Genetic variation in plant below-ground response to elevated CO2 and two herbivore species

Aims

It is unclear how changing atmospheric conditions, including rising carbon dioxide concentration, influence interactions between above and below-ground systems and if intraspecific variation exists in this response.

Methods

We assessed interactive effects of atmospheric CO₂ concentration, above-ground herbivory, and plant genotype on root traits and mycorrhizal associations. Plants from five families of Asclepias syriaca, a perennial forb, were grown under ambient and elevated atmospheric CO₂ concentrations. Foliar herbivory by either lepidopteran caterpillars or phloem-feeding aphids was imposed. Mycorrhizal colonization, below-ground biomass, root biomass, and secondary defensive chemistry in roots were quantified.

Results

We observed substantial genetic variation among A. syriaca families in their mycorrhizal colonization levels in response to elevated CO₂ and herbivory treatments. Elevated CO₂ treatment increased root biomass in all genetic families, whereas foliar herbivory tended to decrease root biomass. Root cardenolide concentration and composition varied greatly among plant families, and elevated CO₂ treatment increased root cardenolides in two of the five plant families. Moreover, herbivores differentially affected the composition of cardenolides expressed below ground.

Conclusions

Increased atmospheric CO₂ has the potential to influence interactions among plants, herbivores and mycorrhizal fungi and intraspecific variation suggests that such interactions can evolve.     

Exposure to warming and CO2 enrichment promotes greater above-ground biomass, nitrogen, phosphorus and arbuscular mycorrhizal colonization in newly established grasslands

Aims

In view of the projected increase in global air temperature and CO₂ concentration, the effects of climatic changes on biomass production, CO₂ fluxes and arbuscular mycorrhizal fungi (AMF) colonization in newly established grassland communities were investigated. We hypothesized that above- and below-ground biomass, gross primary productivity (GPP), AMF root colonization and nutrient acquisition would increase in response to the future climate conditions. Furthermore, we expected that increased below-ground C allocation would enhance soil respiration (R_soil).

Methods

Grassland communities were grown either at ambient temperatures with 375?ppm CO₂ (Amb) or at ambient temperatures +3°C with 620?ppm CO₂ (T+CO₂).

Results

Total biomass production and GPP were stimulated under T+CO₂. Above-ground biomass was increased under T+CO₂ while belowground biomass was similar under both climates. The significant increase in root colonization intensity under T+CO₂, and therefore the better contact between roots and AMF, probably determined the higher above-ground P and N content. R_soil was not significantly affected by the future climate conditions, only showing a tendency to increase under future climate at the end of the season.

Conclusions

Newly established grasslands benefited from the exposure to elevated CO₂ and temperature in terms of total biomass production; higher root AMF colonization may partly provide the nutrients required to sustain this growth response.     

Arbuscular mycorrhizas and their role in plant growth, nitrogen interception and soil gas efflux in an organic production system

Background and aims

Roots and mycorrhizas play an important role in not only plant nutrient acquisition, but also ecosystem nutrient cycling.

Methods

A field experiment was undertaken in which the role of arbuscular mycorrhizas (AM) in the growth and nutrient acquisition of tomato plants was investigated. A mycorrhiza defective mutant of tomato (Solanum lycopersicum L.) (named rmc) and its mycorrhizal wild type progenitor (named 76R) were used to control for the formation of AM. The role of roots and AM in soil N cycling was studied by injecting a ¹⁵N-labelled nitrate solution into surface soil at different distances from the 76R and rmc genotypes of tomato, or in plant free soil. The impacts of mycorrhizal and non-mycorrhizal root systems on soil greenhouse gas (CO₂ and ¹⁴⁺¹⁵N₂O and ¹⁵N₂O) emissions, relative to root free soils, were also studied.

Results

The formation of AM significantly enhanced plant growth and nutrient acquisition, including interception of recently applied NO ₃ ^? . Whereas roots caused a small but significant decrease in ¹⁵N₂O emissions from soils at 23?h after labeling, compared to the root-free treatment, arbuscular mycorrhizal fungi (AMF) had little effect on N₂O emissions. In contrast soil CO₂ emissions were higher in plots containing mycorrhizal root systems, where root biomass was also greater.

Conclusions

Taken together, these data indicate that roots and AMF have an important role to play in plant nutrient acquisition and ecosystem N cycling.     

Elevated CO2 improves root growth and cadmium accumulation in the hyperaccumulator Sedum alfredii

Aims

This study examined the effect of elevated CO₂ on plant growth, root morphology and Cd accumulation in S. alfredii, and assessed the possibility of using elevated CO₂ as fertilizer to enhance phytoremediation efficiency of Cd-contaminated soil by S. alfredii.

Methods

Both soil pot culture and hydroponic experiments were carried out to characterize plant biomass, root morphological parameters, and cadmium uptake in S. alfredii grown under ambient (350 μL L^?1) or elevated (800 μL L^?1) CO₂.

Results

Elevated CO₂ prompted the growth of S. alfredii, shoot and root biomass were increased by 24.6–36.7% and 35.0–52.1%, respectively, as compared with plants grown in ambient CO₂. After 10 days growth in medium containing 50 μM Cd under elevated CO₂, the development of lateral roots and root hairs were stimulated, additionally, root length, surface area, root volume and tip number were increased significantly, especially for the finest diameter roots. The total Cd uptake per pot was significantly greater under elevated CO₂ than under ambient CO₂. After 60 d growth, Cd phytoextraction efficiency was increased significantly in the elevated CO₂ treatment.

Conclusions

Results suggested that the use of elevated CO₂ may be a useful way to improve phytoremediation efficiency of Cd-contaminated soil by S. alfredii.     

Carbon allocation in Larrea tridentata plant-soil systems as affected by elevated soil moisture and N availability

Background and Aims

Global change will likely express itself in southwestern United States arid lands through changes in amounts and timing of precipitation in response to elevated CO₂ concentrations. In addition, increased nitrogen (N) deposition may occur due to increased urban development. This study addressed the effects of water and N availability on C allocation in arid land soil-plant systems.

Methods

Columns filled with Mojave Desert topsoil containing Larrea tridentata seedlings with two treatment levels each of N and soil moisture were labeled by exposure to ¹³C-enriched CO₂.

Results

Increased soil moisture increased plant biomass, total ¹³C uptake, ¹³C levels in leaves, soil organic matter, and soil respiration, decreased relative C allocation to stems but increased allocation to soil organic matter. Increased soil N availability increased N uptake but decreased C allocation to soil respiration presumably due to decreased substrate supply for microbes. There was no detectable label in carbonate C, suggesting that this pool does not significantly contribute to ecosystem C fluxes.

Conclusions

Our study indicates that increased water availability causes increased C uptake with increased C allocation to soil organic matter in Larrea tridentata-dominated communities while increased N deposition will have a minimal impact on C sequestration.     

Crop residue incorporation negates the positive effect of elevated atmospheric carbon dioxide concentration on wheat productivity and fertilizer nitrogen recovery

Background and purpose

Rapid increases in atmospheric carbon dioxide concentration ([CO₂]) may increase crop residue production and carbon: nitrogen (C:N) ratio. Whether the incorporation of residues produced under elevated [CO₂] will limit soil N availability and fertilizer N recovery in the plant is unknown. This study investigated the interaction between crop residue incorporation and elevated [CO₂] on the growth, grain yield and the recovery of ¹⁵N-labeled fertilizer by wheat (Triticum aestivum L. cv. Yitpi) under controlled environmental conditions.

Methods

Residue for ambient and elevated [CO₂] treatments, obtained from wheat grown previously under ambient and elevated [CO₂], respectively, was incorporated into two soils (from a cereal-legume rotation and a cereal-fallow rotation) 1 month before the sowing of wheat. At the early vegetative stage ¹⁵N-labeled granular urea (10.22 atom%) was applied at 50 kg?N ha^?1 and the wheat grown to maturity.

Results

When residue was not incorporated into the soil, elevated [CO₂] increased wheat shoot (16 %) and root biomass (41 %), grain yield (19 %), total N uptake (4 %) and grain N removal (8 %). However, the positive [CO₂] fertilization effect on these parameters was absent in the soil amended with residue. In the absence of residue, elevated [CO₂] increased fertilizer N recovery in the plant (7 %), but when residue was incorporated elevated [CO₂] decreased fertilizer N recovery.

Conclusions

A higher fertilizer application rate will be required under future elevated [CO₂] atmospheres to replenish the extra N removed in grains from cropping systems if no residue is incorporated, or to facilitate the [CO₂] fertilization effect on grain yield by overcoming N immobilization resulting from residue amendment.     

Fertilizer N uptake of paddy rice in two soils with different fertility under experimental warming with elevated CO2

Background and aims

Only limited information is available in the research area on the effect of elevated CO₂ concentration ([CO₂]) and air temperature (T_air) on the fertilizer N uptake by rice. This study was conducted to investigate changes in rice uptake of N derived from fertilizer (NDFF) and soil (NDFS) as well as fertilizer N uptake efficiency (FUE) with elevated [CO₂] and T_air in two soils with different fertility.

Methods

Rice (Oryza sativa L.) plants were grown with ¹⁵N-urea for two growing seasons (2007 in the less fertile and 2008 in the more fertile soil) in temperature gradient chambers under two (ambient and elevated) levels of [CO₂] and T_air regimes. At harvest, dry matter (DM) and N uptake amount of rice compartments (root, shoot, and grain) were determined.

Results

The DM of whole rice increased (P?<?0.01) with co-elevation of [CO₂] and T_air in both years (by 28.0 % in 2007 and by 27.4 % in 2008). The DM in 2008 was greater than that in 2007 by 48.1 to 63.1 % probably due to better soil fertility as well as longer sunshine hours (456 h vs. 568 h). Co-elevation of [CO₂] and T_air increased total N uptake, NDFF, and NDFS by 19.4 to 29.1 % in general compared to the ambient conditions. The FUE increased with co-elevation of [CO₂] and T_air from 46.5 to 59.5 % in 2007 and from 36.7 to 43.8 % in 2008.

Conclusions

The projected global warming with elevated [CO₂] is expected to increase FUE via enhanced DM accumulation with less increments in the soils that have higher indigenous soil N availabilities.     

Elevated CO2 affects plant responses to variation in boron availability

Aim

Effects of elevated CO₂ on N relations are well studied, but effects on other nutrients, especially micronutrients, are not. We investigated effects of elevated CO₂ on response to variation in boron (B) availability in three unrelated species: seed geranium (Pelargonium x hortorum), barley (Hordeum vulgare), and water fern (Azolla caroliniana).

Methods

Plants were grown at two levels of CO₂ (370, 700?ppm) and low, medium, and high B. Treatment effects were measured on biomass, net photosynthesis (P_n) and related variables, tissue nutrient concentrations, and B transporter protein BOR1.

Results

In geranium, there were interactive effects (P?<?0.05) of B and CO₂ on leaf, stem, and total plant mass, root:shoot ratio, leaf [B], B uptake rate, root [Zn], and P_n. Elevated CO₂ stimulated growth at 45?μM B, but decreased it at 450?μM B and did not affect it at 4.5?μM B. P_n was stimulated by elevated CO₂ only at 45?μM B and chlorophyll was enhanced only at 450?μM B. Soluble sugars increased with high CO₂ only at 4.5 and 45?μM B. High CO₂ decreased leaf [B] and B uptake rate, especially at 450?μM B. Though CO₂ and B individually affected the concentration of several other nutrients, B x CO₂ interactions were evident only for Zn in roots, wherein [Zn] decreased under elevated CO₂. Interactive effects of B and CO₂ on growth were confirmed in (1) barley grown at 0, 30, or 1,000?μM B, wherein growth at high CO₂ was stimulated more at 30?μM B, and (2) Azolla grown at 0, 10, and 1,000?μM B, wherein growth at high CO₂ was stimulated at 0 and 10?μM B.

Conclusion

Thus, low and high B both may limit growth stimulation under elevated vs. current [CO₂], and B deficiency and toxicity, already common, may increase in the future.     

Grassland root demography responses to multiple climate change drivers depend on root morphology

Aims

We examine how root system demography and morphology are affected by air warming and multiple, simultaneous climate change drivers.

Methods

Using minirhizotrons, we studied root growth, morphology, median longevity, risk of mortality and standing root pool in the upper soil horizon of a temperate grassland ecosystem for 3 years. Grassland monoliths were subjected to four climate treatments in a replicated additive design: control (C); elevated temperature (T); combined T and summer precipitation reduction (TD); combined TD and elevated atmospheric CO₂ (TDCO₂).

Results

Air warming (C vs T) and the combined climate change treatment (C vs TDCO₂) had a positive effect on root growth rate and standing root pool. However, root responses to climate treatment varied depending on diameter size class. For fine roots (≤ 0.1 mm), new root length and mortality increased under warming but decreased in response to elevated CO₂ (TD vs TDCO₂); for coarse roots (> 0.2 mm), length and mortality increased under both elevated CO₂ and combined climate change drivers.

Conclusions

Our data suggest that the standing roots pool in our grassland system may increase under future climatic conditions. Contrasted behaviour of fine and coarse roots may correspond to differential root activity of these extreme diameter classes in future climate.     

Effects of single and mixed inoculation with two arbuscular mycorrhizal fungi in two different levels of phosphorus supply on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers

Aims

This study aimed to determine the effect of arbuscular mycorrhizal (AM) fungi and phosphorus (P) supply levels on β-carotene concentrations in sweet potato (Ipomoea batatas L.) tubers.

Methods

Two commercial AM fungal isolates of Glomus intraradices (IFP Glintra) and Glomus mosseae (IFP Glm) which differ in their life cycles were used. Sweet potato plants were grown in a horizontal split-root system that consisted of two root compartments. A root-free fungal compartment that allowed the quantification of mycelial development was inserted into each root compartment. The two root compartments were inoculated either with the same or with different AM isolates, or remained free of mycorrhizal propagules. Each fungal treatment was carried out in two P supply levels.

Results

In the low P supply level, mycorrhizal colonization significantly increased β-carotene concentrations in sweet potato tubers compared with the non-mycorrhizal plants. Glomus intraradices appeared to be more efficient in increasing β-carotene concentrations than G. mosseae. Dual inoculation of the root system with the two mycorrhizal fungi did not result in a higher increase in tuber β-carotene concentrations than inoculation with the single isolates. Improved P nutrition led to higher plant tuber biomass but was not associated with increased β-carotene concentrations.

Conclusions

The results indicate a remarkable potential of mycorrhizal fungi to improve β-carotene concentrations in sweet potato tubers in low P fertilized soils. These results also suggest that β-carotene metabolism in sweet potato tubers might be specifically activated by root mycorrhizal colonization.     

The symbiotic recapture of nitrogen from dead mycorrhizal and non-mycorrhizal roots of tomato plants

Aims

The aim was to quantify the nitrogen (N) transferred via the extra-radical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices from both a dead host and a dead non-host donor root to a receiver tomato plant. The effect of a physical disruption of the soil containing donor plant roots and fungal mycelium on the effectiveness of N transfer was also examined.

Methods

The root systems of the donor (wild type tomato plants or the mycorrhiza-defective rmc mutant tomato) and the receiver plants were separated by a 30 μm mesh, penetrable by hyphae but not by the roots. Both donor genotypes produced a similar quantity of biomass and had a similar nutrient status. Two weeks after the supply of ¹⁵?N to a split-root part of donor plants, the shoots were removed to kill the plants. The quantity of N transferred from the dead roots into the receiver plants was measured after a further 2 weeks.

Results

Up to 10.6 % of donor-root ¹⁵N was recovered in the receiver plants when inoculated with the arbuscular mycorrhizal fungus (AMF). The quantity of ¹⁵N derived from the mycorrhizal wild type roots clearly exceeded that from the only weakly surface-colonised rmc roots. Hyphal length in the donor rmc root compartments was only about half that in the wild type compartments. The disruption of the soil led to a significantly increased AMF-mediated transfer of N to the receiver plants.

Conclusions

The transfer of N from dead roots can be enhanced by AMF, especially when the donor roots have been formerly colonised by AMF. The transfer can be further increased with higher hyphae length densities, and the present data also suggest that a direct link between receiver mycelium and internal fungal structures in dead roots may in addition facilitate N transfer. The mechanical disruption of soil containing dead roots may increase the subsequent availability of nutrients, thus promoting mycorrhizal N uptake. When associated with a living plant, the external mycelium of G. intraradices is readily able to re-establish itself in the soil following disruption and functions as a transfer vessel.     

Combined effects of warming and elevated CO2 on the impact of drought in grassland species

Aims

Drought is a major growth limiting factor in the majority of terrestrial ecosystems and is expected to become more frequent in the future. Therefore, resolving the drought response of plants under changing climate conditions is crucial to our understanding of future ecosystem functioning. This study responds to the need for experimental research on the combined effects of warming, elevated CO₂ and drought, and aims to determine whether the response to drought is altered under future climate conditions.

Methods

Two grassland species, Lolium perenne L. and Plantago lanceolata L., were grown in sunlit climate-controlled chambers. Four climates were simulated: (1) current climate, (2) current climate with drought, (3) a warmer climate with drought, and (4) a climate with combined warming, elevated CO₂ and drought.

Results

Warming did not alter the drought response, neither directly through photosynthesis nor indirectly through changes in water consumption. Also for combined warming and elevated CO₂ there were no effects on the plant response to drought for any of the measured parameters. However, simultaneous warming and elevated CO₂ mitigated the biomass response to drought through a positive pre-drought effect on photosynthesis and biomass response.

Conclusions

Our results indicate that a positive pre-drought effect of combined warming and elevated CO₂ has the potential to compensate for drought-induced biomass losses under future climate conditions.     

Root hairs explain P uptake efficiency of soybean genotypes grown in a P-deficient Ferralsol

Background and aims

Incorporating soybean (Glycine max) genotypes with a high nitrogen fixation potential into cropping systems can sustainably improve the livelihoods of smallholder farmers in Western Kenya. Nitrogen fixation is, however, often constrained by low phosphorus (P) availability. The selection of soybean genotypes for increased P efficiency could help to overcome this problem. This study investigated the contribution of different root traits to variation in P efficiency among soybean genotypes.

Methods

Eight genotypes were grown in a Ferralsol amended with suboptimal (low P) and optimal (high P) amounts of soluble P. Root hair growth was visualized by growing plants in a novel agar system where P intensity was buffered by Al₂O₃ nanoparticles.

Results

In the pot trial, P uptake was unaffected among the genotypes at high P but differed about 2-fold at low P. The genotypes differed in P uptake efficiency but not in P utilization efficiency. Regression analysis and mechanistic modeling indicated that P uptake efficiencies were to a large extent related to root hair development (length and density) and, to a lower extent, to colonization by mycorrhizal fungi.

Conclusion

Breeding for improved root hair development is a promising way to increase P uptake efficiency in soybean.     

Phosphorus supply enhances the response of legumes to elevated CO2 (FACE) in a phosphorus-deficient vertisol

Background & aims

Understanding the mechanism of how phosphorus (P) regulates the response of legumes to elevated CO₂ (eCO₂) is important for developing P management strategies to cope with increasing atmospheric CO₂ concentration. This study aimed to explore this mechanism by investigating interactive effects of CO₂ and P supply on root morphology, nodulation and soil P fractions in the rhizosphere.

Methods

A column experiment was conducted under ambient (350?ppm) (aCO₂) and eCO₂ (550?ppm) in a free air CO₂ enrichment (FACE) system. Chickpea and field pea were grown in a P-deficient Vertisol with P addition of 0–16?mg?P?kg^?1.

Results

Increasing P supply increased plant growth and total P uptake with the increase being greater under eCO₂ than under aCO₂. Elevated CO₂ increased root biomass and length, on average, by 16?% and 14?%, respectively. Nodule biomass increased by 46?% in response to eCO₂ at 16?mg P kg^?1, but was not affected by eCO₂ at no P supply. Total P uptake was correlated with root length while N uptake correlated with nodule number and biomass regardless of CO₂ level. Elevated CO₂ increased the NaOH-extractable organic P by 92?% when 16?mg P kg^?1 was applied.

Conclusion

The increase in P and N uptake and nodule number under eCO₂ resulted from the increased biomass production, rather than from changes in specific root-absorbing capability or specific nodule function. Elevated CO₂ appears to enhance P immobilization in the rhizosphere.     

Arbuscular mycorrhiza improve growth,nitrogen uptake,and nitrogen use efficiency in wheat grown under elevated CO<Subscript>2</Subscript>

Effects of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis on plant growth, carbon (C) and nitrogen (N) accumulation, and partitioning was investigated in Triticum aestivum L. plants grown under elevated CO₂ in a pot experiment. Wheat plants inoculated or not inoculated with the AM fungus were grown in two glasshouse cells with different CO₂ concentrations (400 and 700 ppm) for 10 weeks. A ¹⁵N isotope labeling technique was used to trace plant N uptake. Results showed that elevated CO₂ increased AM fungal colonization. Under CO₂ elevation, AM plants had higher C concentration and higher plant biomass than the non-AM plants. CO₂ elevation did not affect C and N partitioning in plant organs, while AM symbiosis increased C and N allocation into the roots. In addition, plant C and N accumulation, ¹⁵N recovery rate, and N use efficiency (NUE) were significantly higher in AM plants than in non-AM controls under CO₂ enrichment. It is concluded that AM symbiosis favors C and N partitioning in roots, increases C accumulation and N uptake, and leads to greater NUE in wheat plants grown at elevated CO₂.     

Seasonal carbon allocation to arbuscular mycorrhizal fungi assessed by microscopic examination, stable isotope probing and fatty acid analysis

Background and Aim

Climate change models are limited by lack of baseline data, in particular carbon (C) allocation to – and dynamics within – soil microbial communities. We quantified seasonal C-assimilation and allocation by plants, and assessed how well this corresponds with intraradical arbuscular mycorrhizal fungal (AMF) storage and structural lipids (16:1ω5 NLFA and PLFA, respectively), as well as microscopic assessments of AMF root colonization.

Methods

Coastal Hypochoeris radicata plants were labeled with ¹³CO₂ in February, July and October, and ¹³C-allocation to fine roots and NLFA 16:1ω5, as well as overall lipid contents and AM colonization were quantified.

Results

C-allocation to fine roots and AMF storage lipids differed seasonally and mirrored plant C-assimilation, whereas AMF structural lipids and AM colonization showed no seasonal variation, and root colonization exceeded 80 % throughout the year. Molecular analyzes of the large subunit rDNA gene indicated no seasonal AMF community shifts.

Conclusions

Plants allocated C to AMF even at temperatures close to freezing, and fungal structures persisted in roots during times of low C-allocation. The lack of seasonal differences in PLFA and AM colonization indicates that NLFA analyses should be used to estimate fungal C-status. The implication of our findings for AM function is discussed.     

Soil microorganisms respond to five years of climate change manipulations and elevated atmospheric CO2 in a temperate heath ecosystem

Background and aims

Soil microbial responses to global change can affect organic matter turnover and nutrient cycling thereby altering the overall ecosystem functioning. In a large-scale experiment, we investigated the impact of 5 years of climate change and elevated atmospheric CO₂ on soil microorganisms and nutrient availability in a temperate heathland.

Methods

The future climate was simulated by increased soil temperature (+0.3 °C), extended pre-summer drought (excluding 5–8 % of the annual precipitation) and elevated CO₂ (+130 ppm) in a factorial design. Soil organic matter and nutrient pools were analysed and linked to microbial measures by quantitative PCR of bacteria and fungi, chloroform fumigation extraction, and substrate-induced respiration to assess their impact of climate change on nutrient availability.

Results

Warming resulted in higher measures of fungi and bacteria, of microbial biomass and of microbial growth potential, however, this did not reduce the availability of nitrogen or phosphorus in the soil. Elevated CO₂ did not directly affect the microbial measures or nutrient pools, whereas drought shifted the microbial community towards a higher fungal dominance.

Conclusions

Although we were not able to show strong interactive effects of the global change factors, warming and drought changed both nutrient availability and microbial community composition in the heathland soil, which could alter the ecosystem carbon and nutrient flow in the long-term.     

The plasticity of the growth and proliferation of wheat root system under elevated CO2

Background and Aims

Understanding crop responses to increasing atmospheric CO₂ requires knowledge of how their root systems grow, proliferate and function. The effect of elevated CO₂ on the growth and proliferation of wheat root system (Triticum aestivum L.), was examined.

Methods

Two pairs of sister lines of wheat contrasting in vigour (CV97 and CV207) and tillering (7750N and 7750PF) were grown in rhizo-boxes under ambient (380 μl L^?1) and elevated CO₂ (700 μl L^?1), and the root growth and proliferation mapped.

Results

Elevated CO₂ effects on shoot and root biomass were observed in the lines contrasting for vigour, but not in the lines contrasting for tillering. Root biomass was reduced by 67 % in the high vigour line CV97, reducing total plant biomass by 26 % compared to the low vigour line, CV207. This was due to a reduction in root length down the 1 m soil profile and root proliferation in the top 0.2 m layer. The reduction in root biomass was not compensated by an increase in shoot biomass.

Conclusions

The reduction in root biomass under elevated CO₂ in the vigour line CV97 can be explained through its inability to increase the sink strength due to the failure to increase tiller number to which the plant presumably responded by increasing losses of the newly assimilated carbon by respiration.     

Fertilization and forb:graminoid ratio affect arbuscular mycorrhiza in seedlings but not adult plants of Plantago lanceolata

Aims

Nutrients play a key role in arbuscular mycorrhizal (AM) symbiosis. We quantified the response of AM symbiosis of seedlings and adult plants of Plantago lanceolata to fertilization under field conditions in managed grasslands differing in nutrient availability and soil moisture.

Methods

The AM symbiosis was measured as the total extent of AM fungal colonization and frequency of arbuscules or vesicles, and as the relative proportions of morphotypes. We further examined the effects of the surrounding vegetation upon AM symbiosis.

Results

Fertilization decreased total AM colonization and relative arbuscular frequency of the whole mycorrhizal community and of Acaulospora and “fine endophyte” morphotypes in seedling roots, but it had no effect upon the mycorrhiza in adult plants. The decline in arbuscular frequency in seedling roots due to fertilization was greater at the sites with higher nutrient availability and lower N:P ratio. Seedlings surrounded by more forbs had a greater total AM colonization and higher vesicular frequency.

Conclusions

Increased nutrient availability in the initial stages of seedling development has a prominent effect upon AM symbiosis development, but these effects seem to diminish over the long term, as evidenced by the results obtained for adult plants and from the limited effects of parameters characterizing long-term nutrient availability.