂], which averaged 350 μmol mol^?1 in the daytime, and at ambient + 350 μmol mol^?1[CO₂] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing g_s measured at 700 μmol mol^?1[CO₂] for plants grown at the two [CO₂]. The direct effect of increasing [CO₂] from 350 to 700 μmol mol^?1 was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with g_s. For all species, growth at the higher [CO₂] significantly reduced g_s measured at 700 μmol mol^?1[CO₂]. The reduction in g_s caused by growth at the higher [CO₂] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO₂] on conductance. For barley, there was no other evidence for stomatal acclimation, despite consistent down‐regulation of photosynthetic rate in plants grown at the higher [CO₂]. In wheat and potato, in addition to the vapour pressure difference interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down‐regulation of photosynthetic rate through the season. In sorghum, g_s consistently exhibited acclimation, but there was no down‐regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in g_s when averaged over measurement dates. The net effect of growth at elevated [CO₂] on mid‐day g_s resulted from unique combinations of direct and acclimatory responses in the various species.     

Genotype-specific effects of elevated CO2 on fecundity in wild radish (Raphanus raphanistrum)

Rising atmospheric CO₂ may lead to natural selection for genotypes that exhibit greater fitness under these conditions. The potential for such evolutionary change will depend on the extent of within-population genetic variation in CO₂ responses of wild species. We tested for heritable variation in CO₂-dependent life history responses in a weedy, cosmopolitan annual, Raphanus raphanistrum. Progeny from five paternal families were grown at ambient and twice ambient CO₂ using outdoor open-top chambers (160 plants per CO₂ treatment). Elevated CO₂ stimulated net assimilation rates, especially in plants that had begun flowering. Across paternal families, elevated CO₂ led to significant increases in flower and seed production (by 22% and 13% respectively), but no effect was seen on time to bolting, leaf area at bolting, fruit set, or number of seeds per fruit. Paternal families differed in their response to the CO₂ treatment: in three families there were no significant CO₂ effects, while in one family lifetime fecundity increased by >50%. These genotype-specific effects altered fitness rankings among the five paternal families. Although we did not detect a significant genotype x CO₂ interaction, our results provide evidence for heritable responses to elevated CO₂. In a subset of plants, we found that the magnitude of CO₂ effects on fecundity was also influenced by soil fertility.     

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     

A meta-analysis of leaf gas exchange and nitrogen in trees grown under elevated carbon dioxide

The response of trees to rising atmospheric CO₂ concentration ([CO₂]) is of concern to forest ecologists and global carbon modellers and is the focus of an increasing body of research work. I review studies published up to May 1994, and several unpublished works, which reported at least one of the following: net CO₂ assimilation (A), stomatal conductance (g_s), leaf dark respiration (R_d) leaf nitrogen or specific leaf area (SLA) in woody plants grown at <400 μmol mol^?1 CO₂ or at 600–800 μmol mol^?1 CO₂. The resulting data from 41 species were categorized according to growth conditions (unstressed versus stressed), length of CO₂ exposure, pot size and exposure facility [growth chamber (GC), greenhouse (GH), or open-top chamber (OTC)] and interpreted using meta-analytic methods. Overall, A showed a large and significant increase at elevated [CO₂] but length of CO₂ exposure and the exposure facility were important modifiers of this response. Plants exposed for < 50 d had a significantly greater response, and those from GCs had a significantly lower response than plants from longer exposures or from OTC studies. Negative acclimation of A was significant and general among stressed plants, but in unstressed plants was influenced by length of CO₂ exposure, the exposure facility and/or pot size. Growth at elevated [CO₂] resulted in moderate reductions in gs in unstressed plants, but there was no significant effect of CO₂ on gs in stressed plants. Leaf dark respiration (mass or area basis) was reduced strongly by growth at high [CO₂] > while leaf N was reduced only when expressed on a mass basis. This review is the first meta-analysis of elevated CO₂ studies and provides statistical confirmation of several general responses of trees to elevated [CO₂]. It also highlights important areas of continued uncertainty in our understanding of these responses.     

Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO₂ alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO₂ was 35% that of uninfected, control plants, while at elevated CO₂, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO₂ supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO₂. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO₂. There were significant CO₂ and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO₂] (A/C_i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO₂ had significantly lower CO₂ saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO₂‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO₂, but had no impact on photosynthesis of hosts grown at ambient CO₂. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO₂ had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO₂. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO₂ or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO₂ concentrations. These results demonstrate that elevated CO₂ concentrations can alleviate the impact of infection with Striga on the growth of C₃ hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO₂.     

Competition and patterns of resource use among seedlings of five tropical trees grown at ambient and elevated CO2

Summary Seedlings of five tropical trees, Cecropia obtusifolia, Myriocarpa longipes, Piper auritum, Senna multijuga and Trichospermum mexicanum, were grown both as individuals, and in competition with each other at ambient (350) and two levels of elevated CO₂ (525 and 700 l l^-1) for a period of 111 days. Growth, allocation, canopy architecture, mid-day leaf water potential and soil moisture content were assessed three times over this period for individually grown plants, and at the end of the experiment for competitively grown plants. In addition, leaf photosynthesis and conductance were assessed for the individually grown plants midway through the experiment, and light profile curves were determined for the competitive arrays at three stages of development. Elevated CO₂ did not affect photosynthesis or overall growth of the individually-grown plants but did affect canopy architecture; mean canopy height increased with CO₂ in Piper and Trichospermum and decreased in Senna. Stomatal conductance decreased slightly as CO₂ increased from 350 to 525 l l^-1 but this had no significant effect upon whole plant water use of leaf water potential. Soil moisture content for the individuals increased marginally as CO₂ increased, but this did not occur in the competitive arrays. There was a marked effect of CO₂ upon species composition of the competitive arrays; Senna decreased in importance as CO₂ increased while Cecropia, Trichospermum and Piper increased in importance. Stepwise regression analysis using competitive performance as the independent variable, and the various morphological and physiological parameters measured on the individually grown plants as independent variables, suggested that canopy height was the single most important variable determining competitive ability. Also significant were photosynthetic rate (particularly at low light levels) and allocation to roots early in the experiment. Light profiles in the canopy revealed that less than 15% of incident light penetrated to the level of mean canopy height. Results suggest that competition for light was the major factor determining community composition, and that CO₂ affected competitive outcome through its affect upon canopy architecture.This study was supported by a grant from the US Department of Energy     

Photosynthetic acclimation to elevated CO2 in Phaseolus vulgaris L. is altered by growth response to nitrogen supply

Abstract Long‐term exposure of plants to elevated CO₂ often leads to downward photosynthetic acclimation. Nitrogen (N) deficiency could potentially exacerbate this response by reducing growth rate and the sink for photosynthates, but this has not always been observed. Experimentally, the interpretation of N effects on CO₂ responses can be confounded by increasing severity of tissue N deficiency over time when N supply is not adjusted as demand increases. In this study, N supply ranged from sub‐ to supra‐optimal (20–540 kgN ha^–l equivalent), and relatively stable levels of tissue N concentration were obtained in all treatments by varying twice‐weekly application rates in proportion to plant growth. The effects of N on photosynthesis and growth of beans (Phaseolus vulgaris L.) raised at ambient (35 Pa) and three elevated (70, 105, 140 Pa) CO₂ partial pressures (pCO₂) were evaluated. Averaging across N treatments, leaf total non‐structural carbohydrates (TNC) were 2.5‐ to 3‐fold higher and leaf N concentrations were 31–35% lower at elevated compared to ambient pCO₂. Light‐saturated net CO₂ assimilation rates measured at growth pCO₂ (A_satg) were significantly higher (26–40% depending on N supply) in plants grown at elevated compared to ambient pCO₂. When measured at a common pCO₂ of 35 Pa, the A_sat of plants grown at elevated CO₂ was 15–29% less than that of plants grown at 35 Pa, indicative of downward photosynthetic acclimation. The magnitude of downward photosynthetic acclimation to elevated CO₂ was greater in plants grown at high (180 and 540 kgN ha^–l) compared to low (20 and 60 kgN ha^–l) N supply, and this was associated with a higher A_sat at growth pCO₂, higher leaf area ratio (leaf area/total biomass), and higher TNC in leaves of high‐N plants. Our results indicate that the effect of N on acclimation to CO₂ will depend on the balance between supply and demand for N during the growing period, and the effect this has on biomass allocation and source‐sink C balance at the whole‐plant level.     

Growth and nitrogen accretion of dinitrogen-fixing Alnus glutinosa (L.) Gaertn. under elevated carbon dioxide

Short-term studies of tree growth at elevated CO₂ suggest that forest productivity may increase as atmospheric CO₂ concentrations rise, although low soil N availability may limit the magnitude of this response. There have been few studies of growth and N₂ fixation by symbiotic N₂-fixing woody species under elevated CO₂ and the N inputs these plants could provide to forest ecosystems in the future. We investigated the effect of twice ambient CO₂ on growth, tissue N accretion, and N₂ fixation of nodulated Alnus glutinosa (L.) Gaertn. grown under low soil N conditions for 160 d. Root, nodule, stem, and leaf dry weight (DW) and N accretion increased significantly in response to elevated CO₂. Whole-plant biomass and N accretion increased 54% and 40%, respectively. Delta-¹⁵N analysis of leaf tissue indicated that plants from both treatments derived similar proportions of their total N from symbiotic fixation suggesting that elevated CO₂ grown plants fixed approximately 40% more N than did ambient CO₂ grown plants. Leaves from both CO₂ treatments showed similar relative declines in leaf N content prior to autumnal leaf abscission, but total N in leaf litter increased 24% in elevated compared to ambient CO₂ grown plants. These results suggest that with rising atmospheric CO₂ N₂-fixing woody species will accumulate greater amounts of biomass N through N₂ fixation and may enhance soil N levels by increased litter N inputs.     

Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice 1

In order to predict the potential impacts of global change, it is important to understand the impact of increasing global atmospheric [CO₂] on the growth and yield of crop plants. The objectives of this study were to determine the interaction of N fertilization rates and atmospheric [CO₂] on radiation interception and radiation-use efficiency of rice (Oryza sativa L. cv. IR72) grown under tropical field conditions. Rice plants were grown inside open top chambers in a lowland rice field at the International Rice Research Institute in the Philippines at ambient (about 350 μmol mol^-1) or elevated (about 600 μmol mol^-1 during the 1993 wet season and 700 μmol mol^-1 during the 1994 dry season) in combination with three levels of applied N (0, 50 or 100 kg N ha^-1 in the wet season; 0, 90 or 200 kg N ha-1 in the dry season). Light interception was not directly affected by [CO₂], but elevated [CO₂] indirectly increased light interception through increasing total absorbed N. Plant N requirement for radiation interception was similar for rice grown under ambient [CO₂] or elevated [CO₂] treatments. The conversion efficiency of intercepted radiation to dry matter, radiation-use efficiency (RUE), was about 35% greater at elevated [CO₂] than at ambient [CO₂]. The relationship between leaf N and RUE was curvilinear. At ambient [CO₂], RUE was fairly stable across levels of leaf N, but leaf N less than about 2.5% resulted in lower RUE for plants grown with elevated [CO₂] than for plant grown at ambient [CO₂]. Decreased leaf N with increased [CO₂], therefore decreased RUE of rice plants grown at elevated [CO₂]. When predicting responses of rice to elevated [CO₂], RUE should be adjusted with a decrease in leaf N. This revised version was published online in June 2006 with corrections to the Cover Date.