Subnanomolar concentrations of membrane chitolipooligosaccharides from Rhizobium leguminosarum biovar trifolii are fully capable of eliciting symbiosis-related responses on white clover

Axenic seedling bioassays were performed on white clover, vetch, and alfalfa to assess the variety and dose responses of biological activities exhibited by membrane chitolipooligosaccharides (CLOSs) from wild type Rhizobium leguminosarum bv. trifolii ANU843. Subnanomolar concentrations of CLOSs induced deformation of root hairs (Had) and increased the number of foci of cortical cell divisions (Ccd) in white clover, some of which developed into nodule meristems. In contrast, ANU843 CLOSs were unable to induce Had in alfalfa and required a 10⁴-fold higher threshold concentration to induce this response in vetch. Also, ANU843 CLOSs were not mitogenic on either of these non-host legumes. In addition, CLOS action also increased chitinase activity in white clover root exudate. Thus, the membrane CLOSs from wild type R. leguminosarum bv. trifolii are fully capable of eliciting various symbiosis-related responses in white clover in the same concentration range as extracellular CLOSs of other rhizobia on their respective legume hosts. These results and our earlier studies indicate that membrane CLOSs represent one of many different classes of bioactive metabolites made by R. leguminosarum bv. trifolii which elicit more intense symbiosis-related responses in white clover than in other legumes. Therefore, CLOSs evidently play an important role in symbiotic development, but they may not be the sole determinant of host-range in the Rhizobium-clover symbiosis.Abbreviations Ccd cortical cell division - CLOS chitolipooligosaccharide - Had root hair deformation     

N2 fixation by Acacia species increases under elevated atmospheric CO2

In the present study the effect of elevated CO₂ on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol^?1) or elevated (700 µmol mol^?1) CO₂ concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO₂. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO₂, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO₂ increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.     

Influence ofRhizobium leguminosarum biovartrifolii host specific nodulation genes on the ontogeny of clover nodulation

Summary The infection of white clover seedlings byRhizobium strains with different host range properties was assessed using various microscopic techniques. Several wild-type andRhizobium leguminosarum biovarvicias hybrid strains containing definedR. l. bv.trifolii host range genes were used. The morphological changes in the root tissue of uninoculated and rhizobia inoculated white clovers were identified and compared. In particular, changes were observed in the induction of inner cortical cell division, alterations to nodule development and lateral root formation. The responses of the infected roots and the types of structures formed support the hypothesis that lateral roots and nodules may be physiologically homologous structures. To establish a normal pattern of nodulation on white clover roots, both sets of known host specific nodulation genes (operonsnod FERL andnod MNX) ofR. l. bv.trifolii were required. However, some nodule development occurred when only thenod FERL genes were present in the hybrid strain.     

Microscopic analysis of the effect ofRhizobium leguminosarum biovartrifolii host specific nodulation genes in the infection of white clovers

Summary A microscopic assessment is presented of the comparative infection capacity of wild-type and hybrid strains ofRhizobium leguminosarum bv.viciae withR. l. bv.trifolii strain ANU 843 on white clover seedlings. TheR. l. bv.viciae hybrid strains contained defined DNA segments coding for different combinations ofR. l. bv.trifolii host-specific nodulation genes. White clover plants were examined over a 72 h period to assessRhizobium infectivity, the morphological changes in root hair growth; colonisation ability of rhizobia; infection thread initiation and the ability to induce cortical cell division.R. l. bv.viciae strain 300 induced root hair curling more slowly than strain ANU 843 or any of the hybrid strain 300 bacteria, and when curling had taken place, there was poorer colonization by strain 300 within the folded hair cell, no evidence of infection thread formation and only limited cortical cell division 72 h after inoculation. The addition of the host-specific nodulation genes ofR. l. bv.trifolii to strain 300 was necessary to induce infection threads and establish a normal pattern of nodulation of the roots of white clovers.     

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.     

Effects of elevated CO2 on fine root dynamics in a Mojave Desert community: a FACE study

Fine roots (≤1 mm diameter) are critical in plant water and nutrient absorption, and it is important to understand how rising atmospheric CO₂ will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO₂ on production, mortality, and standing crops of fine root length over 2 years in a free‐air CO₂ enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient (～370 μmol mol^?1) and elevated CO₂ (～550 μmol mol^?1) atmospheric concentrations. Twenty‐eight minirhizotron tubes were placed in each ring to sample three microsite locations: evergreen Larrea shrubs, drought‐deciduous Ambrosia shrubs, and along systematic community transects (primarily in shrub interspaces which account for ～85% of the area). Seasonal dynamics were similar for ambient and elevated CO₂: fine root production peaked in April–June, with peak standing crop occurring about 1 month later, and peak mortality occurring during the hot summer months, with higher values for all three measures in a wet year compared with a dry year. Fine root standing crop, production, and mortality were not significantly different between treatments except standing crop along community transects, where fine root length was significantly lower in elevated CO₂. Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year^?1, and was not significantly different among CO₂ treatments, except for community transect tubes where it was significantly lower for elevated CO₂. There were no differences in fine root responses to CO₂ between evergreen (Larrea) and drought‐deciduous (Ambrosia) shrubs. Combined with observations of increased leaf‐level water‐use efficiency and lack of soil moisture differences, these results suggest that under elevated CO₂ conditions, reduced root systems (compared with ambient CO₂) appear sufficient to provide resources for modest aboveground production increases across the community, but in more fertile shrub microsites, fine root systems of comparable size with those in ambient CO₂ were required to support the greater aboveground production increases. For community transects, development of the difference in fine root standing crops occurred primarily through lower stimulation of fine root production in the elevated CO₂ treatment during periods of high water availability.     

Allocation responses to CO2 enrichment and defoliation by a native annual plant Heterotheca subaxillaris

Among plants grown under enriched atmospheric CO₂, root:shoot balance (RSB) theory predicts a proportionately greater allocation of assimilate to roots than among ambient‐grown plants. Conversely, defoliation, which decreases the plant's capacity to assimilate carbon, is predicted to increase allocation to shoot. We tested these RSB predictions, and whether responses to CO₂ enrichment were modified by defoliation, using Heterotheca subaxillaris, an annual plant native to south‐eastern USA. Plants were grown under near‐ambient (400 μmol mol^?1) and enriched (700 μmol mol^?1) levels of atmospheric CO₂. Defoliation consisted of the weekly removal of 25% of each new fully expanded, but not previously defoliated, leaf from either rosette or bolted plants. In addition to dry mass measurements of leaves, stems, and roots, Kjeldahl N, protein, starch and soluble sugars were analysed in these plant components to test the hypothesis that changes in C:N uptake ratio drive shifts in root:shoot ratio. Young, rapidly growing CO₂‐enriched plants conformed to the predictions of RSB, with higher root:shoot ratio than ambient‐grown plants (P < 0.02), whereas older, slower growing plants did not show a CO₂ effect on root:shoot ratio. Defoliation resulted in smaller plants, among which both root and shoot biomass were reduced, irrespective of CO₂ treatment (P < 0.03). However, H. subaxillaris plants were able to compensate for leaf area removal through flexible shoot allocation to more leaves vs. stem (P < 0.01). Increased carbon availability through CO₂ enrichment did not enhance the response to defoliation, apparently because of complete growth compensation for defoliation, even under ambient conditions. CO₂‐enriched plants had higher rates of photosynthesis (P < 0.0001), but this did not translate into increased final biomass accumulation. On the other hand, earlier and more abundant yield of flower biomass was an important consequence of growth under CO₂ enrichment.     

Effect of Seed Treatment with Rhizobium leguminosarum on Pythium Damping-off, Seedling Height, Root Nodulation, Root Biomass, Shoot Biomass, and Seed Yield of Pea and Lentil

Field experiments were conducted in 2004 and 2005 to determine the effects of seed treatment with Rhizobium leguminosarum bv. viceae on damping‐off, seedling height, root nodule mass, root biomass, shoot biomass and seed yield of pea and lentil in a field naturally infested with Pythium spp. Compared with the untreated controls, treatment of pea seeds with R. leguminosarum bv. viceae strains R12, R20 or R21 significantly (P < 0.05) reduced incidence of damping‐off, promoted seedling growth and increased root nodule mass, root biomass and shoot biomass. Seed treatments with R12 or R21 also resulted in a significant (P < 0.05) increase in seed yield of pea. The strain R21 was most effective among the four strains of R. leguminosarum bv. viceae tested in peas. Although, the level of disease control by strain R21 was similar to seed treatment with the fungicide Thiram^TM, R21 was more effective in enhancing root nodule production and promoting plant growth. For lentil, treatment of seeds with R. leguminosarum bv. viceae strains R12 or R21 significantly (P < 0.05) reduced incidence of damping‐off compared with the untreated control. All of the four strains of R. leguminosarum bv. viceae tested increased lentil seedling height, root nodule mass and shoot biomass, and all except R20 increased root biomass. Seed yield was higher for the treatments of R12 and R21. The strain R12 was most effective among the four strains of R. leguminosarum bv. viceae tested in lentil. Although, strain R12 was as effective as Thiram^TM for control of damping‐off of lentil, it was more effective than Thiram^TM for the production of root nodules and promotion of plant growth. The study concludes that seed treatment with R. leguminosarum bv. viceae is effective in control of Pythium damping‐off of pea and lentil and that the efficacy of control is strain specific, strain R21 for control of the disease on pea and strain R12 for control of the disease on lentil.     

The response of the Rhizobium leguminosarum bv. trifolii wild-type and exopolysaccharide-deficient mutants to oxidative stress

Aims

The aim of this study was investigation of the response of R. leguminosarum bv. trifolii wild-type and its two rosR and pssA mutant strains impaired in exopolysaccharide (EPS) synthesis to oxidative stress conditions caused by two prooxidants: a superoxide anion generator- menadione (MQ) and hydrogen peroxide (H₂O₂).

Methods

The levels of enzymatic (catalase, superoxide dismutase, pectinase and β-glucosidase) and non-enzymatic (superoxide anion generator, formaldehyde, phenolic compounds) biomarkers were monitored using biochemical methods in both the supernatants and rhizobial cells after treatment with 0.3?mM MQ and 1.5?mM H₂O₂. The viability of bacterial cells was estimated using fluorescent dyes and confocal laser scanning microscopy. In addition, the effect of prooxidants on symbiosis of the R. leguminosarum bv. trifolii strains with clover was established.

Results

The tested stress factors significantly changed enzymatic patterns of the rhizobial strains, and the wild-type strain proved to be more resistant to these prooxidants than both pssA and rosR mutants. Significantly higher activities of both catalase and superoxide dismutase have been detected in these mutants in comparison to the wild-type strain. H₂O₂ and MQ also increased the levels of pectinase and β-glucosidase activities in the tested strains. Moreover, pre-incubation of R. leguminosarum bv. trifolii strains with the prooxidants negatively affected the viability of bacterial cells and the number of nodules elicited on clover plants.

Conclusions

EPS produced in large amounts by R. leguminosarum bv. trifolii plays a significant protective role as a barrier against oxidative stress factors and during symbiotic interactions with clover plants.     

Construction of a Symbiotically Effective Strain of Rhizobium leguminosarum bv. trifolii with Increased Nodulation Competitiveness

Genes involved in nodulation competitiveness (tfx) were inserted by marker exchange into the genome of the effective strain Rhizobium leguminosarum bv. trifolii TA1. Isogenic strains of TA1 were constructed which differed only in their ability to produce trifolitoxin, an antirhizobial peptide. Trifolitoxin production by the ineffective strain R. leguminosarum bv. trifolii T24 limited nodulation of clover roots by trifolitoxin-sensitive strains of R. leguminosarum bv. trifolii. The trifolitoxin-producing exconjugant TA1::10-15 was very competitive for nodulation on clover roots when coinoculated with a trifolitoxin-sensitive reference strain. The nonproducing exconjugant TA1::12-10 was not competitive for nodule occupancy when coinoculated with the reference strain. Tetracycline sensitivity and Southern analysis confirmed the loss of vector DNA in the exconjugants. Trifolitoxin production by TA1::10-15 was stable in the absence of selection pressure. Transfer of tfx to TA1 did not affect nodule number or nitrogenase activity. These experiments represent the first stable genetic transfer of genes involved in nodulation competitiveness to a symbiotically effective Rhizobium strain.     

Enhancement of symbiotic nitrogen fixation by vitamin-secreting fluorescent Pseudomonas

Fluorescent Pseudomonas sp. strain 267 promotes growth of nodulated clover plants under gnotobiotic conditions. In the growth conditions (60 M FeCl₃), the production of siderophores of the pseudobactin-pyoverdin group was repressed. Plant growth enhancement results from secretion of B vitamins by Pseudomonas sp. strain 267. This was proven by stimulation of clover growth by naturally auxotrophic strains of Rhizobium leguminosarum bv. trifolii and marker strains E. coli thi^- and R. meliloti pan^- in the presence of the supernatant of Pseudomonas sp. strain 267. The addition of vitamins to the plant medium increased symbiotic nitrogen fixation by the clover plants.     

Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings

In this study, we investigated the impact of elevated atmospheric CO₂ (ambient + 350 μmol mol^–1) on fine root production and respiration in Scots pine (Pinus sylvestris L.) seedlings. After six months exposure to elevated CO₂, root production measured by root in-growth bags, showed significant increases in mean total root length and biomass, which were more than 100% greater compared to the ambient treatment. This increased root length may have lead to a more intensive soil exploration. Chemical analysis of the roots showed that the roots in the elevated treatment accumulated more starch and had a lower C/N-ratio. Specific root respiration rates were significantly higher in the elevated treatment and this was probably attributed to increased nitrogen concentrations in the roots. Rhizospheric respiration and soil CO₂ efflux were also enhanced in the elevated treatment. These results clearly indicate that under elevated atmospheric CO₂ root production and development in Scots pine seedlings is altered and respiratory carbon losses through the root system are increased.     

Biomass allocation in old-field annual species grown in elevated CO2 environments: no evidence for optimal partitioning

Increased atmospheric carbon dioxide supply is predicted to alter plant growth and biomass allocation patterns. It is not clear whether changes in biomass allocation reflect optimal partitioning or whether they are a direct effect of increased growth rates. Plasticity in growth and biomass allocation patterns was investigated at two concentrations of CO₂ ([CO₂]) and at limiting and nonlimiting nutrient levels for four fast‐ growing old‐field annual species. Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, and Polygonum pensylvanicum were grown from seed in controlled growth chamber conditions at current (350 μmol mol^?1, ambient) and future‐ predicted (700 μmol mol^?1, elevated) CO₂ levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throughout vegetative development. Under nonlimiting nutrient conditions, whole plant growth was increased greatly under elevated [CO₂] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant increases in whole plant growth were observed under limiting nutrient conditions. Plants grown in elevated [CO₂] had lower or unchanged root:shoot ratios, contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analysed, indicating that CO₂‐induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO₂] for these species. The general lack of biomass allocation responses to [CO₂] availability is in stark contrast with known responses of these species to light and nutrient gradients. We conclude that biomass allocation responses to elevated atmospheric [CO₂] are not consistent with optimal partitioning predictions.     

Effects of atmospheric CO2 enrichment and root restriction on leaf gas exchange and growth of banana (Musa)

The effects of atmospheric CO₂ enrichment and root restriction on photosynthetic characteristics and growth of banana (Musa sp. AAA cv. Gros Michel) plants were investigated. Plants were grown aeroponically in root chambers in controlled environment glasshouse rooms at CO₂ concentrations of 350 or 1 000 μmol CO₂ mol^-1. At each CO₂ concentration, plants were grown in large (2001) root chambers that did not restrict root growth or in small (20 1) root chambers that restricted root growth. Plants grown at 350 μmol CO₂ mol^-1 generally had a higher carboxylation efficiency than plants grown at 1 000 μmol CO₂ mol^-1 although actual net CO₂ assimilation (A) was higher at the higher ambient CO₂ concentration due to increased intercellular CO₂ concentrations (C_i resulting from CO₂ enrichment. Thus, plants grown at 1 000 μmol CO₂ mol^-1 accumulated more leaf area and dry weight than plants grown at 350 μmol CO₂ mol^-1. Plants grown in the large root chambers were more photosynthetically efficient than plants grown in the small root chambers. At 350 μmol CO₂ mol^-1, leaf area and dry weights of plant organs were generally greater for plants in the large root chambers compared to those in the small root chambers. Atmospheric CO₂ enrichment may have compensated for the effects of root restriction on plant growth since at 1 000 μmol CO₂ mol^-1 there was generally no effect of root chamber size on plant dry weight.     

The effects of elevated atmospheric CO2 on the amount and depth distribution of plant water uptake in a California annual grassland

Soil moisture profiles can affect species composition and ecosystem processes, but the effects of increased concentrations of atmospheric carbon dioxide ([CO₂]) on the vertical distribution of plant water uptake have not been studied. Because plant species composition affects soil moisture profiles, and is likely to shift under elevated [CO₂], it is also important to test whether the indirect effects of [CO₂] on soil water content may depend on species composition. We examined the effects of elevated [CO₂] and species composition on soil moisture profiles in an annual grassland of California. We grew monocultures and a mixture of Avena barbata and Hemizonia congesta– the dominant species of two phenological groups – in microcosms exposed to ambient (～370 μmol mol^?1) and elevated (～700 μmol mol^?1) [CO₂]. Both species increased intrinsic and yield‐based water use efficiency under elevated [CO₂], but soil moisture increased only in communities with A. barbata, the dominant early‐season annual grass. In A. barbata monocultures, the [CO₂] treatment did not affect the depth distribution of soil water loss. In contrast to communities with A. barbata, monocultures of H. congesta, a late‐season annual forb, did not conserve water under elevated [CO₂], reflecting the increased growth of these plants. In late spring, elevated [CO₂] also increased the efficiency of deep roots in H. congesta monocultures. Under ambient [CO₂], roots below 60 cm accounted for 22% of total root biomass and were associated with 9% of total water loss, whereas in elevated [CO₂], 16% of total belowground biomass was associated with 34% of total water loss. Both soil moisture and isotope data showed that H. congesta monocultures grown under elevated [CO₂] began extracting water from deep soils 2 weeks earlier than plants in ambient [CO₂].     

Trifolitoxin Production and Nodulation Are Necessary for the Expression of Superior Nodulation Competitiveness by Rhizobium leguminosarum bv. trifolii Strain T24 on Clover

Rhizobium leguminosarum bv. trifolii T24 is ineffective in symbiotic nitrogen fixation, produces a potent antibiotic (referred to here as trifolitoxin) that is bacteriostatic to certain Rhizobium strains, and is very competitive for clover root nodulation (EA Schwinghamer, RP Belkengren 1968 Arch Mikrobiol 64: 130-145). The primary objective of this work was to demonstrate the roles of nodulation and trifolitoxin production in the expression of nodulation competitiveness by T24. Unlike wildtype T24, transposon mutants of T24 lacking trifolitoxin production were unable to decrease clover nodulation by an effective, trifolitoxin-sensitive strain of R. leguminosarum bv. trifolii. A non-nodulating transposon mutant of T24 prevented clover nodulation by a trifolitoxin-sensitive R. leguminosarum bv. trifolii when co-inoculated with a T24 mutant lacking trifolitoxin production. Neither mutant alone prevented nodulation by the trifolitoxin-sensitive strain. These results demonstrate that trifolitoxin production and nodulation are required for the expression of nodulation competitiveness by strain T24. A trifolitoxin-sensitive strain of R. meliloti did not nodulate alfalfa when co-inoculated with T24 and a trifolitoxin-resistant strain of R. meliloti. Thus, a trifolitoxin-producing strain was useful in regulating nodule occupancy on a legume host other than clover. Trifolitoxin production was constitutive in both minimal and enriched media. Trifolitoxin was found to inhibit the growth of 95% of all strains of R. leguminosarum bvs. trifolii, viceae, and phaseoli tested. Strains of all 13 biotypes of R. leguminosarum bv. trifolii were inhibited by trifolitoxin. Three strains of R. fredii were also inhibited. Strain T24 ineffectively nodulated 46 clover species, did not nodulate Trifolium ambiguum, and induced partially effective nodules on Trifolium micranthum. Since T24 produced partially effective nodules on T. micranthum and since a trifolitoxin-minus mutant of T24 induced ineffective nodules, trifolitoxin production is not the cause of the symbiotic ineffectiveness of T24.     

Impact of arbuscular mycorrhizal fungi (AMF) and atmospheric CO2 concentration on the biomass production and partitioning in the forage legume alfalfa

The influence of mycorrhizal symbiosis, atmospheric CO₂ concentration and the interaction between both factors on biomass production and partitioning were assessed in nodulated alfalfa (Medicago sativa L.) associated or not with arbuscular mycorrhizal fungi (AMF) and grown in greenhouse at either ambient (392 μmol?mol^?1) or elevated (700 μmol?mol^?1) CO₂ air concentrations. Measurements were performed at three stages of the vegetative period of plants. Shoot and root biomass achieved by plants at the end of their vegetative period were highly correlated to the photosynthetic rates reached at earlier stages, and there was a significant relationship between CO₂ exchange rates and total nodule biomass per plant. In non-mycorrhizal alfalfa, the production of leaves, stems and nodules biomass significantly increased when plants had been exposed to elevated CO₂ concentration in the atmosphere for 4 weeks. Regardless CO₂ concentration at which alfalfa were cultivated, mycorrhizal symbiosis improved photosynthetic rates and growth of alfalfa at early stages of the vegetative period and then photosynthesis decreased, which suggests that AMF shortened the vegetative period of the host plants. At final stages of the vegetative period, AMF enhanced both area and biomass of leaves as well as the leaves to stems ratio when alfalfa plants were cultivated at ambient CO₂. The interaction of AMF with elevated CO₂ improved root biomass and slightly increased the leaves to stems ratio at the end of the vegetative growth. Therefore, AMF may favor both the forage quality of alfalfa when grown at ambient CO₂ and its perennity for next cutting regrowth cycle when grown under elevated CO₂. Nevertheless, this hypothesis needs to be checked under natural conditions in field.     

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.