Studies on the rhizomatous clonal sedge Carex bigelowii, at a fellfield site in subarctic Swedish Lapland examined the ecological potential of rhizomes as nitrogen uptake systems. Direct application of a solution of 15NH415NO3 to the rhizomes of C. bigelowii significantly enriched the 15N content of the plant tissue. The pattern of enrichment indicated movement of labelled nitrogen into rhizomes and adjoining tissue (including roots and shoots), showing both uptake and translocation via the rhizome system. There was a gradient of decreasing 15N enrichment with increasing distance from the point of labelling, and estimation of the total mass of nitrogen taken up via labelled rhizomes showed low levels and rates of uptake. Quantification of the size of rhizome and root systems of C. bigelowii at the study site indicates that 80% of the living biomass may be below ground, and that the surface area of the rhizome system of C. bigelowii is approx. two-thirds that of the roots. The rhizome system of C. bigelowii can therefore act as a route for nitrogen uptake, with the potential to exploit almost as great a volume of soil as the root system. This mechanism of nitrogen uptake may play an important role in the Arctic, where many species have a clonal, rhizomatous growth form. In addition, plant growth in many arctic ecosystems is limited by low soil nutrient availability, a result of low temperatures (leading to slow soil decomposition rates) and patchy resource distribution. Nitrogen uptake via rhizomes may provide plants with the capacity to take advantage of transient nutrient supplies, and may partly compensate for the cost of developing and maintaining persistent rhizome networks in ecosystems where nutrient resources are in short supply.

Applying silicon in the form of metasilicic acid (H4SiO3) or silicic acid (H4SiO3) to Bradyrhizobium-infected, hydroponically grown cowpea seedlings resulted in a significant (P[les ]0.05) increase in the number of nodules, nodule dry matter, and nitrogen fixed on a per plant basis. Total dry matter of plants increased with silicon supply, and the differences were significant (P[les ]0.05) at the higher silicon concentrations. Cowpea plants cultured in sand were also assessed for their response to silicic acid. Nodule number and nodule mass increased with silicon supply to sand cultured plants, though nitrogen fixation was unaltered. Although silicon is not essential for growth of cowpea, it is important for nodule formation and nodule functioning in hydroponically grown plants. Consequently, data collected and conclusions drawn from earlier glasshouse experiments, which have excluded silicon from nutrient solutions, may be flawed. Future studies on nodulation and nitrogen fixation using legumes in liquid culture must therefore include silicon as a nutrient element.

Mature heather (Calluna vulgaris) and bracken (Pteridium aquilinum) turfs, transplanted from the field, were subjected to factorially combined experimental treatments for three consecutive years. Summer drought had the greatest effect, decreasing photosynthesis, growth and reproductive output in both species, and opening the bracken canopy. The timing of the drought relative to plant development was critical to which species was worst affected; bracken was worst affected by an early drought, heather by a later drought. Both species showed physiological damage during drought but, as predicted on the basis of their functional types, heather showed greater acclimation of water-use efficiency to drought stress. Contrary to expectations based on functional types, heather responded more rapidly than bracken to increased nitrogen supply (50 kg N ha−1 yr−1). Added nitrogen caused both species to start above-ground growth earlier in the spring. For bracken this stimulation was short- lived; added nitrogen might be preferentially allocated to the rhizome and the longer-term consequences of this are unknown. For heather, nitrogen promoted growth and flowering throughout the season. There was no positive effect on the photosynthetic physiology of either species; changes in resource partitioning, and thus photosynthate production at the canopy level, are the most likely mechanism for the increase in heather shoot growth. Warmer temperatures increased heather shoot growth from early spring onwards but did not advance bracken crozier emergence, although frond height and the proportion of fertile fronds were subsequently increased. No significant effects of warming on the photosynthetic physiology of either species were found. Predictions of responses of heather and bracken to environmental change are complicated by the strong interactive effects of unpredictable climatic events such as drought and extreme winter temperatures. When drought was imposed, damage to heather was much greater in plants receiving increased nitrogen supply. Stimulation of growth by nitrogen resulted in a water demand that was unsustainable in drought conditions, leading to wilting, reduced shoot growth and some acclimation of water-use efficiency. Additionally, a very cold winter spell proved most damaging to heather that had been droughted in the previous summer. For bracken, winter damage occurred in plants that had been warmed, with significantly fewer fronds emerging in the next spring and thus canopy photosynthetic potential being reduced. We predict that positively managed heather has the potential to limit the bracken problem in conditions of environmental change, provided that high levels of nitrogen deposition do not coincide with increased drought frequency.

Many soil fungi colonize the roots of pines to form symbiotic organs known as ectomycorrhizas. Dichotomous branching of short lateral roots and the formation of coralloid organs are diagnostic of ectomycorrhizas in many pine species, although the regulation of these changes in root morphology is not well understood. We used axenic root cultures of six pine species to examine the role of auxin, cytokinin, ethylene and nutrients in the regulation of root architecture. Surprisingly, extensive dichotomous and coralloid branching of lateral roots occurred spontaneously in Pinus taeda, P. halepensis and P. muricata. In P. sylvestris, P. ponderosa and P. nigra, treatment with auxin transport inhibitors (ATIs), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or the ethylene-releasing compound 2-chloroethylphosphonic acid (CEPA or ethephon) induced extensive dichotomous branching and coralloid organ formation. Formation of both spontaneous and ATI-induced coralloid structures was blocked by treatment with an ethylene synthesis inhibitor L-α-(2-aminoethoxyvinyl)glycine; this inhibition was reversed by either ACC or CEPA. In addition, the induction of this unique morphogenetic pattern in pine root cultures was regulated by nutrient levels. The morphology and anatomical organization of the chemically induced dichotomous and coralloid structures, as well as the regulation of their formation by nutrient levels, show a striking similarity to those of ectomycorrhizas.

Gravitropic responses of oat coleoptiles were measured in different growth media; humid air, natural soil and artificial soil (glass beads). The oat coleoptiles in soil and glass beads were monitored by NMR imaging, while those in humid air were imaged in darkness with an infrared-sensitive charge-coupled device (CCD) camera. The present study shows for the first time that gravitropic experiments can be performed in artificial soil using NMR imaging as a convenient and suitable recording method. Not only was it possible to follow the gravitropic curvatures in natural soil, but the artificial soil allowed plant images of sufficient spatial and temporal resolution to be recorded. The advantages of using artificial soil in magnetic resonance imaging studies are that the iron content of glass beads is very low compared with natural soil, and that the artificial soil matrix can easily be standardized with regard to particle size distribution and nutrient content. Two types of glass beads were used, the diameter of the small and the large beads being 300–400 and 420–840 μm, respectively. The growth rate of the coleoptiles in soil and in big beads was roughly the same and only slightly lower than in humid air, whereas small beads reduced the growth rate by approx. 16%. The bending rate of the coleoptiles during the gravitropic response was reduced by c. 65% in soil and 75% in bead mixtures relative to bending in air. It should be noted, however, that the maximum curvature of the coleoptile tip was of the same order in all cases, about 35°. This value may represent the largest possible curvature of the organ. The potential of NMR imaging to study how plant organs penetrate the soil under the influence of gravitropism, mechanical impedance and thigmotropism is also discussed.

Canopies of heterophyllous trees expand by production of long shoots. We have previously shown in mountain birch (Betula pubescens ssp. czerepanovii) that damage to internode leaves within long shoots does not impede shoot growth, indicating that long-shoot elongation occurs by means of external resources. To study to what extent leaves other than true long-shoot leaves are necessary for the normal growth of mountain birch long shoots, we simulated herbivore damage to the two basal leaves of shoots (which flush simultaneously with short-shoot leaves) and the short-shoot leaves nearest to the long shoot within the branch. Damage to the two basal long-shoot leaves significantly reduced long-shoot growth. Additional damage to short-shoot leaves, situated proximally to the long shoot, did not retard long-shoot growth any more than damage to basal leaves alone. To determine the extent to which short-shoot leaves within a large branch are responsible for the pooled long-shoot production of the branch, we clipped differing proportions of short-shoot leaves from such branches. We found small but significant reduction in the pooled length of the long shoots of the branch, presumably indicating a limited role in long-shoot elongation of current photosynthates within the branch. Our experiments indicate that long shoots are not independent modular units in their carbon economy.

We tested a hypothesis that elevated ozone was an eliciting or contributing factor in outbreaks of the ‘top dying’ (or ‘subtop dying’) syndrome in Norway spruce (Picea abies). Progeny were used from open-pollinated trees within a stand with the ‘top dying’ syndrome. The mother trees were classified in relation to the expression of the ‘top dying’ syndrome, and progeny from the healthiest and least healthy thirds of the population were exposed to high and low concentrations of ozone for three seasons. Elevated ozone did not affect height growth of the trees. It did not measurably affect net photosynthesis, stomatal conductance or instantaneous water use efficiency. Chlorophyll and carotenoid contents were also not significantly affected by ozone concentration. In the first year, instantaneous water use efficiency was lower in the progeny of the unhealthy mother trees than in the healthy mother trees. Furthermore, the unhealthy mother trees tended to produce longer annual shoots and showed more winter damage at the end of the experiment. None of these parameters were related to ozone concentration in the atmosphere. These results do not support a hypothesis that elevated ozone is a significant contributory factor or an eliciting factor in the development of the syndrome.

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.

Grasses and forbs are often classified into separate functional types, although systematic differences between the types have only been verified for a few functional traits. Since leaf longevity has been shown to be a key trait linking plant ecophysiology, whole-plant growth and ecosystem resource cycling, we compared the leaf longevity of 14 species to determine if there were consistent differences between grasses and forbs or other functional classifications, such as persistence of leaves into winter. Leaf longevity was assessed in 6-yr-old monoculture plots in central North America by tagging and sequentially monitoring the phenological states of whole forb leaves and sections of grass leaves. This new approach enables a calculation of leaf longevity unbiased by the manner in which grass leaves grow and provides a more accurate comparison between grasses and forbs. Lupinus perennis had the shortest leaf longevity (4 wk) and Koeleria cristata, Poa pratensis, and Solidago rigida the longest (13–14 wk). Average leaf longevity for the 14 species was c. 9 wk, with no significant differences between grasses and forbs nor between current alternative functional classifications.

A range of plant species typical of semi-natural grasslands were tested for their sensitivity to short-term ozone injury under normal and reduced irrigation, and in relationship to air vapour pressure deficit. Potted specimens of 24 herbs, legumes and grasses were exposed during two seasons to four O3 treatments in open-top chambers. The ozone treatments were: (a) charcoal-filtered air; (b) charcoal-filtered air plus ozone to match ambient levels; (c) charcoal-filtered air plus O3 to ambient levels 1.5 and (d) charcoal-filtered air with ozone added to twice ambient levels during selected episodes of 7–13 d. During these ozone episodes, half of the plants in each ozone treatment received reduced irrigation (dry treatment) while the rest was kept under full irrigation (wet treatment). Type and date of first occurrence of leaf injury were noted during individual growth periods. Plants were harvested three times per year, and the percentage of injured leaves was recorded. Depending on species, injury symptoms were expressed as flecking (O3-specific injury), leaf yellowing or anthocyanin formation. Carum carvi and most species of the Fabaceae family (Onobrychis sativa, Trifolium repens, Trifolium pratense) were found to be most responsive to O3, injury occurring after only a few days of exposure in treatment (b). An episodic reduction in irrigation tended to reduce the expression of O3-specific symptoms, but only in species for which a reduction in soil moisture potential and an associated reduction in stomatal conductance during the dry episodes were observed. In other species, the protection from O3 injury seemed to be of little importance. Using artificial neural networks the injury response of nine species was analysed in relation to Species, stomatal conductance, ozone as AOT40 (accumulated exposure above a threshold of 0.04 ppm for periods with global radiation [ges ]50 W m−2 (Fuhrer et al., 1997)), mean relative growth rate, air vapour pressure deficit and global radiation. In the model with all factors, Species was most important, and when Species was omitted, stomatal conductance was the most important determinant for leaf injury to occur, whereas mean relative growth rate was less important. With no plant-related factors included, air vapour pressure deficit and AOT40 were of highest importance. Only in eight species was a positive relationship found between these two factors during the five days before the onset of injury, indicating increasing protection from ozone with increasing air vapour pressure deficit in some but not all species. These data show that across a range of grassland species, leaf injury caused by elevated levels of ozone is most likely to occur in species with high stomatal conductance and that protection from ozone during dry periods is species-specific and depends on a reduction in stomatal conductance due to a decrease in soil moisture potential. Protection under increased vapour pressure deficit can occur in some but not all species, depending on the relationship between stomatal conductance and air vapour pressure deficit.

A fructan polymerase activity was partially purified and concentrated by sequential acid and salt precipitation from extracts of excised, illuminated leaves of timothy grass (Phleum pratense). The polymerase catalysed the de novo synthesis of oligo- and polyfructan from sucrose as sole substrate at near-physiological rates (0.5–0.9 mg g−1 fresh mass h−1; 0.9–1.5 nkat g−1). Rates of in vitro polymerisation were high, at up to 4.1 mg cm−3 h−1 (7.1 nkat cm−3) of total products of degree of polymerization greater than 2 (DP>2). The trisaccharides 1-kestose and 6-kestose together with oligosaccharides of up to DP[les ]c. 10 were synthesized in under 2 h at 30°C. In longer incubations, ethanol-precipitable polymers of DP=c. 10–35 (1.6–5.7 kDa) were detected by anion-exchange chromatography and pulsed amperometry. When this polymeric product was used as a primer and re-incubated with fresh enzyme and sucrose, abundant polymers of up to DP=50 (8.1 kDa) were formed. The structure of the polymeric enzyme product was compared with native fructan from timothy leaves and with standard inulin, using glycosyl-linkage analysis followed by identification of partially methylated alditol acetate derivatives by GC–MS. The deduced structure was a linear (unbranched) 2,6-linked fructose chain terminated with glucose and fructose. The linkage structures of the native and enzyme-generated polymers were identical, increasing confidence in the physiological relevance of the activity. After ultracentrifugation of tissue homogenates at 265 000 gav, the polymerase remained in the supernatant, demonstrating no tight association with particulate components. The polymerizing reaction was dependent on enzyme concentration, requiring at least 3 g fresh mass equivalent cm−3 (c. 2.7 nkat cm−3) for the efficient in vitro generation of fructans of DP>3. In common with other trisaccharide-synthesizing and oligofructan-glycosylating enzymes from grasses, the polymerase reaction exhibited both a maximal velocity at pH 5.0–5.5 and a low affinity for sucrose. The polymerization reaction did not saturate fully even at 1.5 M sucrose, and the concentration causing half maximal velocity (apparent Km) was c. 560 mM. The preparation contained substantial invertase activity (1.8 mg sucrose g−1 fresh mass h−1=1.5 nkat g−1 fresh mass) with a Km for sucrose hydrolysis of 5 mM. A single peak of polymerase activity with an Mr of 51 kDa was recovered from size-exclusion chromatography (SEC). Invertases of Mr 51 and 110 kDa were identified in the preparation. The 110-kDa invertase isoform exhibited no polymerase activity, but synthesized trisaccharide (mainly 1-kestose) from sucrose. The 51-kDa isoform co-eluted with the polymerase. The trisaccharide fraction produced by this isoform contained abundant 1- and 6-kestose. After SEC, the purification of the polymerase was 41-fold relative to the original tissue homogenate. The properties of enzymatic polymerization of fructan are discussed with respect to the physiology of accumulation in grass leaves and other systems.

Eupatorium makinoi plants with or without geminivirus infection were grown in shading frames with 70, 15 and 5.5% sunlight. Growth characteristics of these plants in the early vegetative phase were compared by means of growth analysis. We also measured leaf photosynthetic gas exchange rates and examined relationships between leaf photosynthesis and whole-plant growth. Relative growth rate (RGR=(1/W)×(dW/dt), where W is plant dry mass) of virus-infected plants was lower than that of uninfected plants under all three light conditions. The reduction of RGR by infection was increased with irradiance. The net assimilation rate (NAR=(1/A)×(dW/dt), where A is total leaf area of the plant) was also reduced both by infection and shading. NARs that were estimated from light-response curves of leaf photosynthesis, in situ measurements of irradiance, and respiration rates of leaves, stems and below-ground parts, agreed very well with the values obtained by conventional growth analysis techniques. Decreases in the estimated NAR value from infection and shading were mostly explained by the decreases in leaf photosynthesis. These results clearly showed that lowered RGR in virus-infected plants was attributed mainly to impaired photosynthesis in virus-infected leaves.

Plant species typical of semi-natural grasslands were tested for their sensitivity to long-term ozone effects on growth under two irrigation regimes, and the relationship between ozone sensitivity, relative growth rate, and plant strategy was analysed. Individuals of 24 grasses, herbs and legumes were grown in pots and exposed for two seasons to four levels of ozone in open-top chambers: (a) CF, charcoal-filtered air, (b) CFAA, charcoal-filtered air plus ozone to match ambient levels, (c) CF+, charcoal-filtered air plus ozone added to match 1.5 times ambient levels and (d) CF++, charcoal-filtered air with ozone added to match twice ambient levels during selected episodes of 7–13 d. During the ozone episodes, one half of the plants received reduced irrigation (dry treatment) while the rest was kept well watered (wet treatment). The effect of ozone on the relative growth rate was determined during the initial growth period during the first season. Above-ground plant dry weight was determined three times during each season, and stubble dry weight at the end of the experiment. In most species, the effect of ozone on relative growth rate was not significant, except in Centaurea jacea and Lychnis flos-cuculi for which a significant stimulation of relative growth rate was found. There was a significant negative relationship between percentage relative growth rate and relative growth rate measured in the CF treatment, indicating that in species growing faster in clean air relative growth rate tended to be more sensitive to ozone than in slower-growing species. After the first re-growth period during the first season, a significant effect of ozone on above-ground biomass was found in five species, i.e. a decrease in Chrysanthemum leucanthemum, Trifolium pratense and T. repens, and an increase in Silene dioïca. During the second season no significant ozone effect was detected, except for an increase in stubble mass in C. leucanthemum. In 10 species, the dry treatment caused a significant decrease in biomass, but in Plantago lanceolata during the first year, and in Knautia arvensis and T. repens during the second year, an ozone×irrigation interaction was observed. Exposure–response patterns in the wet treatment varied between species. In some species growth tended to be increased in charcoal-filtered air plus ozone to ambient levels, in others either a continuous increase or decrease in biomass was observed with increasing ozone. Trifolium repens showed the lowest AOT40 (ozone accumulated above a threshold of 40 ppb) corresponding to a 10% change in biomass. The comparison between exposure–response pattern between the wet and dry treatments revealed reduced, increased, or unchanged ozone sensitivity. The shift was related to plant strategy and not to the specific sensitivity to the dry treatment. The specific response to both ozone and dry treatment tended to be related to Grime's triangular classification of plant strategies, i.e. the C-S-R model. In species with a large component of C (competitor strategy), and/or R (ruderal strategy), biomass was more sensitive to both stresses, compared with species with a high component of S (stress tolerators), and ozone×drought interactions were more pronounced. The results indicate that specific relative growth rate is an important determinant for the potential sensitivity of grassland species to long-term effects of ozone, and that reduced irrigation causes no general protection from the effects of ozone on growth.

Colonization of roots by arbuscular mycorrhizal (AM) fungi in several annual crops in two consecutive seasons was compared with, in the second season, the density of fungal propagules in the soil with the use of a bioassay. Root density decreased down the soil profile in both years in all crops, and a high proportion of roots were mycorrhizal throughout the profile. AM colonization decreased down the profile in cotton and lablab in the second season only. The bioassay indicated that most propagules of AM fungi in soils under cotton were located near the surface, with virtually no propagules at 1 m. The absence of propagules at depth indicates a lack of mycelium deep in the soil, and suggests that mycorrhizas are primarily initiated in the surface soil and that the fungi colonize the root system mostly through secondary spread down the profile. The use of AM colonization in the field as an indicator of propagule density and symbiotic function should be qualified by an understanding of the depth in the soil from which roots were extracted.

Simulated swards of perennial ryegrass (Lolium perenne) growing in 1-m3 soil blocks in the glasshouse were either well watered or deprived of water for 57 d and then rewatered. The first aim was to measure effects of drought on sugar (water-soluble carbohydrate) composition of laminae and sheaths of mature laminae, and bases and laminae of young (growing) leaves. The second aim was to use pulse labelling with 14CO2 to follow the partitioning of recently-fixed assimilates, and the assembly and consumption of reserve sugars (fructans). Over the last 7 d of drought growth almost stopped, old leaves died faster than they were replaced, and total sugar (which had doubled in concentration during drought) was rapidly consumed. Leaf laminae had lower content of total sugars and of large fructan (DP>5) than did growing bases and sheaths. Drought greatly reduced the rate at which sugar was exported from the laminae to the sheaths and growing leaf bases, and the rate at which it was converted to fructan. Nevertheless, fructan accumulated over the first 50 d of drought. Rewatering did not result in depolymerization and remobilization of sugars that had been formed during the last 7 d of drought, but stimulated their further assembly into high-DP fructans. Our hypothesis, that accumulation of neo-kestose (a DP-3 fructan) in droughted laminae was a symptom of sugar remobilization just before death, was disproved. It is concluded that sugar reserves contribute to drought resistance only under extreme conditions. The specific role of fructan in dry environments might be to improve regrowth when drought is relieved, rather than to enhance growth during drought.

Natural abundances of 15N and N concentrations of 34 fruit bodies from 24 species of ectomycorrhizal and saprophytic fungi were measured in a temperate Central European mixed forest stand. The fungi of the two life forms are known to be capable of utilizing different types of N sources (organic N compounds from the humus, inorganic N from the soil and N from litter or wood) differing by their 15N natural abundance values. Based on the two life forms and the three different N sources, four functional groups of fungi were distinguished: (1) ectomycorrhizal fungi capable of utilizing organic N from the humus; (2) ectomycorrhizal fungi known to depend on inorganic N compounds in the soil; (3) saprophytes capable of utilizing organic N from the humus; and (4) saprophytes utilizing N from dead wood or litter. Large differences were found between species in the δ15N values (−3.0 to 3.3‰) and in the N concentrations (0.84 to 6.61 mmol eq N g dw−1) of the fruit bodies. In most cases fungi were more enriched in 15N than their respective bulk N source was. Fungi living in humus, and presumably having access to organic N compounds (groups 1 and 3), were significantly more enriched in 15N than fungi which are known to depend on inorganic N (e.g. Laccaria, group 2), or fungi living on litter or wood (group 4), irrespective of whether they were ectomycorrhizal or saprophytic species. Fungi living in humus had significantly higher N concentrations than fungi living on litter or wood.

In an experiment to determine the influence of mycorrhizal colonization on root development, fertilized seedlings of cotton were grown in long tubular pots. Inoculation of soil with an isolate of Glomus mosseae at 10, 25 or 40 cm below the soil surface resulted in spread of arbuscular mycorrhizas up and down the root system, and localized elongation of roots following colonization. Specific root length was not influenced by colonization. Increases in local root density at the point of inoculation were observed, though total root mass in relation to shoot mass declined following initiation of mycorrhizas.

Functional morphological patterns in root apices of tomato (Lycopersicon esculentum) dependent on growth, ageing and infection by the arbuscular mycorrhizal (AM) fungus Glomus mosseae and/or by the soilborne pathogenic fungus Phytophthora nicotianae var parasitica (P. parasitica) were studied. Uninfected root apices were characterized by closed, tri-layered meristems with nonreticulate nuclei; however, some apices of each treatment lost their meristematic nature, stopped growing and differentiated, becoming ‘parenchymatized’. The pathogenic fungus reduced the apex diameter and the number of mitotically active and viable apices inducing plasmolysis, cell and nucleus degeneration, and necrosis. The AM fungus, on the other hand, produced an increase in apex size and reduced the percentage of necrosis both in uninfected roots and in roots infected by P. parasitica. Thus, the AM fungus protected the apices from the pathogenic infection, allowing normal root growth. Furthermore, larger apices, which produce thicker roots, might indirectly contribute to plant protection. Increased volumes of colonizable tissues favour the spreading of the symbiont, and P. parasitica hyphae are always excluded from arbuscule-containing cells.

A low auxin concentration (0.1% naphthalene acetic acid) induced tracheids with longitudinal polarity parallel to the hypocotyl axis in young Pinus pinea seedlings. Application of 0.1% ethrel laterally and 0.1% naphthalene acetic acid apically disturbed axial tracheid polarity and promoted the differentiation of tracheids with a lateral orientation. Ethrel by itself, with no auxin background, did not affect tracheid differentiation. Apical application of 1% gibberellic acid with the low auxin, reversed the polarity disorder induced by ethrel. Disturbance of axial tracheid polarity was observed under a high auxin concentration (0.5% naphthalene acetic acid) which was similar to the combined effect of ethrel and auxin. The high auxin concentration increased tracheid number significantly. This effect was curtailed following treatment with inhibitors of ethylene formation (Co2+; 1-aminoethoxy- vinylglycine) and action (Ag2+). The role of ethylene in controlling the differentiation of radial tracheids, which characterize the vascular rays of pines, is discussed.

The influence of Glomus intraradices (BEG87) on Pseudomonas fluorescens DF57 in hyphosphere and rhizosphere soil was examined. Cucumis sativus (Aminex, F1 hybrid) was grown in symbiosis with the arbuscular mycorrhizal fungus G. intraradices in PVC tubes, consisting of a central root compartment and two lateral root-free compartments. Two Tn5-luxAB-marked strains of P. fluorescens DF57 were used. Strain DF57-P2, which has an insertion of Tn5[ratio ][ratio ]luxAB in a phosphate starvation-inducible locus, was used as a phosphate starvation reporter. Another lux-tagged strain DF57-40E7, which carries a constitutively expressed luxAB fusion, was used as control for strain DF57-P2 and for measuring the metabolic activity of P. fluorescens DF57. A strain of P. fluorescens DF57, which carries a constitutively expressed gfp gene, was used in studies of attachment between the bacteria and the hyphae. G. intraradices decreased the culturability of P. fluorescens DF57 significantly, both in rhizosphere and hyphosphere soil, whereas the total number of P. fluorescens DF57 measured by immunofluorescence microscopy was decreased in hyphosphere soil only. G. intraradices did not induce a phosphorus starvation response in P. fluorescens DF57, and the metabolic activity of the bacteria was not affected by the fungus after 48 h. P. fluorescens DF57 did not attach to G. intraradices hyphae and was not able to use the hyphae as carbon substrate. The negative effect of G. intraradices on culturability and on number of P. fluorescens DF57 in hyphosphere soil is discussed.