Seasonal changes in the effects of elevated CO2 on rice at three levels of nitrogen supply: a free air CO2 enrichment (FACE) experiment

Over time, the stimulative effect of elevated CO₂ on the photosynthesis of rice crops is likely to be reduced with increasing duration of CO₂ exposure, but the resultant effects on crop productivity remain unclear. To investigate seasonal changes in the effect of elevated CO₂ on the growth of rice (Oryza sativa L.) crops, a free air CO₂ enrichment (FACE) experiment was conducted at Shizukuishi, Iwate, Japan in 1998–2000. The target CO₂ concentration of the FACE plots was 200 µmol mol^?1 above that of ambient. Three levels of nitrogen (N) were supplied: low (LN, 4 g N m^?2), medium [MN, 8 (1998) and 9 (1999, 2000) g N m^?2] and high N (HN, 12 and 15 g N m^?2). For MN and HN but not for LN, elevated CO₂ increased tiller number at panicle initiation (PI) but this positive response decreased with crop development. As a result, the response of green leaf area index (GLAI) to elevated CO₂ greatly varied with development, showing positive responses during vegetative stages and negative responses after PI. Elevated CO₂ decreased leaf N concentration over the season, except during early stage of development. For MN crops, total biomass increased with elevated CO₂, but the response declined linearly with development, with average increases of 32, 28, 21, 15 and 12% at tillering, PI, anthesis, mid‐ripening and grain maturity, respectively. This decline is likely to be due to decreases in the positive effects of elevated CO₂ on canopy photosynthesis because of reductions in both GLAI and leaf N. Up to PI, LN‐crops tended to have a lower response to elevated CO₂ than MN‐ and HN‐crops, though by final harvest the total biomass response was similar for all N levels. For MN‐ and HN‐crops, the positive response of grain yield (ca. 15%) to elevated CO₂ was slightly greater than the response of final total biomass while for LN‐crops it was less. We conclude that most of the seasonal changes in crop response to elevated CO₂ are directly or indirectly associated with N uptake.     

Response of wheat canopy CO2 and water gas-exchange to soil water content under ambient and elevated CO2

The nature of the interaction between drought and elevated CO₂ partial pressure (pC_a) is critically important for the effects of global change on crops. Some crop models assume that the relative responses of transpiration and photosynthesis to soil water deficit are unaltered by elevated pC_a, while others predict decreased sensitivity to drought at elevated pC_a. These assumptions were tested by measuring canopy photosynthesis and transpiration in spring wheat (cv. Minaret) stands grown in boxes with 100 L rooting volume. Plants were grown under controlled environments with constant light (300 µmol m^?2 s^?1) at ambient (36 Pa) or elevated (68 Pa) pC_a and were well watered throughout growth or had a controlled decline in soil water starting at ear emergence. Drought decreased final aboveground biomass (?15%) and grain yield (?19%) while elevated pC_a increased biomass (+24%) and grain yield (+29%) and there was no significant interaction. Elevated pC_a increased canopy photosynthesis by 15% on average for both water regimes and increased dark respiration per unit ground area in well‐watered plants, but not drought‐grown ones. Canopy transpiration and photosynthesis were decreased in drought‐grown plants relative to well‐watered plants after about 20–25 days from the start of the drought. Elevated pC_a decreased transpiration only slightly during drought, but canopy photosynthesis continued to be stimulated so that net growth per unit water transpired increased by 21%. The effect of drought on canopy photosynthesis was not the consequence of a loss of photosynthetic capacity initially, as photosynthesis continued to be stimulated proportionately by a fixed increase in irradiance. Drought began to decrease canopy transpiration below a relative plant‐available soil water content of 0.6 and canopy photosynthesis and growth below 0.4. The shape of these responses were unaffected by pC_a, supporting the simple assumption used in some models that they are independent of pC_a.     

Interactive effects of diversity, nutrients and elevated CO2 on experimental plant communities

The effects of species richness and elevated CO₂ on community productivity under altered nutrient levels were studied in experimental herbaceous communities composed of species from the Midwestern United States annual community, which consists of three functional groups C₃, C₄ and N‐fixers. Aboveground and belowground biomass were measured at flowering stage and at the end of the experiment when fruits of most plants were ripe. At the low nutrient level, species richness did not have a significant effect on community productivity. However, at the high nutrient level, the community biomass decreased with decreasing species richness at both ambient and elevated CO₂ in the first harvest, and at elevated CO₂ in the second harvest. At low nutrient level, CO₂ slightly increased community biomass at medium and high species richness. At high nutrient level, CO₂ significantly increased community biomass in all species‐richness treatments in the first harvest, but a significant response was observed only in the high richness treatment in the second harvest. At the functional group level, biomass of C₃ responded positively to CO₂, and C₄ responded very negatively to CO₂. The N‐fixers responded positively to CO₂ at low and medium species richness, but negatively at high species richness, showing a CO₂×richness interaction. CO₂ increased species evenness in the communities, depending on nutrient level. Species varied in the responses of light‐saturated net photosynthesis (P_max) to elevated CO₂, even within functional groups. Our findings suggest that (1) the relationship between productivity and species diversity was dependent on nutrient levels. (2) Species diversity enhances responses of communities to elevated CO₂. (3) Harvest time can affect the results of diversity‐productivity experiments. (4) Responses of C₃, C₄, and N‐fixers to elevated CO₂ in communities did not follow the prediction based on functional groups or plants grown individually, rather it depended on species richness.     

The density dependence of plant responses to elevated CO2

1 Stands of the annual Brassica kaber were grown at a range of six densities in both ambient and elevated CO₂ environments, and measurements of shoot growth were made from seedling emergence through to reproduction.
2 Early in stand development (21 days following emergence), CO₂ enhancement (β) for above-ground biomass was highly density-dependent, ranging from 1.41 at the lowest density (20 plants m−2) to 0.59 at the highest density (652 plants m−2).
3 As stands matured and total biomass exceeded a relatively low threshold level (<10.0 g m^{−2; c.}  20% of final yield), the density-dependence of β disappeared. Above this shoot biomass threshold, β-values remained remarkably stable (β = 0.34) across a broad range of stand biomass, independent of a stand's initial density or age.
4 Average stand-level reproductive β-values at a final harvest were very similar to biomass values (β = 0.38) and, as with biomass values at later stages, showed no apparent density-dependence.
5 These results highlight the importance of considering density and the time-course of stand development simultaneously when assessing the potential for CO₂-induced growth enhancements in plants.     

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.     

Biosynthesis of plant phenolic compounds in elevated atmospheric CO2

Experiments were carried out to determine the effects of elevated atmospheric carbon dioxide (CO₂) on phenolic biosynthesis in four plant species growing over three generations for nine months in a model plant community. Results were compared to those obtained when the same species were grown individually in pots in the same soils and controlled environment. In the model herbaceous plant community, only two of the four species showed any increase in biomass under elevated CO₂, but this occurred only in the first generation for Spergula arvensis and in the second generation for Poa annua. Thus, the effects of CO₂ on plant biomass and carbon and nitrogen content were species‐ and generation‐specific. The activity of the principle phenolic biosynthetic enzyme, phenylalanine ammonia lyase (PAL), increased under elevated CO₂ in Senecio vulgaris only in Generation 1, but increased in three of the four plant species in Generation 2. There were no changes in the total phenolic content of the plants, except for P. annua in Generation 1. Lignin content decreased under elevated CO₂ in Cardamine hirsuta in Generation 1, but increased in Generation 2, whilst the lignin content of P. annua showed no change, decreased, then increased in response to elevated CO₂ over the three generations. When the species were grown alone in pots, elevated CO₂ increased PAL activity in plants grown in soil taken from the Ecotron community after nine months of plant growth, but not in plants grown in the soil used at the start of the experiment (‘initial' soil). In P. annua, phenolic biosynthesis decreased under elevated CO₂ in initial soil, and in both P. annua and S. vulgaris there was a significant interaction between effects of soil type and CO₂ level on PAL activity. In this study, plant chemical composition altered more in response to environmental factors such as soil type than in response to carbon supply. Results were species‐specific and changed markedly between generations.     

Carbon gains by desiccation-tolerant plants at elevated CO2

1. There have been no reports of the long-term responses of the desiccation-tolerant (DT) plants to elevated CO₂. Xerophyta scabrida is a DT woody shrub, which loses chlorophylls and thylakoids during desiccation: a so-called poikilochlorophyllous desiccation-tolerant species (PDT). When the leaves of X. scabria are allowed to desiccate, the species shows many of the normal features of (P)DT plants.
2. However, the duration of photosynthesis in X. scabria is prolonged by 300% when the measurements are made at 700 as opposed to 350p.p.m. CO₂. The implication is that the carboxylating enzymes must still have been active at this time to enable appreciable photosynthetic activity. This response could have far-reaching implications for the success of such species in a future climate.
3. Lichens and mosses, representing the homoiochlorophyllous DTs (HDT), retain their chlorophyll content and photosynthetic apparatus during desiccation. We show the desiccation responses of two common HDT species ( Cladonia convoluta and Tortula ruralis ) to elevated CO₂ for comparison. Both HDT species showed increased net CO₂ uptake in the material grown at high CO₂ by more than 30% in moss and by more than 50% in lichen. It is concluded that desiccation-tolerant plants will be among the main beneficiaries of a high CO₂ future.     

Growth at elevated CO2: photosynthetic responses mediated through Rubisco

Abstract. The global uptake of CO₂ in photosynthesis is about 120 gigatons (Gt) of carbon per year. Virtually all passes through one enzyme, ribulose bisphosphate carboxylase/oxygenase (rubisco), which initiates both the photosynthetic carbon reduction, and photorespiratory carbon oxidation, cycles. Both CO₂ and O₂ are substrates; CO₂ also activates the enzyme. In C₃ plants, rubisco has a low catalytic activity, operates below its K_m (CO₂), and is inhibited by O₂. Consequently, increases in the CO₂/O₂ ratio stimulate C₃ photosynthesis and inhibit photorespiration. CO₂ enrichment usually enhances the productivity of C₃ plants, but the effect is marginal in C₄ species. It also causes acclimation in various ways: anatomically, morphologically, physiologically or biochemically. So, CO₂ exerts secondary effects in growth regulation, probably at the molecular level, that are not predictable from its primary biochemical role in carboxylation. After an initial increase with CO₂ enrichment, net photosynthesis often declines. This is a common acclimation phenomenon, less so in field studies, that is ultimately mediated by a decline in rubisco activity, though the RuBP/P_i-regeneration capacities of the plant may play a role. The decline is due to decreased rubisco protein, activation state, and/or specific activity, and it maintains the rubisco fixation and RuBP/P_i regeneration capacities in balance. Carbohydrate accumulation is sometimes associated with reduced net photosynthesis, possibly causing feedback inhibition of the RuBP/P_iregeneration capacities, or chloroplast disruption. As exemplified by field-grown soybeans and salt marsh species, a reduction in net photosynthesis and rubisco activity is not inevitable under CO₂ enrichment. Strong sinks or rapid translocation may avoid such acclimation responses. Over geological time, aquatic autotrophs and terrestrial C₄ and CAM plants have genetically adapted to a decline in the external CO₂/O₂ ratio, by the development of mechanisms to concentrate CO₂ internally; thus circumventing O₂ inhibition of rubisco. Here rubisco affinity for CO₂ is less, but its catalytic activity is greater, a situation compatible with a high-CO₂ internal environment. In aquatic autotrophs, the CO₂ concentrating mechanisms acclimate to the external CO₂, being suppressed at high-CO₂. It is unclear, whether a doubling in atmospheric CO₂ will be sufficient to cause a de-adaptive trend in the rubisco kinetics of future C₃ plants, producing higher catalytic activities.     

The effects of reduced and elevated CO2 and O2 on the seaweed Lomentaria articulata

We grew a non-bicarbonate using red seaweed, Lomentaria articulata (Huds.) Lyngb., in media aerated with four O₂ concentrations between 10 and 200% of current ambient [O₂] and four CO₂ concentrations between 67 and 500% of current ambient [CO₂], in a factorial design, to determine the effects of gas composition on growth and physiology. The relative growth rate of L. articulata increased with increasing [CO₂] up to 200% of current ambient [CO₂] but was unaffected by [O₂]. The relative growth enhancement, on a carbon basis, was 52% with a doubling of [CO₂] but fell to 23% under 5× ambient [CO₂]. Plants collected in winter responded more extremely to [CO₂] than did plants collected in the summer, although the overall pattern was the same. Discrimination between stable carbon isotopes (Δ¹³C) increased with increasing [CO₂] as would be expected for diffusive CO₂ acquisition. Tissue C and N were inversely related to [CO₂]. Growth in terms of biomass appeared to be limited by conversion of photosynthate to new biomass rather than simply by diffusion of CO₂, suggesting that non-bicarbonate-using macroalgae, such as L. articulata, may not be directly analogous to C3 higher plants in terms of their responses to changing gas composition.     

Nitrogen nutrition of C3 plants at elevated atmospheric CO2 concentrations

The atmospheric CO₂ concentration has risen from the preindustrial level of approximately 290 μl l⁻¹ to more than 350 μl l⁻¹ in 1993. The current rate of rise is such that concentrations of 420 μl l⁻¹ are expected in the next 20 years. For C₃ plants, higher CO₂ levels favour the photosynthetic carbon reduction cycle over the photorespiratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthetic cycles appears to alter nitrogen and carbon metabolism in the leaf, possibly causing decreases in nitrogen concentrations in the leaf. This may result from increases in the concentration of storage carbohydrates of high molecular weight (soluble or insoluble) and/or changes in distribution of protein or other nitrogen containing compounds. Uptake of nitrogen may also be reduced at high CO₂ due to lower transpiration rates. Decreases in foliar nitrogen levels have important implications for production of crops such as wheat, because fertilizer management is often based on leaf chemical analysis, using standards estimated when the CO₂ levels were considerably lower. These standards will need to be re-evaluated as the CO₂ concentration continues to rise. Lower levels of leaf nitrogen will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.     

Elevated CO2 affects the interactions between aphid pests and host plant flowering

1 Broad beans (Vicia faba L.) were grown at either ambient (350 μL/L) or elevated (700 μL/L) CO₂. Elevated CO₂ increased shoot weight by 14% and root weight by 24% compared to ambient, but did not affect flowering. 2 A single pea aphid (Acyrthosiphon pisum (Harris)) and its progeny decreased shoot and root weights by 20 and 24%, respectively, at ambient CO₂ after 20 days, but did not affect flower number. At elevated CO₂A. pisum decreased shoot and root weights by 27 and 34% and flower number decreased by 73%. 3 A single glasshouse and potato aphid (Aulacorthum solani (Kaltenbach)) and its progeny had no effect on the growth of bean plants after 20 days at ambient CO₂. At elevated CO₂, A. solani decreased shoot and root weights by 20 and 18%, and flower number by 60%. 4 The large reduction in flowering caused by aphids at elevated CO₂ suggests a change in resource allocation within the plants to compensate for aphid infestation. 5 Aphid density was unaffected by elevated CO₂, although there were significant effects of CO₂ on the resulting population structure of both A. pisum and A solani. We suggest that at elevated CO₂, aphids appear not to achieve their maximum reproductive potential and their populations are limited by the lower carrying capacity of their host plants.     

A mechanistic evaluation of photosynthetic acclimation at elevated CO2

Plants grown at elevated pCO₂ often fail to sustain the initial stimulation of net CO₂ uptake rate (A). This reduced, acclimated, stimulation of A often occurs concomitantly with a reduction in the maximum carboxylation velocity (V_c,max) of Rubisco. To investigate this relationship we used the Farquhar model of C₃ photosynthesis to predict the minimum V_c,max capable of supporting the acclimated stimulation in A observed at elevated pCO₂. For a wide range of species grown at elevated pCO₂ under contrasting conditions we found a strong correlation between observed and predicted values of V_c,max. This exercise mechanistically and quantitatively demonstrated that the observed acclimated stimulation of A and the simultaneous decrease in V_c,max observed at elevated pCO₂ is mechanistically consistent. With the exception of plants grown at a high elevated pCO₂ (> 90 Pa), which show evidence of an excess investment in Rubisco, the failure to maintain the initial stimulation of A is almost entirely attributable to the decrease in V_c,max and investment in Rubisco is coupled to requirements.     

Interannual climatic variation mediates elevated CO2 and O3 effects on forest growth

We analyzed growth data from model aspen (Populus tremuloides Michx.) forest ecosystems grown in elevated atmospheric carbon dioxide ([CO₂]; 518 μL L^?1) and ozone concentrations ([O₃]; 1.5 × background of 30–40 nL L^?1 during daylight hours) for 7 years using free‐air CO₂ enrichment technology to determine how interannual variability in present‐day climate might affect growth responses to either gas. We also tested whether growth effects of those gasses were sustained over time. Elevated [CO₂] increased tree heights, diameters, and main stem volumes by 11%, 16%, and 20%, respectively, whereas elevated ozone [O₃] decreased them by 11%, 8%, and 29%, respectively. Responses similar to these were found for stand volume and basal area. There were no growth responses to the combination of elevated [CO₂+O₃]. The elevated [CO₂] growth stimulation was found to be decreasing, but relative growth rates varied considerably from year to year. Neither the variation in annual relative growth rates nor the apparent decline in CO₂ growth response could be explained in terms of nitrogen or water limitations. Instead, growth responses to elevated [CO₂] and [O₃] interacted strongly with present‐day interannual variability in climatic conditions. The amount of photosynthetically active radiation and temperature during specific times of the year coinciding with growth phenology explained 20–63% of the annual variation in growth response to elevated [CO₂] and [O₃]. Years with higher photosynthetic photon flux (PPF) during the month of July resulted in more positive growth responses to elevated [CO₂] and more negative growth responses to elevated [O₃]. Mean daily temperatures during the month of October affected growth in a similar fashion the following year. These results indicate that a several‐year trend of increasingly cloudy summers and cool autumns were responsible for the decrease in CO₂ growth response.