Combined effects of elevated CO2 and soil drought on carbon and nitrogen allocation of the desert shrub Caragana intermedia

Impacts of either elevated CO₂ or drought stress on plant growth have been studied extensively, but interactive effects of these on plant carbon and nitrogen allocation is inadequately understood yet. In this study the response of the dominant desert shrub, Caragana intermedia Kuanget H.c.Fu, to the interaction of elevated CO₂ (700 ± 20 μmol mol⁻¹) and soil drought were determined in two large environmental growth chambers (18 m²). Elevated CO₂ increased the allocation of biomass and carbon into roots and the ratio of carbon to nitrogen (C:N) as well as the leaf soluble sugar content, but decreased the allocation of biomass and carbon into leaves, leaf nitrogen and leaf soluble protein concentrations. Elevated CO₂ significantly decreased the partitioning of nitrogen into leaves, but increased that into roots, especially under soil drought. Elevated CO₂ significantly decreased the carbon isotope discrimination (Δ) in leaves, but increased them in roots, and the ratio of Δ values between root and leaf, indicating an increased allocation into below-ground parts. It is concluded that stimulation of plant growth by CO₂ enrichment may be negated under soil drought, and under the future environment, elevated CO₂ may partially offset the negative effects of enhanced drought by regulating the partitioning of carbon and nitrogen.     

Effects of elevated CO2 and temperature-grown red and sugar maple on gypsy moth performance

Few studies have investigated how tree species grown under elevated CO₂ and elevated temperature alter the performance of leaf‐feeding insects. The indirect effects of an elevated CO₂ concentration and temperature on leaf phytochemistry, along with potential direct effects on insect growth and consumption, may independently or interactively affect insects. To investigate this, we bagged larvae of the gypsy moth on leaves of red and sugar maple growing in open‐top chambers in four CO₂/temperature treatment combinations: (i) ambient temperature, ambient CO₂; (ii) ambient temperature, elevated CO₂ (+ 300 μL L^?1 CO₂); (iii) elevated temperature (+ 3.5°C), ambient CO₂; and (iv) elevated temperature, elevated CO₂. For both tree species, leaves grown at elevated CO₂ concentration were significantly reduced in leaf nitrogen concentration and increased in C: N ratio, while neither temperature nor its interaction with CO₂ concentration had any effect. Depending on the tree species, leaf water content declined (red maple) and carbon‐based phenolics increased (sugar maple) on plants grown in an enriched CO₂ atmosphere. The only observed effect of elevated temperature on leaf phytochemistry was a reduction in leaf water content of sugar maple leaves. Gypsy moth larval responses were dependent on tree species. Larvae feeding on elevated CO₂‐grown red maple leaves had reduced growth, while temperature had no effect on the growth or consumption of larvae. No significant effects of either temperature or CO₂ concentration were observed for larvae feeding on sugar maple leaves. Our data demonstrate strong effects of CO₂ enrichment on leaf phytochemical constituents important to folivorous insects, while an elevated temperature largely has little effect. We conclude that alterations in leaf chemistry due to an elevated CO₂ atmosphere are more important in this plant–folivorous insect system than either the direct short‐term effects of temperature on insect performance or its indirect effects on leaf chemistry.     

Strong effects of hydrologic environment and weak effects of elevated CO<Subscript>2</Subscript> on the invasive weed <Emphasis Type="Italic">Alternanthera philoxeroides</Emphasis> and the biocontrol beetle <Emphasis Type="Italic">Agasicles hygrophila</Emphasis>

Global change, such as elevated CO₂, may alter interactions between invasive plants and biocontrol agents, impacting biocontrol efficacy. Here, we conducted four experiments in Texas, USA to test how elevated CO₂ influences an invasive plant (Alternanthera philoxeroides) and its interactions with an introduced biocontrol beetle (Agasicles hygrophila) in terrestrial (well-watered) and flooded environments. We grew plants for 9 months in ambient or elevated CO₂ (800 ppm) chambers in continuously flooded or well-watered conditions. In no-choice trials, flooding increased leaf toughness and decreased beetle consumption but beetles only oviposited on ambient CO₂ leaves. In choice trials, beetles preferred to feed and oviposit on terrestrial plants but were also less likely to damage elevated CO₂ leaves. Caged beetle populations were larger in terrestrial conditions than aquatic conditions for a second set of plants grown in the chambers. With a third set of plants grown in the ambient or elevated CO₂ chambers, damage for plants placed in the field (aquatic setting) was higher for plants grown in terrestrial conditions vs. flooded conditions at ambient CO₂. Our results suggest that elevated CO₂ will have minor effects on the efficacy of this biocontrol agent by decreasing oviposition and number of leaves damaged, and hydrologic environment may affect invasive plant performance by altering herbivore oviposition and feeding preferences. A broader understanding of the effects of global change on biocontrol will help prevent and manage future spread of invasive plants.     

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.     

Accumulation of phenolic compounds in birch leaves is changed by elevated carbon dioxide and ozone

Atmospheric change may affect plant phenolic compounds, which play an important part in plant survival. Therefore, we studied the impacts of CO₂ and O₃ on the accumulation of 27 phenolic compounds in the short‐shoot leaves of two European silver birch (Betula pendula Roth) clones (clones 4 and 80). Seven‐year‐old soil‐grown trees were exposed in open‐top chambers over three growing seasons to ambient and twice ambient CO₂ and O₃ concentrations singly and in combination in central Finland. Elevated CO₂ increased the concentration of the phenolic acids (+25%), myricetin glycosides (+18%), catechin derivatives (+13%) and soluble condensed tannins (+19%) by increasing their accumulation in the leaves of the silver birch trees, but decreased the flavone aglycons (?7%) by growth dilution. Elevated O₃ increased the concentration of 3,4′‐dihydroxypropiophenone 3‐β‐d ‐glucoside (+22%), chlorogenic acid (+19%) and flavone aglycons (+4%) by inducing their accumulation possibly as a response to increased oxidative stress in the leaf cells. Nevertheless, this induction of antioxidant phenolic compounds did not seem to protect the birch leaves from detrimental O₃ effects on leaf weight and area, but may have even exacerbated them. On the other hand, elevated CO₂ did seem to protect the leaves from elevated O₃ because all the O₃‐derived effects on the leaf phenolics and traits were prevented by elevated CO₂. The effects of the chamber and elevated CO₂ on some compounds changed over time in response to the changes in the leaf traits, which implies that the trees were acclimatizing to the altered environmental conditions. Although the two clones used possessed different composition and concentrations of phenolic compounds, which could be related to their different latitudinal origin and physiological characteristics, they responded similarly to the treatments. However, in some cases the variation in phenolic concentrations caused by genotype or chamber environment was much larger than the changes caused by either elevated CO₂ or O₃.     

The influence of elevated CO2 and temperature on biomass production of continuously defoliated white clover

Clonal plants of white clover (Trifolium repens L.), grown singly in pots of Perlite and solely dependent for nitrogen on root nodule N₂ fixation, were maintained in controlled environments which provided four environments: 18/13 °C day/night temperature at 340 and 680 μmol mol^?1 CO₂ and 20·5/15·5°C day/night temperature at 340 and 680 μmol mol^?1 CO₂. The daylength was 12 h and the photon flux density 500±25 μmol m^?2 s^?1 (PFD). All plants were defoliated for about 80d, nominally every alternate day, to leave the youngest expanded leaf intact on 50% of stolons, plus expanding leaves (simulated grazing). Elevated CO₂ increased the yield of biomass removed at defoliation by a constant 45% during the second 40d of the experiment and by a varying amount in the first half of the experiment. Elevated temperature had little effect on biomass yield. Nitrogen, as a proportion of the harvested biomass, was only fractionally affected by elevated CO₂ or temperature. In contrast, N₂ fixation increased in concert with the promoting effect of elevated CO₂ on biomass production. The increased yield of biomass harvested in 680 μmol mol^?1 CO₂ was primarily due to the early development and continued maintenance of more stolons. However, the stolons of plants grown in elevated CO₂ also developed leaves which were heavier and slightly larger in area than their counterparts in ambient CO₂. The conclusion is that, when white clover plants are maintained at constant mass by simulated grazing, they continue to respond to elevated CO₂ in terms of a sustained increase in biomass production.     

Structural characteristics and chemical composition of birch (Betula pendula) leaves are modified by increasing CO2 and ozone

Impacts of ozone and CO₂ enrichment, alone and in combination, on leaf anatomical and ultrastructural characteristics, nutrient status and cell wall chemistry in two European silver birch (Betula pendula Roth) clones were studied. The young soil‐growing trees were exposed in open‐top chambers over three growing seasons to 2 × ambient CO₂ and/or ozone concentrations in central Finland. The trees were measured for changes in altogether 35 variables of leaf structure, nutrients and cell wall chemistry of green leaves, and 20 of the measured variables were affected by CO₂ and/or O₃. Elevated CO₂ increased the size of chloroplasts and starch grains, number of mitochondria, P : N ratio, and contents of cell wall hemicellulose. Elevated CO₂ decreased the total leaf thickness, specific leaf area, concentrations of N, K, Cu, S and Fe, and contents of cell wall α‐cellulose, uronic acids, acid‐soluble lignin and acetone‐soluble extractives. Elevated ozone led to thinner leaves, higher palisade to spongy ratio, increased number of peroxisomes and mitochondria, reduced content of Mn, Zn, Cu, hemicellulose and uronic acids, and lower Mn : N and Zn : N ratios. In the combined exposure, interactions were antagonistic. Ultrastructural changes became more evident towards the end of the exposure. Young leaves were tolerant against ozone‐caused oxidative stress, whereas oxidative H₂O₂ accumulation was found in older leaves. CO₂ enrichment improved ozone tolerance not only through increased photosynthesis rates, but also through changes in cell wall chemistry (hemicellulose, in particular). However, nutrient imbalances due to ozone and/or CO₂ may predispose the trees to other biotic and abiotic stresses. Down‐regulation and up‐regulation of photosynthesis under elevated CO₂ through anatomical changes is discussed.     

Effect of CO2 enrichment on non-structural carbohydrates in leaves, stems and ears of spring wheat

Field-grown spring wheat (Triticum aestivum L. cv. Dragon) was exposed to ambient and elevated CO₂ concentrations (1.5 and 2 times ambient) in open-top chambers. Contents of non-structural carbohydrates were analysed enzymatically in leaves, stems and ears six times during the growing season. The impact of elevated CO₂ on wheat carbohydrates was non-significant in most harvests. However, differences in the carbohydrate contents due to elevated CO₂ were found in all plant compartments. Before anthesis, at growth stage (GS) 30 (the stem is 1 cm to the shoot apex), the plants grown in elevated CO₂ contained significantly more water soluble carbohydrates (WSC), fructans, starch and total non-structural carbohydrates (TNC) in the leaves in comparison with the plants grown in ambient CO₂. It is hypothesised that the plants from the treatments with elevated CO₂ were sink-limited at GS30. After anthesis, the leaf WSC and TNC contents of the plants from elevated CO₂ started to decline earlier than those of the plants from ambient CO₂. This may indicate that the leaves of plants grown in the chambers with elevated CO₂ senesced earlier. Elevated CO₂ accelerated grain development: 2 weeks after anthesis, the plants grown in elevated CO₂ contained significantly more starch and significantly less fructans in the ears compared to the plants grown in ambient CO₂. Elevated CO₂ had no effect on ear starch and TNC contents at the final harvest. Increasing the CO₂ concentration from 360 to 520 μmol mol^?1 had a larger effect on wheat non-structural carbohydrates than the further increase from 520 to 680 μmol mol^?1. The results are discussed in relation to the effects of elevated CO₂ on yield and yield components.     

The effects of elevated CO2 and temperature on the resistance of winter-dormant birch seedlings (Betula pendula) to hares and voles

The effects of elevated CO₂ and temperature on the resource allocation pattern and resistance against mammalian herbivores of silver birch (Betula pendula Roth) were studied. Birch seedlings were grown through two growing seasons in closed‐top chambers exposed to four different treatments: ambient CO₂ and temperature, elevated atmospheric CO₂ (700 ppm) and ambient temperature, elevated temperature (+3°C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. After winter hardening of the seedlings, the growth of the seedlings was measured and the concentration of secondary compounds such as phenolics and papyriferic acid determined. The top parts of the stem were fed to hares, and the basal parts of the same stems were offered to voles. Elevated CO₂ increased the height and basal diameter of the shoots, shoot biomass and total biomass of the seedlings but did not have any effect on secondary chemistry. Elevated temperature increased the height and shoot biomass, but did not have a significant effect on the total biomass of the seedlings. Elevated temperature decreased the concentration of condensed tannins and their precursor, (+)‐catechin, in the top part of the stems, but only the concentration of (+)‐catechin in the basal part of the stems. There were no significant interactive effects between CO₂ and temperature on phenolics in the stems, while the concentration of papyriferic acid showed significant interaction in the top part of the stems. This indicates high accumulation of papyriferic acid in ambient CO₂ under increased temperature. Consequently, elevated temperature increased the resistance of birch against hares, but did not affect the resistance of the basal parts of the same birches to voles. Our results indicate that the predicted climatic change will not necessarily lead to increased browsing damage by the mountain hare and the field vole to silver birch.     

Photosynthetic responses to elevated CO2and O3 in field-grown potato(Solanum tuberosum)

Potato plants (Solanum tuberosum L. cv. Bintje) were grown to maturity in open-top chambers under three carbon dioxide (CO₂; ambient and 24 h d⁻¹ seasonal mean concentrations of 550 and 680 μmol mol⁻¹) and two ozone levels (O₃; ambient and an 8 h d⁻¹ seasonal mean of 50 nmol mol⁻¹). Chlorophyll content, photosynthetic characteristics, and stomatal responses were determined to test the hypothesis that elevated atmospheric CO₂ may alleviate the damaging influence of O₃ by reducing uptake by the leaves. Elevated O₃ had no detectable effect on photosynthetic characteristics, leaf conductance, or chlorophyll content, but did reduce SPAD values for leaf 15, the youngest leaf examined. Elevated CO₂ also reduced SPAD values for leaf 15, but not for older leaves; destructive analysis confirmed that chlorophyll content was decreased. Leaf conductance was generally reduced by elevated CO₂, and declined with time in the youngest leaves examined, as did assimilation rate (A). A generally increased under elevated CO₂, particularly in the older leaves during the latter stages of the season, thereby increasing instantaneous transpiration efficiency. Exposure to elevated CO₂ and/or O₃ had no detectable effect on dark-adapted fluorescence, although the values decreased with time. Analysis of the relationships between assimilation rate and intercellular CO₂ concentration and photosynthetically active photon flux density showed there was initially little treatment effect on CO₂-saturated assimilation rates for leaf 15. However, the values for plants grown under 550 μmol mol⁻¹ CO₂ were subsequently greater than in the ambient and 680 μmol mol⁻¹ treatments, although the beneficial influence of the former treatment declined sharply towards the end of the season. Light-saturated assimilation was consistently greater under elevated CO₂, but decreased with time in all treatments. The values decreased sharply when leaves grown under elevated CO₂ were measured under ambient CO₂, but increased when leaves grown under ambient CO₂ were examined under elevated CO₂. The results obtained indicate that, although elevated CO₂ initially increased assimilation and growth, these beneficial effects were not necessarily sustained to maturity as a result of photosynthetic acclimation and the induction of earlier senescence.     

The fraction of expanding to expanded leaves determines the biomass response of Populus to elevated CO2

We examined whether the effects of elevated CO₂ on growth of 1-year old Populus deltoides saplings was a function of the assimilation responses of particular leaf developmental stages. Saplings were grown for 100 days at ambient (approximately 350 ppm) and elevated (ambient + 200 ppm) CO₂ in forced-air greenhouses. Biomass, biomass distribution, growth rates, and leaf initiation and expansion rates were unaffected by elevated CO₂. Leaf nitrogen (N), the leaf C:N ratio, and leaf lignin concentrations were also unaffected. Carbon gain was significantly greater in expanding leaves of saplings grown at elevated compared to ambient CO₂. The Rubisco content in expanding leaves was not affected by CO₂ concentration. Carbon gain and Rubisco content were significantly lower in fully expanded leaves of saplings grown at elevated compared to ambient CO₂, indicating CO₂-induced down-regulation in fully expanded leaves. Elevated CO₂ likely had no overall effect on biomass accumulation due to the more rapid decline in carbon gain as leaves matured in saplings grown at elevated compared to ambient CO₂. This decline in carbon gain has been documented in other species and shown to be related to a balance between sink/source balance and acclimation. Our data suggest that variation in growth responses to elevated CO₂ can result from differences in leaf assimilation responses in expanding versus expanded leaves as they develop under elevated CO₂. Received: 28 September 1998 / Accepted: 23 June 1999     

The effect of elevated CO2 and temperature on the secondary chemistry of Betula pendula seedlings

The effect of elevated atmospheric CO₂ and temperature on resource allocation and secondary chemistry of white birch (Betula pendula Roth) under a non-limiting nutrient and water supply was investigated. Birch seedlings were grown in closed-top chambers exposed to ambient CO₂ and temperature, elevated atmospheric CO₂ (700 ppm), elevated temperature (2°C above ambient) and a combination of elevated CO₂ and temperature for one growing season. Elevated CO₂ significantly increased the total biomass of the seedlings. The combined effect of the elevated CO₂ and temperature treatments further increased the total biomass, but not significantly. The content of nitrogen and water decreased, while some secondary compounds (such as condensed tannins and flavonol glycosides) increased in leaves subjected to CO₂ enrichment. Elevated temperature increased the concentration of total flavone aglycones and decreased that of total HPLC-phenolics in the leaves, due to the decrease in individual flavonol glycosides, cinnamoylquinic acids and (+)-catechin. There were no significant interactive effects between CO₂ and temperature in the phenolic concentrations of the leaves and in the stems, while the number of resin droplets in the top part of the stems showed significant interaction. This clearly implies that carbon allocation into secondary metabolites in the leaves and stems differ under enhanced CO₂ and temperature, and the combined effect of CO₂ and temperature on the herbivore resistance of birches, is lower than that of CO₂ and temperature alone.     

Effects of elevated CO2 and temperature on the leaf chemistry of birch Betula pendula (Roth) and the feeding behaviour of the weevil Phyllobius maculicornis

Abstract 1 The effect of elevated CO₂ and temperature on the foliar chemistry Betula pendula Roth and the feeding performance of polyphagous weevils Phyllobius maculicornis Germ. was studied. Birch seedlings were grown during one growing season in chamber‐less field conditions and in closed‐top chambers exposed to four different treatments: ambient CO₂ (350 p.p.m) and temperature, elevated atmospheric CO₂ (700 p.p.m) and ambient temperature, elevated temperature +3 °C above ambient) and ambient CO₂, and a combination of elevated CO₂ and temperature. 2 In leaves under CO₂ enrichment, the concentration of nitrogen and some flavonol glycosides significantly decreased, whereas the concentration of total phenolics, condensed tannins and (+)‐catechin significantly increased. The total concentration of cinnamoylquinic acids was significantly increased by CO₂ and decreased by temperature. The concentration of salidroside increased under elevated temperature. 3 Weevil‐feeding experiments were carried out in a five‐choice arrangement, one leaf from each of the five treatments (chamber‐less field controls and four different treatments in chambers) being placed in random order in a plastic box. The weevils preferred the leaves grown under elevated CO₂, which had low nitrogen, high phenolics and the highest condensed tannin concentrations. Whether the reason for this trend is due to the stimulating effect of condensed tannins and/or a change in other secondary compounds, remains unknown. The weevils did not show any obviously different response in feeding performance to temperature and the combination of elevated CO₂ and temperature.     

Interactive effects of elevated atmospheric CO2, mycorrhization and drought on long-distance transport of reduced sulphur in young pedunculate oak trees (Quercus robur L.)

Pedunculate oak (Quercus robur L.) was germinated and grown under nutrient non-limiting conditions for a total of 10–15 weeks at ambient CO₂ concentration and 1100 μmol mol^–1 CO₂ either in the presence or the absence of the mycorrhizal fungus Laccaria laccata. Half of the oak trees of these treatments were exposed to drought during final growth by suspending the water supply for 21 d. Mycorrhization and elevated atmospheric CO₂ each enhanced total plant biomass per tree. Whereas additional biomass accumulation of trees grown under elevated CO₂ was mainly attributed to increased growth of lateral roots, mycorrhization promoted shoot growth. Water deficiency reduced biomass accumulation without affecting relative water content, but this effect was more pronounced in mycorrhizal as compared to non-mycorrhizal trees. Elevated CO₂ partially prevented the development of drought stress, as indicated by leaf water potential, but did not counteract the negative effects of water deficiency on growth during the time studied. Enhanced biomass accumulation requires adaption in protein synthesis and, as a consequence, enhanced allocation of reduced sulphur produced in the leaves to growing tissues. Therefore, allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH, the main long-distance transport form of reduced sulphur, to mature oak leaves. Export of radiolabel proceeded almost exclusively in basipetal direction to the roots. The rate of export of radioactivity out of the fed leaves was significantly enhanced under elevated CO₂, irrespective of mycorrhization. A higher proportion of the exported GSH was transported to the roots than to basipetal stem sections under elevated CO₂ as compared to ambient CO₂. Mycorrhization did not affect ³⁵S export out of the fed leaves, but the distribution of radiolabel between stem and roots was altered in preference of the stem. Trees exposed to drought did not show appreciable export of the ³⁵S radioactivity fed to the leaves when grown under ambient CO₂. Apparently, drought inhibited basipetal transport of reduced sulphur at the level of phloem loading and/or phloem transport. Elevated CO₂ seemed to counteract this effect of drought stress to some extent, since higher leaf water potentials and improved ³⁵S export out of the fed leaves was observed in oak trees exposed to drought and elevated CO₂ as compared to trees exposed to drought and ambient CO₂.     

Development of gypsy moth larvae feeding on red maple saplings at elevated CO<Subscript>2</Subscript> and temperature

Predicted increases in atmospheric CO₂ and global mean temperature may alter important plant-insect associations due to the direct effects of temperature on insect development and the indirect effects of elevated temperature and CO₂ enrichment on phytochemicals important for insect success. We investigated the effects of CO₂ and temperature on the interaction between gypsy moth (Lymantria dispar L.) larvae and red maple (Acer rubrum L.) saplings by bagging first instar larvae within open-top chambers at four CO₂/temperature treatments: (1) ambient temperature, ambient CO₂, (2) ambient temperature, elevated CO₂ (+300 l l^-1 CO₂), (3) elevated temperature (+3.5°C), ambient CO₂, and (4) elevated temperature, elevated CO₂. Larvae were reared to pupation and leaf samples taken biweekly to determine levels of total N, water, non-structural carbohydrates, and an estimate of defensive phenolic compounds in three age classes of foliage: (1) immature, (2) mid-mature and (3) mature. Elevated growth temperature marginally reduced (P <0.1) leaf N and significantly reduced (P <0.05) leaf water across CO₂ treatments in mature leaves, whereas leaves grown at elevated CO₂ concentration had a significant decrease in leaf N and a significant increase in the ratio of starch:N and total non-structural carbohydrates:N. Leaf N and water decreased and starch:N and total non-structural carbohydrates:N ratios increased as leaves aged. Phenolics were unaffected by CO₂ or temperature treatment. There were no interactive effects of CO₂ and temperature on any phytochemical measure. Gypsy moth larvae reached pupation earlier at the elevated temperature (female =8 days, P <0.07; male =7.5 days, P <0.03), whereas mortality and pupal fresh weight of insects were unrelated to either CO₂, temperature or their interaction. Our data show that CO₂ or temperature-induced alterations in leaf constituents had no effect on insect performance; instead, the long-term exposure to a 3.5°C increase in temperature shortened insect development but had no effect on pupal weight. It appears that in some tree-herbivorous insect systems the direct effects of an increased global mean temperature may have greater consequences for altering plant-insect interactions than the indirect effects of an increased temperature or CO₂ concentration on leaf constituents.     

Lower incidence and severity of tomato virus in elevated CO2 is accompanied by modulated plant induced defence in tomato

Elevation in atmospheric CO₂ concentration broadly affects plant phenology and physiology, and these effects may alter the performance of plant viruses. The effects of elevated CO₂ on the susceptibility of tomato plants to Tomato yellow leaf curl virus (TYLCV) were examined for two successive years in open top chambers (OTC) in the field. We experimentally tested the hypothesis that elevated CO₂ would reduce the incidence and severity of TYLCV on tomato by altering plant defence strategies. Our results showed that elevated CO₂ decreased TYLCV disease incidence (by 14.6% in 2009 and 11.8% in 2010) and decreased disease severity (by 20.0% in 2009 and 10.4% in 2010). Elevated CO₂ also decreased the level of TYLCV coat protein in tomato leaves. Regardless of virus infection, elevated CO₂ increased plant height and aboveground biomass. Additionally, elevated CO₂ increased the leaf C:N ratio of tomato, but decreased soluble protein content in leaves. Notably, elevated CO₂ increased the salicylic acid (SA) level in uninfected and infected plants. In contrast, elevated CO₂ reduced jasmonic acid (JA) in uninfected plants while it increased JA and abscisic acid (ABA) in virus‐infected plants. Furthermore, combined exogenous SA and JA application enhanced resistance to TYLCV more than application of either SA or JA alone. Our results suggest that the modulated antagonistic relationship between SA and JA under elevated CO₂ makes a great contribution to increased tomato resistance to TYLCV, and the predicted increases in tomato productivity may be enhanced by reduced plant virus susceptibility under projected rising CO₂ conditions.     

Decomposition of soybean grown under elevated concentrations of CO2 and O3

A critical global climate change issue is how increasing concentrations of atmospheric CO₂ and ground‐level O₃ will affect agricultural productivity. This includes effects on decomposition of residues left in the field and availability of mineral nutrients to subsequent crops. To address questions about decomposition processes, a 2‐year experiment was conducted to determine the chemistry and decomposition rate of aboveground residues of soybean (Glycine max (L.) Merr.) grown under reciprocal combinations of low and high concentrations of CO₂ and O₃ in open‐top field chambers. The CO₂ treatments were ambient (370 μmol mol^?1) and elevated (714 μmol mol^?1) levels (daytime 12 h averages). Ozone treatments were charcoal‐filtered air (21 nmol mol^?1) and nonfiltered air plus 1.5 times ambient O₃ (74 nmol mol^?1) 12 h day^?1. Elevated CO₂ increased aboveground postharvest residue production by 28–56% while elevated O₃ suppressed it by 15–46%. In combination, inhibitory effects of added O₃ on biomass production were largely negated by elevated CO₂. Plant residue chemistry was generally unaffected by elevated CO₂, except for an increase in leaf residue lignin concentration. Leaf residues from the elevated O₃ treatments had lower concentrations of nonstructural carbohydrates, but higher N, fiber, and lignin levels. Chemical composition of petiole, stem, and pod husk residues was only marginally affected by the elevated gas treatments. Treatment effects on plant biomass production, however, influenced the content of chemical constituents on an areal basis. Elevated CO₂ increased the mass per square meter of nonstructural carbohydrates, phenolics, N, cellulose, and lignin by 24–46%. Elevated O₃ decreased the mass per square meter of these constituents by 30–48%, while elevated CO₂ largely ameliorated the added O₃ effect. Carbon mineralization rates of component residues from the elevated gas treatments were not significantly different from the control. However, N immobilization increased in soils containing petiole and stem residues from the elevated CO₂, O₃, and combined gas treatments. Mass loss of decomposing leaf residue from the added O₃ and combined gas treatments was 48% less than the control treatment after 20 weeks, while differences in decomposition of petiole, stem, and husk residues among treatments were minor. Decreased decomposition of leaf residues was correlated with lower starch and higher lignin levels. However, leaf residues only comprised about 20% of the total residue biomass assayed so treatment effects on mass loss of total aboveground residues were relatively small. The primary influence of elevated atmospheric CO₂ and O₃ concentrations on decomposition processes is apt to arise from effects on residue mass input, which is increased by elevated CO₂ and suppressed by O₃.     

Effects of elevated CO2 and temperature on leaf characteristics,photosynthesis and carbon storage in aboveground biomass of a boreal bioenergy crop (Phalaris arundinacea L.) under varying water regimes

This work examined the effects of elevated CO₂ and temperature and water regimes, alone and in interaction, on the leaf characteristics [leaf area (LA), specific leaf weight (SLW), leaf nitrogen content (N_L) based on LA], photosynthesis (light‐saturated net carbon fixation rate, P_sat) and carbon storage in aboveground biomass of leaves (C_l) and stem (C_s) for a perennial reed canary grass (Phalaris arundinacea L., Finnish local cultivar). For this purpose, plants were grown under different water regimes (ranging from high to low soil moisture) in climate‐controlled growth chambers under the elevated CO₂ and/or temperature (following a factorial design) over a whole growing season (May–September in 2009). The results showed that the elevated temperature increased the leaf growth, photosynthesis and carbon storage of aboveground biomass the most in the early growing periods, compared with ambient temperature. However, the plant growth declined rapidly thereafter with a lower carbon storage at the end of growing season. This was related to the accelerated phenology regulation and consequent earlier growth senescence. Consequently, the elevation of CO₂ increased the P_sat, LA and SLW during the growing season, with a significant concurrent increase in the carbon storage in aboveground biomass. Low soil moisture decreased the P_sat, leaf stomatal conductance, LA and carbon storage in above ground biomass compared with high and normal soil moisture. This water stress effect was the largest under the elevated temperature. The elevated CO₂ partially mitigated the adverse effects of high temperature and low soil moisture. However, the combination of elevated temperature and CO₂ did not significantly increase the carbon storage in aboveground biomass of the plants.