bluuebunny/crossref_metadata_2025 huggingface dataset:
Elevated carbon dioxide ameliorates the effects of ozone on photosynthesis and growth: species respond similarly regardless of photosynthetic pathway or plant functional group
Authors: VOLIN JOHN C. , REICH PETER B. , GIVNISH THOMAS J.
Published: March 2003

Due to their different physiological effects, elevated carbon dioxide and elevated ozone might have interactive impacts on plants, and differentially so on plants differing in photosynthetic pathway and growth rate. To test several hypotheses related to these issues, we examined the physiological, morphological and growth responses of six perennial species grown at various atmospheric concentrations of carbon dioxide and ozone. The species involved (two C3 trees: Populus tremuloides Michx., Quercus rubra L.; two C3 grasses: Agropyron smithii Rybd., Koeleria cristata L.; two C4 grasses: Bouteloua curtipendula Michx., Schizachyrium scoparium Michx.) differed in growth form, stomatal conductance and photosynthetic pathway. In situ photosynthesis, relative growth rate (RGR) and its determinants (leaf area ratio, specific leaf area, leaf weight ratio and root weight ratio) were determined via sequential harvests of seedlings that were grown in all combinations of 366 or 672 μmol mol−1 CO2 and 3 or 95 nmol mol−1 O3 over a 101‐d period. Elevated CO2 had minimal effect on either photosynthesis or RGR. By contrast, RGR for all six species was lower in high O3 concentrations at ambient CO2, significantly so in A. smithii and P. tremuloides. Five of the six species also exhibited reductions in in situ photosynthesis at ambient CO2 in high‐O3‐grown compared with low‐O3‐grown plants. For all species, these O3‐induced reductions in RGR and photosynthesis were absent in the elevated CO2 environment. Root weight ratio was significantly reduced by elevated O3 in A. smithii and P. tremuloides in ambient but not elevated CO2. Species with high stomatal conductance were the most susceptible to oxidant injury, while those with low stomatal conductance, such as the C4 species and Q. rubra, were not as detrimentally affected by O3. Elevated levels of CO2 will reduce stomatal conductance and O3 uptake, and might therefore reduce the potential for oxidant damage. However, there was a stronger relationship of the percent reduction in whole‐plant mass due to O3, related to the ratio of photosynthesis to stomatal conductance. In general, results of this study of six functionally diverse plant species suggest that O3 pollution effects on carbon balance and growth are likely to be ameliorated by elevated concentrations of atmospheric CO2.
Daily embolism and refilling of xylem vessels in the roots of field‐grown maize
Authors: McCULLY M. E. , HUANG C. X. , LING L. E. C.
Published: March 2003

Embolisms in the vessels of maize axile roots of different types were observed directly after rapid freezing of intact, functioning roots in the field, by cryo‐scanning electron microscopy. Quantification of the degree of embolization in each root was made by counting empty and full vessels of both the late and early metaxylem (LMX & EMX), and expressed as percent embolized vessels of the LMX, and %EMX poles containing embolized vessels. Contents of the connecting xylem (CX) at branch root junctions, and of xylem in branch roots were observed also, but not systematically quantified. Records of % embolized vessels were made from dawn to dusk on summer days in Ottawa under moderate irradiance, and in Canberra under high irradiance. Measurements in Canberra were supported by estimates of irradiance, of stomatal conductance, and of chamber balance pressure of bagged and unbagged leaves. Soon after sunrise embolisms appeared in all types of vessel, at balance pressures c. 300–400 kPa, and increased rapidly with increasing irradiance. During the middle of the day % embolized vessels reached a maximum (LMX ≈70% in Ottawa, and ≈80% in Canberra). At all times the EMX vessels were less embolized. The midday maximum was brief in Ottawa, and % embolized vessels fell to a low value during the afternoon. In Canberra the maximum was prolonged into late afternoon. By dusk nearly all vessels were once again filled with sap. The balance pressures measured during vessel refilling in Canberra ranged from 500 kPa to 1200 kPa. At all times of the day sap was seen entering some embolized vessels. Almost all were refilling by mid‐ to late‐afternoon. Such refilling was especially frequent at junctions of branch roots with the axile roots. X‐ray microanalysis of the sap entering the vessels, and of the liquid filling or partly filling vessels, showed the concentration of mineral solutes present in the sap was below the threshold of detection (≈12 mM). These results are discussed in relation to current opinions about embolisms and vessel refilling.
Growth and survival of Azolla filiculoides in Britain I. Vegetative production
Authors: JANES RACHEL
Published: March 2003

Azolla filiculoides Lam. causes serious weed problems in Britain, but its long‐term survival might be limited by winter death. The aim of this study was to establish the low temperature responses and limitations of A. filiculoides sporophytes.

In the laboratory, normal vegetative growth was shown to continue at 5°C. Reddening of plants was a response to low temperature and high light conditions which could be prevented by shading. Adult plants died after short (18 h) exposure to −4°C but survived sub‐zero temperatures >−4°C. Evidence was found of seasonal changes in chill tolerance, but not in freeze tolerance.

In outdoor culture, plants survived encasement in ice and air temperatures to −5°C. Additional evidence suggested that natural populations can readily survive air temperatures much lower than this. Microclimatic effects are likely to be responsible for this discrepancy between laboratory and outdoor culture results.

Three phenotyes were identified; survival, colonizing and mat forms.
Sensitivity to and requirement for iron in Plantago species
Authors: SCHMIDT W. , FÜHNER C.
Published: March 2003

The relative importance of some aspects of iron nutrition for the distribution of six Plantago species (P. maritima L., P. coronopus L., P. major ssp. major L., P. serpentina All., P. media L. and P. lanceolata L.) with different habitat requirements for soil pH and moisture was evaluated. Iron efficiency and Fe tolerance of hydroponically‐grown plants were assessed by determining the decrease in relative growth rates caused by suboptimal and supra‐optimal external Fe concentrations. Marked interspecific differences were observed in visual symptoms of Fe deficiency and Fe toxicity and in the external Fe concentrations leading to 50% inhibition of maximal relative growth rates (rgr50%). Whereas P. serpentina displayed a clear preference for low external Fe concentrations, growth rates of P. maritima and P. media were found to be increased at higher concentrations. Severe growth restriction at both low and high Fe concentrations was evident in P. lanceolata and P. major. A broad optimum was observed in P. coronopus exhibiting a low external Fe requirement and a high tolerance to supra‐optimal Fe concentrations. Iron efficiency and Fe tolerance differed in a way which was only partly correlated with the expected Fe availability at the natural habitat of the species, suggesting that Fe concentrations are of minor importance for their distribution, or that some of the mechanisms that render Fe oxides available for uptake in situ are masked in solution experiments.

To gain insight into the impact of the physiological and morphological characteristics of the species on Fe efficiency, the effect of Fe status on shoot∶root ratio, relative root surface area, root Fe(III) reductase and proton extrusion capacity were investigated. Roots of all species showed increased Fe chelate reduction activity upon growth at suboptimal Fe concentrations; marked differences were observed with respect to the kinetic parameters of the reductase. Maximal velocity of the reduction was positively correlated with relative growth rate and was not related to the Fe efficiency of the species. By contrast, Km corresponded to Fe efficiency ranking and can therefore be regarded as an important parameter for the uptake of Fe at low availability. Enhanced ability to acidify the rhizosphere was only observed in P. major. From the morphological characteristics investigated, root surface area appears to be the most important parameter in the uptake of Fe at suboptimal external concentrations.
Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens
Authors: DALE HEATHER , PRESS MALCOLM C.
Published: March 2003

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO2 concentrations (350 and 550 μmol mol−1). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root∶shoot ratios) and within tissues∶infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO2. Elevated CO2 stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO2 on growth, with infected plants showing a greater response, such that elevated CO2 partly alleviated the effects of the parasite on host growth. Elevated CO2 did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO2 but was unaffected by O. minor. There was no evidence of down‐regulation of photosynthesis in T. repens grown at elevated CO2 in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO2 on other parasitic angiosperm‐host associations and factors which control plant responses to elevated CO2.
Survival of an introduced ectomycorrhizal Laccaria bicolor strain in a European forest plantation monitored by mitochondrial ribosomal DNA analysis
Authors: SELOSSE MARC‐ANDRÉ , MARTIN FRANCIS , LE TACON FRANÇOIS
Published: March 2003

Mitochondrial and nuclear genes have different inheritance, thus studies of fungal populations should use both mitochondrial and nuclear markers. Using nuclear markers, the S238N strain of the ectomycorrhizal basidiomycete Laccaria bicolor ((Maire) Orton) has been previously shown to persist for at least 10 yr after outplanting in a plantation of Douglas fir (Pseudotsuga menziesii (Mir.) Franco) inoculated with this strain. In the present study, we have sampled 539 sporophores of Laccaria spp. from this plantation, some of which had the S238N nuclear genotype, to study mitochondrial DNA polymorphism and persistence of the inoculated S238N mitochondrial genome. Length polymorphism in fragments of the large subunit of mitochondrial ribosomal DNA (LrDNA) allowed distinction of the haplotypes present in the plantation at the species level. In addition, heteroduplex analysis and sequencing revealed intraspecific polymorphism of LrDNA among the L. bicolor sporophores and enabled specific identification of S238N LrDNA. This haplotype was only retained in sporophores carrying the S238N nuclear genome, confirming the survival of this introduced strain in a natural population.
Response of antioxidant systems and leaf water relations to NaCl stress in pea plants
Authors: HERNÁNDEZ J. A. , CAMPILLO A. , JIMÉNEZ A. , ALARCÓN J. J. , SEVILLA F.
Published: March 2003

A pea (Pisum sativum cv. Puget) cultivar was grown on a medium containing different NaCl concentrations (0–160 mol m−3) in order to study the effects of salt stress on leaf water relations and on the activity of antioxidant enzymes. NaCl stress caused a rapid decline in chlorophyll content. Both leaf water (ψl) and osmotic potentials (ψs) decreased progressively with the severity of the stress (from 90–160 mol m−3 NaCl) whereas leaf turgor pressure (ψp) increased in treated plants. Pea leaves contained an iron‐containing superoxide dismutase (Fe‐SOD) isozyme in chloroplasts alongside a copper‐zinc‐containing (CuZn‐SOD) form (CuZn‐SOD II). The lowest NaCl concentration (70 mol m−3) had no effect on the activity of these antioxidant enzymes while higher concentrations (110–130 mol m−3) enhanced the activity of cytosolic CuZn‐SOD I and chloroplastic CuZn‐SOD II as well as that of mitochondrial and/or peroxisomal manganese‐containing superoxide dismutase (Mn‐SOD). These inductions were matched by increases in the activity of ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR). The increased activities coincided with decreased stomatal conductance and were unaffected by the severity of stress except in the case of CuZn‐SOD II which fell to control values under the highest stress conditions (140–160 mol m−3 NaCl), when a concomitant increase in chloroplastic Fe‐SOD activity was observed. Glutathione reductase (GR) and dehydroascorbate reductase (DHAR) activities were only induced under severe NaCl stress (130–160 mol m−3) and were accompanied by losses in the ascorbate and glutathione pools, lower ASC/DHA and GSH/GSSG ratios and increases in GSSG. Electron microscopy showed that the thylakoidal structure of the chloroplasts became disorganized and their starch content decreased in plants treated with 160 mol m−3 NaCl. Overall, the results suggest that salt stress is accompanied by oxidative stress, perhaps at the cell compartment level. The capacity of Puget cultivar to ensure cell turgor and to enhance the activity of enzymes involved in the defence against oxidative stress seems to be important in determining adaptation to moderate NaCl stress conditions. In plants exposed to severe NaCl stress (130–160 mol m−3) it seems that such resistance to oxidative stress is overcome, which might contribute to the deleterious effects of salt and significant growth reduction in these conditions.
How well do plant traits correlate with establishment ability? Evidence from a study of 16 calcareous grassland species
Authors: LEISHMAN MICHELLE R.
Published: March 2003

The ability of 16 calcareous grassland species to establish in competition with adult neighbours of two different growth forms was assessed in a glasshouse experiment. The questions asked were: (1) how well is establishment ability related to plant traits such as seed mass, DNA content, relative growth rate and adult height; and (2) are the results consistent across different neighbours? Two measures of establishment ability were calculated, survival relative yield and biomass relative yield, for seedlings grown with a dicot. (Leontodon hispidus) and with a graminoid (Festuca ovina). The results depended on which measure of establishment ability was used. When establishment ability was measured in terms of biomass relative yield, seed mass was a good predictive trait, accounting for 38–55% of variation. However, when establishment ability was measured as survival relative yield, adult height was found to be the best predictor, capable of accounting for 20% of variation, although this was not consistent across neighbours. Thus, no clear relationship between plant traits and establishment ability was found: results depended on the measure of establishment ability used, and in the case of adult height, on the identity of the neighbour species. Survival relative yield was strongly associated with target and neighbour species growth form type, with stronger suppression of like than of non‐like growth forms. The lack of strong relationships between plant traits and establishment ability, and the dependence on neighbour identity, suggests that the links between plant traits and establishment might be context‐dependent; thus an understanding of the mechanisms underlying any such links is essential.
Effects of drought, transgenic expression of a fructan synthesizing enzyme and of mycorrhizal symbiosis on growth and soluble carbohydrate pools in tobacco plants
Authors: SCHELLENBAUM L. , SPRENGER N. , SCHÜEPP H. , WIEMKEN A. , BOLLER T.
Published: March 2003

The effects of three conditions likely to affect soluble carbohydrate pools, namely drought, expression of barley sucrose: fructan 6‐fructosyl transferase (6‐SFT, EC 2.4.1.10) and the establishment of the arbuscular mycorrhizal symbiosis with Glomus mosseae were studied in a multifactorial experiment using tobacco (Nicotiana tabacum). Tobacco, a plant naturally unable to form fructan, accumulated fructan in leaves, and to a larger extent in the roots, when transformed with 6‐SFT. Under drought conditions, growth was considerably reduced, but neither expression of 6‐SFT nor mycorrhiza formation had an effect on growth rate. However, in response to drought, carbon partitioning was significantly altered towards accumulation of soluble sugars. In plants exposed to drought, pools of sucrose were greater than those of unstressed plants, particularly in their roots. In the transgenic plants expressing 6‐SFT, there were also increased contents of the products of 6‐SFT, namely fructan, most probably because of the increased availability of the substrate, sucrose. These effects were the same in the presence or absence of mycorrhiza. Hexoses (glucose and fructose) also increased in response to drought, primarily in the leaves. This effect of drought was little affected by the expression of 6‐SFT, except that it slightly enhanced drought‐induced glucose accumulation in roots. However, the presence of mycorrhiza led to a considerable reduction in drought‐induced accumulation of hexoses in the leaves. The content of the fungal disaccharide trehalose was greatly increased in the roots of all mycorrhizal plants upon exposure to drought, particularly in some of the transgenic plants expressing 6‐SFT.
Nitrogen isotope fractionation during nitrogen uptake by ectomycorrhizal and non‐mycorrhizal Pinus sylvestris
Authors: HÖGBERG PETER , HÖGBERG MONA N. , QUIST MAUD E. , EKBLAD ALF , NÄSHOLM TORGNY
Published: March 2003

An experiment was performed to find out whether ectomycorrhizal (ECM) fungi alter the nitrogen (N) isotope composition, δ15N, of N during the transport of N from the soil through the fungus into the plant. Non‐
mycorrhizal seedlings of Pinus sylvestris were compared with seedlings inoculated with either of three ECM fungi, Paxillus involutus, Suillus bovinus and S. variegatus. Plants were raised in sand in pots supplied with a nutrient solution with N given as either NH4+ or NO3−. Fractionation against 15N was observed with both N sources; it decreased with increasing plant N uptake, and was larger when NH4+ was the source. At high ratios of Nuptake/Nsupplied there was no (NO3−), or little (NH4+), fractionation. There seemed to be no difference in fractionation between ECM and non‐mycorrhizal plants, but fungal rhizomorphs were sometimes enriched in 15N (up to 5‰ at most) relative to plant material; they were also enriched relative to the N source. However, this enrichment of the fungal material was calculated to cause only a marginal decrease (−0.1‰ in P. involutus) in δ15N of the N passing from the substrate through the fungus to the host, which is explained by the small size of the fungal N pool relative to the total N of the plant, i.e. the high efficiency of transfer. We conclude that the relatively high 15N abundance observed in ECM fungal species should be a function of fungal physiology in the ECM symbiosis, rather than a reflection of the isotopic signature of the N source(s) used. This experiment also shows that the δ15N of plant N is a good approximation of δ15N of the available N source(s), provided that N is limiting growth.
Modelling leaf expansion in a fluctuating environment: are changes in specific leaf area a consequence of changes in expansion rate?
Authors: TARDIEU F. , GRANIER C. , MULLER B.
Published: March 2003

Leaf expansion rate varies with leaf temperature, photon flux density (PPFD), evaporative demand and soil water status. In most simulation models, it is calculated every day by multiplying the amount of carbohydrate available to leaves by specific leaf area (SLA). However, leaf expansion rate is considerably reduced by mild water deficits which do not affect photosynthesis, and is not affected by a reduction in the PPFD intercepted during rapid leaf expansion. Specific leaf area undergoes a several‐fold variability depending on PPFD, soil water status and time of day. It is increased when environmental conditions have a greater depressive effect on expansion rate than on photosynthesis, and is decreased in the opposite case. It is therefore appropriate to model leaf expansion independently of the plant carbon budget. Consistent characteristics can be deduced from a series of experiments, allowing a model of leaf expansion to be proposed. (i) Time courses of relative leaf expansion rate and of epidermal cell division rate are well conserved within a plant and across a large range of environmental conditions, provided that durations and rates are expressed in thermal time. Maximum relative rates are common to all zones of a leaf and to all leaves of a plant, in maize and sunflower. (ii) A water deficit, or a reduction in intercepted PPFD, imposed in the first half of the period of leaf development affects the relative expansion rate in the deficit only, but permanently affects the absolute expansion rate. In contrast, a reduction in PPFD causes no effect on leaf expansion if imposed in the rapid expansion period when the leaf is autotrophic. (iii) Expansion rate is related to evaporative demand and to the concentration of ABA in the xylem sap with relationships that apply under both field and laboratory conditions. (iv) Tissue expansion and epidermal cell division behave as independent processes which determine epidermal cell area at each time.
Contribution of carbohydrate pools to the variations in leaf mass per area within a tomato plant
Authors: BERTIN N. , TCHAMITCHIAN M. , BALDET P. , DEVAUX C. , BRUNEL B. , GARY C.
Published: March 2003

The contribution of the starch and soluble carbohydrate pools to the diurnal variations of leaf mass per unit area (LMA) has been investigated in tomato leaves. A glasshouse experiment was carried out with plants pruned to two or five fruits per truss. Leaflets were sampled at sunrise, noon and sunset at different positions within the leaf (basal or terminal), and on different sympods along the stem. Carbohydrate contents and LMA were significantly higher in the terminal than in the basal leaflets, except at sunrise. During the day, differences in starch accumulation between terminal and basal leaflets increased with leaf height on the plant. Among sympods, the soluble carbohydrate content of the terminal leaflets did not vary significantly, whereas at 13.00 h the LMA was minimum in the middle of the plant and maximum at the top, and the leaf starch content significantly increased half‐way up the plant. The plant fruit load had only small and non‐significant effects on the LMA and carbohydrate contents. The response of LMA and carbohydrate contents to changing source activity was observed under controlled climatic conditions. The starch pool of fully expanded leaves was rapidly filled and emptied under increasing and decreasing source activity. In young expanding leaves, this pool was hardly filled during daylight. On average the soluble carbohydrates did not contribute significantly to the diurnal variations in LMA, whereas fluctuations in starch explained c. 70% and 44% of these variations in the upper and lower leaves, respectively. The results are discussed with respect to the modelling of LMA at the level of individual tomato leaves or sympods.
Pollen‐tube growth rate and seed‐siring success among Betula pendula clones
Authors: PASONEN H. L. , PULKKINEN P. , KÄPYLÄ M. , BLOM A.
Published: March 2003

The aim of this study was to investigate whether genetically different pollen donors (Betula pendula clones) differed in pollen‐tube growth rate across 11 maternal plants and in vitro, and whether the differences between the donors were consistent across the recipients. To compare the seed‐siring success of competing pollen donors, a two‐donor hand‐pollination experiment with six donors and six recipients was conducted. The experiments were performed at a plastic‐house seed orchard. The donors showed significant variation in pollen‐tube growth rate on all the 11 recipients. The rankings of the pollen donors were statistically consistent across different maternal plants. A significant positive correlation between pollen tube growth in vivo and in vitro was found. The seed‐siring success of two competing pollen donors was unequal in 20 of 29 cases and there was a significant positive correlation between seed‐siring success and pollen‐tube growth rate in vivo and in vitro. The results show that fertilizations are not random and pollen competition operates in a B. pendula seed orchard population.
Drought resistance of Quercus pubescens as a function of root hydraulic conductance, xylem embolism and hydraulic architecture
Authors: NARDINI ANDREA , PITT FRANCO
Published: March 2003

Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field‐grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.
Resistance to complete submergence in Rumex species with different life histories: the influence of plant size and light
Authors: NABBEN R. H. M. , BLOM C. W. P. M. , VOESENEK L. A. C. J.
Published: March 2003

Resistance to complete submergence was tested in three Rumex species that occur in the Dutch river forelands. The species differ in both habitat and life history characteristics. The annual or biennial R. maritimus and the biennial or short lived perennial R. palustris grow on frequently flooded mud flats of low elevation, while the perennial R. thyrsiflorus can be found on dykes and river dunes that are seldom flooded. The flooding characteristics of the habitats of the three species were determined. These data were used to design experiments to determine the survival and biomass development of the three species during submergence and the influence of plant size and light level on these parameters. It was shown in all three species that plants submerged during daytime were much more resistant to flooding than those submerged at night. This is most probably due to the generation of oxygen or carbohydrates by underwater photosynthesis. Mature plants of the three species showed higher survival after submergence than juvenile plants, which might be caused by higher carbohydrate levels in the taproots of mature plants. In addition, the three species clearly differed in survival and biomass development during submergence. Rumex thyrsiflorus, the species least subjected to flooding, is least tolerant to complete submergence. Rumex maritimus, which can avoid the floods by having a short life cycle, is less tolerant to submergence than R. palustris, which has to survive the floods as a vegetative plant. It was noted that some plants that survived the flooding period itself, still died in the following period of drained conditions, possibly due to post‐anoxic injury.
Pollen tube competition as a mechanism of prezygotic reproductive isolation between Mimulus nasutus and its presumed progenitor M. guttatus
Authors: DIAZ A. , MACNAIR M. R.
Published: March 2003

This paper considers the extent to which differences in pollen tube growth rates can provide prezygotic reproductive isolation between Mimulus nasutus and its presumed progenitor, Mimulus guttatus. Mimulus nasutus is partially cleistogamous, but its larger chasmogamous flowers offer appreciable opportunity for outcrossing. Mimulus nasutus was found to have smaller pollen grains and shorter styles than M. guttatus. No differences were observed in pollen grain germination on conspecific and heterospecific stigmas. However, pollen tube growth rates of M. nasutus were found to be much slower than those of M. guttatus in the styles of that species. Consequently, any M. nasutus pollen transferred to an M. guttatus stigma was found to be competitively disadvantaged in an M. gutattus style. By contrast, no difference in pollen tube growth rate was detected between the species when growing in M. nasutus styles, possibly because M. nasutus styles are unable to support fast pollen tube growth. We tested the prediction from the pollen tube studies that a 50∶50 mix of M. guttatus and M. nasutus pollen would produce 50% hybrid seeds when M. nasutus was the maternal parent, and near to 0% hybrid seed when M. guttatus was the maternal parent. The results were found to support this prediction. We conclude that pollen–pistil interactions can effect strong reproductive isolation between these species, as M. guttatus pollen tubes have a competitive advantage over those of M. nasutus in an M. guttatus style, but not in an M. nasutus style.
Growth responses of Quercus petraea, Fraxinus excelsior and Pinus sylvestris to elevated carbon dioxide, ozone and water supply
Authors: BROADMEADOW MARK S. J. , JACKSON S. B.
Published: March 2003

Seedlings of Quercus petraea (oak), Fraxinus excelsior (ash) and Pinus sylvestris (Scots pine) were grown at two CO2
concentrations with two O3 and two water supply treatments for 3 yr in a factorial experiment. Oak was the most
responsive species to all three treatments; elevated CO2 and irrigation increased biomass by an average of 79% and
41%, respectively, whereas the ozone treatment resulted in a 30% reduction in growth. Significant treatment
interactions in this species demonstrated that CO2 ameliorated and irrigation exacerbated the effects of ozone. For
Scots pine and ash, irrigation and elevated CO2 increased growth by approx. 60% and 20%, respectively, whereas
ozone had no detectable effect on ash and resulted in a 15% reduction in growth of Scots pine. Carbon partitioning
to the shoot was enhanced by both the CO2 and H2O treatments in oak, while branching was also increased in this
species in response to elevated O3, resulting in changes to the allometric relationships. CO2 enhanced leaf
production in oak and Scots pine, and together with the promotion of shoot allocation, this indicates an increased
susceptibility to windthrow. Biomass accumulation expressed as relative growth rate, suggested three different
time‐dependent growth responses to elevated CO2; the CO2 fertilization effect was maintained through the third
year of growth in oak, had disappeared in Scots pine and a negative effect was evident in ash. Foliar nitrogen and
chlorophyll concentrations indicated a CO2‐induced nitrogen deficiency in oak and ash, but not in Scots pine.
Chlorophyll degradation in response to ozone was observed in oak, an effect that was enhanced by irrigation and
reduced by CO2, presumably through stomatal mediated changes in effective ozone dose. These results therefore
suggest that elevated atmospheric CO2 concentrations will enhance growth of some UK forest tree species,
although this might only be apparent during the juvenile phase. However, nitrogen deficiencies might limit this
enhancement on some sites while changes in allocation and leaf area might promote susceptibility to windthrow.
Elevated CO2 also provides some protection against ozone pollution, especially in combination with limited soil
moisture availability. These interactions between CO2, ozone and water supply should be taken into account when predicting the effects of environmental change on tree growth and forest productivity.
The control of carbon acquisition by roots
Authors: FARRAR J. F. , JONES D. L.
Published: March 2003

We review four hypotheses for the control of carbon acquisition by roots, and conclude that the functional equilibrium hypothesis can offer a good description of C acquisition by roots relative to shoots, but is deficient mechanistically. The hypothesis that import into roots is solely dependent on export from the shoot, itself determined by features of the shoot alone (the ‘push’ hypothesis), is supported by some but not all the evidence. Similarly, the idea that root demand, a function of the root alone, determines import into it (the ‘pull’ hypothesis), is consonant with some of the evidence. The fourth, general, hypothesis (the ‘shared control’ hypothesis) – that acquisition of C by roots is controlled by a range of variables distributed between root and shoot – accords with both experiment and theory. Top‐down metabolic control analysis quantifies the control of C flux attributable to root relative to source leaf. We demonstrate that two levels of mechanistic control, short‐term regulation of phloem transport and control of gene expression by compounds such as sugars, underlie distributed control. Implications for the impact of climate change variables are briefly discussed.
The representation of root processes in models addressing the responses of vegetation to global change
Authors: WOODWARD F. I. , OSBORNE C. P.
Published: March 2003

The representation of root activity in models is here confined to considerations of applications assessing the impacts of changes in climate or atmospheric [CO2]. Approaches to modelling roots can be classified into four major types: models in which roots are not considered, models in which there is an interplay between only selected above‐ground and below‐ground processes, models in which growth allocation to all parts of the plants depends on the availability and matching of the capture of external resources, and models with explicit treatments of root growth, architecture and resource capture. All models seem effective in describing the major root activities of water and nutrient uptake, because these processes are highly correlated, particularly at large scales and with slow or equilibrium dynamics. Allocation models can be effective in providing a deeper, perhaps contrary, understanding of the dynamic underpinning to observations made only above ground. The complex and explicit treatment of roots can be achieved only in small‐scale highly studied systems because of the requirements for many initialized variables to run the models.
Spatial differences in acquisition of soil phosphate between two arbuscular mycorrhizal fungi in symbiosis with Medicago truncatula
Authors: SMITH F. A. , JAKOBSEN I. , SMITH S. E.
Published: March 2003

Responses of Medicago truncatula to colonization by two arbuscular mycorrhizal fungi, Scutellospora calospora
isolate WUM 12(2) and Glomus caledonium isolate RIS 42, were compared in the light of previous findings that
the former fungus can be ineffective as a beneficial microsymbiont with some host plants. The plants were grown
individually in two‐compartment systems in which a lateral side arm containing soil labelled with 33P was
separated from the main soil compartment by a nylon mesh that prevented penetration by roots but not fungal
hyphae. Fungal inoculum was applied as a root–soil mixture in a band opposite the side arm. Nonmycorrhizal
controls were set up similarly, without inoculum. There were harvests at 28, 35, 42 and 49 d. Both sets of
mycorrhizal plants grew better than nonmycorrhizal plants and initially had higher concentrations of P in shoots
and roots. Plants grown with S. calospora grew better than plants grown with G. caledonium, and this was
associated with somewhat greater fungal colonization in terms of intraradical hyphae and numbers of arbuscules.
Scutellospora calospora formed denser hyphae at root surfaces than G. caledonium. By 28 d there were extensive
hyphae of both fungi in the side arms, and after 35 d S. calospora produced denser hyphae there than G.
caledonium. Nevertheless, there was very little transfer of 33P via S. calospora to the plant at 28 d, and thereafter
its transfer increased at a rate only c. 33% of that via G. caledonium. The results showed that plants colonized by
S. calospora preferentially obtained P from sites in the main soil chamber relatively close to the roots, compared
with plants colonized by G. caledonium. Hence formation of a highly beneficial arbuscular mycorrhizal symbiosis
does not necessarily depend on development of hyphae at a distance from the roots or on large‐scale translocation
of P from distant sites. The results are discussed in relation to previous studies with compartmented systems that
have involved the same fungi. Possible causes of the variable effects of S. calospora in symbiosis with different host plants are briefly assessed. Differences in spatial abilities of individual arbuscular mycorrhizal fungi to acquire P might have strong ecological implications for plant growth in soils low in P.
Experimental evidence for a persistent spore bank in Sphagnum
Authors: SUNDBERG S. , RYDIN H.
Published: March 2003

Spore capsules of four Sphagnum species were buried at different depths in peat on a bog. Spore viability was
determined after 0, 1, 2 and 3 yr. Viability generally declined with time, but viable spores were still found at all
depths after 3 yr. The light‐coloured spores of S. balticum and S. tenellum retained their viability better than the
darker spores of S. fuscum and S. lindbergii. Survival was highest under wet but aerobic conditions, but was also
high under humid or periodically desiccated conditions. By contrast, most spores stored under wet, anaerobic
conditions died within 2–3 yr. These results, and predictions from them, are not consistent with earlier results for
spores of long‐lived and dominant bryophytes, or for seeds of phanerogams of undisturbed wetlands and forests.
There was no correlation between spore size and longevity across species, but the small spores from small capsules
of S. balticum and S. tenellum generally showed higher viability than those from the medium‐sized and large
capsules of the same species. This suggests a positive intraspecific relationship between longevity and dispersal
distance. There was an indication of conditional dormancy, controlled by weather, in Sphagnum spores. The
experiments indicate that Sphagnum spores can form a long‐term persistent spore bank under suitable conditions,
with a half‐life of between 1 and 20 yr (mean across species of 2.6 and 5.0 yr at two depths studied), and with
potential values in individual spore capsules of several decades, or even of centuries. Sphagnum spores kept
refrigerated showed 15–35% viable spores after 13 yr. The capacity to form a persistent spore bank that can be
activated whenever favourable conditions occur might help explain the wide geographical distribution of many
Sphagnum species in the boreal and temperate zones, where they have managed to colonize almost every suitable patch of acidic, nutrient‐poor wetland.
Arbuscular mycorrhizal fungi might alleviate aluminium toxicity in banana plants
Authors: RUFYIKIRI G. , DECLERCK S. , DUFEY J. E. , DELVAUX B.
Published: March 2003

Some mycorrhizal plants exhibit greater resistance than nonmycorrhizal plants to aluminium toxicity. This has not yet been shown for banana despite its importance as a cash and food crop in tropical regions, although bananas are sensitive to aluminium stress. We studied the effects of the arbuscular mycorrhizal fungus Glomus intraradices in alleviating aluminium toxicity in the banana cultivar Grande Naine grown in a continuous‐nutrient‐flow cultivation system using dilute solution. The micropropagated plants, some of which were inoculated with arbuscular mycorrhizal fungus, were grown for 40 d in pots filled with sand, and continuously irrigated with a nutrient solution containing up to 180 μM of aluminium. Water and nutrient uptake were measured once a week for 24 h, and root arbuscular mycorrhizal fungal colonization, biomass production, and mineral content of roots and shoots were measured at harvest. The root arbuscular mycorrhizal fungal colonization was large, and not significantly influenced by aluminium treatment. The effects of aluminium on both mycorrhizal and nonmycorrhizal plants were: decrease in biomass production, water and nutrient uptake, and magnesium content of roots and shoots; greater aluminium content in roots than in shoots; and increase in potassium and phosphorus content, particularly in roots. A significant positive effect of arbuscular mycorrhizal fungi on plant growth was observed with aluminium treatment, and was most pronounced at the highest concentration. The benefits, compared with nonmycorrhizal plants, included: increase in shoot dry weight, uptake of water and of most nutrients, and in calcium, magnesium and phosphorus content, particularly in roots; decrease in aluminium content in root and shoot; and delay in the appearance of aluminium‐induced leaf symptoms. These results indicate that arbuscular mycorrhizal fungi could be effective in alleviating aluminium toxicity to banana plants.
Nuclear ribosomal sequence analysis ofOidiodendron: towards a redefinition of ecologically relevant species
Authors: Lacourt Isabelle , Girlanda Mariangela , Perotto Silvia , Del Pero Massimiliano , Zuccon Dario , Luppi Anna Maria
Published: March 2003

• Nuclear ribosomal sequence analysis was performed to investigate delimitation of commonOidiodendronspecies comprising endomycorrhizal symbionts and close associates of ectomycorrhizal plants.

• Neighbour‐joining, maximum likelihood and parsimony analyses were used to compare 35 ribosomal DNA (internal transcribed spacer (ITS) and 5.8S) sequences (including sequences available in databases) from 15 putative species.

• Oidiodendron citrinumformed a monophyletic group nested withinO. maius, whereasO. tenuissimumandO. griseumdid not appear either as distinct groups or as a single complex. Pairwise nucleotide divergence values betweenO. citrinumandO. maiuswere very low and comparable to intraspecific values obtained for both species; values forO. griseumandO. tenuissimum, although higher, overlapped those observed at the intraspecific level for the two species.

• Molecular data indicate thatO. maiusandO. citrinum, which were described as distinct, though related species, could be moved to a subspecific level; however, the delimitation ofO. griseumandO. tenuissimumis still open to question. Taxonomic rank assignment to groups determined from sequence data analysis is discussed.
Productivity of Trifolium subterraneum and Phalaris aquatica under warmer, high CO2 conditions
Authors: Lilley J. M. , Bolger T. P. , Gifford R. M.
Published: March 2003

Despite the importance of grass‐legume pastoral ecosystems worldwide, there is little known about the impacts of concurrent increase in temperature and atmospheric CO2 concentration on their productivity.

 Pure and mixed swards of subterranean clover (Trifolium subterraneum) and phalaris (Phalaris aquatica) were established under ambient and warmed (+3.4°C) air temperatures, at ambient and 690 µmol mol−1 CO2 concentrations in field tunnels in temperate south‐eastern Australia.

 Over one year, elevated CO2 increased clover foliage growth in the monoculture by 19%, and by 31% in the mixture. Warming reduced clover monoculture herbage production at ambient CO2 by 28% and reduced the growth enhancement by elevated CO2 to +8%. Forage growth of phalaris monoculture was not affected significantly by either factor. Forage growth of the mixture was increased by 34% in response to higher CO2, but unaffected by warming. Elevated CO2 combined with warming increased forage growth of the mixed sward by 23%.

 Concurrent rise in atmospheric CO2 concentration and temperature increased productivity of subterranean clover‐phalaris swards. However, longer term effects on species competition and persistence may modify this conclusion.
Differential expression of the hexose‐regulated fungal genes AmPAL and AmMst1 within Amanita/Populus ectomycorrhizas
Authors: Nehls Uwe , Bock Andrea , Ecke Margret , Hampp Rüdiger
Published: March 2003

At least three fungal ‘networks’ are formed in ectomycorrhizal symbiosis: the Hartig net, the sheath and extramatrical hyphae. Since these structures have different functions regarding uptake, storage and partitioning of carbohydrates and nutrients, regulation and coordination of metabolic fluxes are of special interest for ectomycorrhizal functioning.

 To get some insight into structure‐specific functions, the Amanita fungal sheath was separated by microdissection from the remaining Populus roots that still contained the Hartig net. Expression of the genes coding for phenylalanine ammonium lyase (AmPAL) and hexose transporter (AmMst1) was investigated separately for both structures by RT‐PCR.

 When constitutively expressed fungal genes were used as a control, the expression of AmMst1 was enhanced about six‐fold in hyphae of the Hartig net compared with those of the fungal sheath. By contrast, AmPAL was only barely detectable in the Hartig net, but highly expressed in the fungal sheath.

 Since in pure fungal culture both genes are regulated by monosaccharides, it is supposed that an apoplastic hexose gradient, between hyphae of the Hartig net and those of the fungal sheath, might regulate fungal gene expression and thus hyphal physiology within ectomycorrhizas.
Isolation and phylogenetic identification of a dark‐septate fungus associated with the alpine plantRanunculus adoneus
Authors: Schadt Christopher W. , Mullen Renée B. , Schmidt Steven K.
Published: March 2003

Dark‐septate endophytic (DSE) fungi are ubiquitous in the roots of Arctic and alpine plants, yet very little is known about their phylogenetic identities or effects on their host plants.

 Several such fungi were isolated from the alpine snowbed plantRanunculus adoneusin the Front Range of Colorado, USA; one isolate was chosen for detailed study. The ability of this isolate to re‐colonize plant roots in pot cultures was assessed, and phylogenetic analyses were performed using small‐subunit (SSU), 5.8S and internal transcribed spacer (ITS) 2 ribosomal DNA sequences.

 This isolate had the ability to produce root endophytic structures in pot cultures similar to those reported from other sources and observed inR. adoneusroots. SSU phylogenetic analyses showed this isolate to be related to a clade within the Euascomycetes containing the Leotiales and Erysiphales. In addition, SSU and 5.8S–ITS2 sequences showed high phylogenetic similarity to a variety of isolates reported from other plants of diverse geographical origins. Although most of these isolates remain unidentified, one closely related isolate was the anamorphic taxonPhialophora gregata.

 The results suggest that this DSE isolate might belong to the fairly closely related group of plant endophytes that have varied effects on the plants that they inhabit.