Water accessibility to plant roots in different soil structures occurring in the same soil type

The capacity of a soil to supply roots with water and nutrients for crop growth is important when defining sustainable land management which implies maintenance of production and reduction of production risks. Not only the amount of available water is important but also its accessibility, which differs among different soil structures. Different structures within one soil series were associated with three types of management: (i) conventional, temporary grassland (Conv), (ii) biodynamic, temporary grassland (Bio) and (iii) conventional permanent grassland (Perm). Transpiration of barley plants, under identical circumstances, and the associated rooting patterns, were measured in five large undisturbed cores from each of the three soil structures. Management had significantly changed bulk density, organic matter content and porosity. Measured transpiration showed significant differences with highest amounts for Perm followed by Conv and lowest amounts for Bio. Rooting pattern characteristics, defined as the relation between a series of hypothetical extraction zones around each root and the volumes of excluded soil were determined for the three structures. These rooting pattern characteristics were most favourable for Perm, followed by Bio and Conv, respectively. The water supply characteristics, defined as the number of days the soil can satisfy a transpiration demand of 5 mm d^-1 as a function of a hypothetical extraction zone, reflects the capacity of the soil to supply roots with water. These water supply characteristics combined with the rooting pattern characteristics were used to quantify the accessibility of soil water. Accessibility was highest for Perm and Conv with 95% and 94% respectively, followed by Bio with 68%. When used in a simulation model and compared with simulations implicitly assuming total accessibility, measured transpirations were better simulated by introducing the expression for water accessibility.     

A global analysis of root distributions for terrestrial biomes

Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient to the data for each biome (Gale and Grigal 1987). is a simple numerical index of rooting distribution based on the asymptotic equation Y=1-^d, where d = depth and Y = the proportion of roots from the surface to depth d. High values of correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (=0.913, 0.943, and 0.943, respectively), with 80–90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (=0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m^-2. Tropical evergreen forests had the highest root biomass (5 kg m^-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m^-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil. (=0.952), while shrubs had only 21% in the same depth increment (=0.978). The rooting distribution of all temperate and tropical trees was =0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was =0.966 (r ²=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.     

Root characteristics of selected field crops: Data from the Wageningen Rhizolab (1990–2002)

Since being built in 1990, the rhizotron facility in Wageningen, the Wageningen Rhizolab, has been used for experiments on crops (e.g. Alfalfa, Brussels sprouts, common velvet grass, field bean, fodder radish, leeks, lupins, maize, potato, beetroot, ryegrass, spinach, spring wheat, winter rye and winter wheat). In the experiments, horizontal glass minirhizotron tubes combined with auger sampling were used to assess rooting characteristics. For this paper we took the root data from these experiments and looked for a general relationship between thermal time/time after planting and rooting depth, the velocity of the root front and root proliferation. For certain depths (fixed by the depth at which the horizontal minirhizotrons were installed) a simple linear regression was established between the average root number per cm² minirhizotron surface area and thermal time after planting. The compartments selected for each crop were those in which there had been a control treatment and/or in which conditions for rooting were considered to be optimal. We performed regression analyses per compartment and per depth, but only for the period after planting in which a linear increase of root numbers vs. thermal time was observed. After averaging the results, the regression procedure yielded two parameters of rooting for each crop: (a) the actual or thermal time at which the first root appeared at a certain depth and (b) the root proliferation rate after the first root had appeared. In this way, inherent crop differences in rooting behaviour (rooting depth and root proliferation) became apparent. For each crop, the velocity of the root front after planting could be established (calculated in cm(°C day)^–1). This parameter differed greatly between crops. Some crops (such as leeks and common velvet grass) explored the soil profile slowly: the root front moved at a velocity of only 0.07cm(°C day)^–1. Among the crops whose roots grew down much faster (0.18–0.26cm (°C day)^–1) were cereals and fodder radish. For a day with an average temperature of 15°C these rates would have corresponded with the root front travelling approximately 1–4cm per day. In the crops studied the root front velocity did not correlate with the root proliferation rate.     

Patterns of adventitious root formation in English ivy

Adventitious root formation by debladed petiole cuttings of English ivy (Hedera helix L.) proceeds via a direct rooting pattern for the easy-to-root juvenile phase, while the difficult-to-root mature phase roots through an indirect rooting pattern. Juvenile petiole cuttings treated with -naphthaleneacetic acid (NAA, 100 M) plus the polyamine biosynthesis inhibitor, difluoromethylarginine (DFMA, 1 mM), formed an increased number of roots per cutting initiated by the indirect rooting pattern. The increased root formation and change in rooting pattern were reversed by the addition of putrescine (1 mM). Delaying auxin application to petiole cuttings for 15 days also induced juvenile petioles to root by the indirect pattern. This could be reversed by rewounding the base of the cutting prior to auxin application after day 15. The data support the use of the terms competent root-forming cells and induced competent root-forming cells to describe the target cells for the initial events of root formation for the direct and indirect rooting patterns, respectively.Kentucky Experiment Station publication 90-10-122.     

Multiple shoot regeneration from root organ cultures of Populus alba x P. grandidentata

Excised roots of various ages from Crandon and Hansen clones of Populus alba x P. grandidentata were tested for their regeneration capacity. Sixty-day-old excised roots that contained root tips were found to be most suitable. The highest number of shoots (an average of 111 shoots/root segment with Crandon and 98 with Hansen) was obtained by adding 22M and 14M zeatin to the medium, respectively. The two clones of hybrid poplar responded similarly to growth regulator treatments; however, the number of shoots produced was greater from the root organs derived from Crandon clones. Regenerated shoots were rooted in basal Woody Plant Medium without any growth regulators. Successful transplantation into soil and growth was achieved with all plants.     

Root growth and nitrate utilization of maize cultivars under field conditions

In a 2-year field study conducted on a high fertilized Gleyic Luvisol in Stuttgart-Hohenheim significant differences among 10 maize cultivars were observed in soil nitrate depletion. The different capability of the cultivars to utilize nitrate particularly from the subsoil was positively correlated with (a) shoot N uptake at maturity, and (b) root length density (L_v) in the subsoil layers at silking. Critical root length densities for nitrate uptake were estimated by (a) calculating uptake rates per unit root length (U), (b) subsequent calculation of needed nitrate concentration in soil solution (C₁) to sustain calculated U according to the Baldwin formula, and (c) reducing measured L_v and proportionate increase of U until needed concentration equaled measured concentration. Uptake rate generally increased with soil depth. Critical root length densities for cultivar Brummi (high measured root length densities and soil nitrate depletion) at 60–90 cm depth ranged from 7 % (generative growth) to 28 % (vegetative growth) of measured L_v Measured root length density of each other cultivar was higher than critical root length density for Brummi indicating that the root system of each cultivar examined would have been able to ensure N uptake of Brummi. Positive relationships between root length density and nitrate utilization as indicated by correlation analysis therefore could not be explained by model calculations. This might be due to simplifying assumptions made in the model, which are in contrast to non-ideal uptake conditions in the field, namely irregular distribution of roots and nitrate in the soil, limited root/soil contact, and differences between root zones in uptake activity. It is concluded from the field experiment that growing of cultivars selected for high N uptake-capacity of the shoots combined with high root length densities in the subsoil may improve the utilization of a high soil nitrate supply.     

Effect of artificial soil acidification and liming on growth and nutrient status of mycorrhizal roots of Norway spruce (Picea abies [L.] Karst.)

Effects of soil acidification and liming on biomass responses and free Al, Ca, K, Mg, Mn and P contents of mycorrhizal roots of mature Norway spruce (Picea abies [L.] Karst.) were studied at Höglwald Forest in Southern Germany.At the untreated site, mycorrhizal root biomass was lower in the acid humus (pH = 3.3) than in the less acid upper (0–5 cm) mineral soil (pH 4.1). Mycorrhizal roots from the humus contained 10% of the level free Al in mycorrhizal roots from the upper mineral soil. During seven years of soil acidification the quantity of mycorrhizal roots remained unaffected in the humus and the upper mineral soil, perhaps due to the high buffering capacity of the humus which prevented a significant alteration of the nutrient status of the roots. However, two years after soil acidification had been terminated, the percentage of mycorrhizal roots in the humus decreased, possibly because the free root concentrations of K had decreased.On the other hand, six years after liming, there was a two-fold increase of the annual mean quantity of mycorrhizal roots in the humus. Compensatory liming (acid irrigation after liming) had a similar effect on mycorrhizal root production in the humus. However, two years after acid irrigation had been terminated a decrease of mycorrhizal roots in the upper mineral soil (0–5 cm) was observed. Since the total amount of mycorrhizal roots in the humus and upper mineral soil remained constant, compensatory liming produced a shift in fine roots to the humus layer.The higher mass of living mycorrhizal roots in the upper mineral soil (0–5 cm) as compared to the humus of the untreated plot as well as the increased mass of mycorrhizal roots in the humus after liming or compensatory liming are both attributed to an increase in pH to 4.5 rather than alleviation of Al toxicity.     

Are differences in root growth of nitrogen catch crops important for their ability to reduce soil nitrate-N content,and how can this be measured?

An experiment was made to measure root growth of nitrogen catch crops, to investigate whether differences in root growth among plant species are related to their ability to deplete the soil nitrate-N pool. Large differences were observed in root growth parameters. Monocot species had rooting depth penetration rates in the range of 1.0 to 1.2 mm d^–1 °C^–1, whereas the non-legume dicot species had rates between 1.5 and 2.3 mm d^–1 °C^–1. Substantial differences were also found in the lag time from sowing until significant root growth was observed. The estimated temperature sum needed for the crops to reach a rooting depth of 1.0 m varied from 750 d °C for fodder radish to 1375 d °C for Italian ryegrass. The depth distribution of the root system varied strongly, and at a depth of 1.0 m the non-legume dicot species generally had root intensities (number of root intersections m^–1 line on the minirhizotrons) 12 times as high as the monocot species.The amount of nitrate left in the topsoil (0–0.5 m) was only weakly correlated to a few of the measured plant and root parameters, whereas nitrate left in the subsoil (0.5–1.0 m) was clearly correlated to several root parameters. Subsoil nitrate residues were well correlated to root intensity, but showed even stronger correlations to more simple estimates of rooting depth. In the deepest soil layer measured (1.0–1.5 m), the soil water nitrate concentration was reduced from 119 g L^–1 without a catch crop to 61 g L^–1 under Italian ryegrass and to only 1.5 g L^–1 under fodder radish.The results show that to identify the important differences in root growth among catch crops, root growth must be measured in deep soil layers. In this study, none of the measurements made aboveground or in the upper soil layers were well related to subsoil nitrate depletion.     

Modelling the effect of varying soil water on root growth dynamics of annual crops

We present a simple framework for modelling root growth and distribution with depth under varying soil water conditions. The framework considers the lateral growth of roots (proliferation) and the vertical extension of roots (root front velocity). The root front velocity is assumed to be constant when the roots descend into an initially wet soil profile. The lateral growth of roots is governed by two factors: (1) the current root mass or root length density at a given depth, and (2) soil water availability at that depth.Under non-limiting soil water conditions, the increase in root mass at any depth is governed by a logistic equation so that the root length density (R _v) cannot exceed the maximum value. The maximumR _v, is assumed to be the same for all depths. Additional dry matter partitioned to roots is initially distributed according to the current root mass at each depth. As the root mass approaches the maximum value, less dry matter is partitioned to that depth.When soil water is limiting, a water deficit factor is introduced to further modify the distribution of root dry matter. It is assumed that the plant is an energy minimiser so that more root mass is partitioned to the wetter regions of the soil where least energy will be expended for root growth. Hence, the model allows for enhanced root growth in areas where soil water is more easily available.Simulation results show that a variety of root distribution patterns can be reproduced due to varying soil water conditions. It has been demonstrated that broad patterns of root distribution reported in the literature can also be simulated by the model.     

Vertical rooting patterns of mature Quercus trees growing on different soil types in northern Germany

At two different sites in northern Germany, the vertical distribution patterns of roots with diameters of up to 10 mm were investigated with the trench profile wall technique beneath mature trees of Quercus petraea (Matt.) Liebl. growing on clayey (cambi-stagnic gleysol) or sandy soil (dystric cambisol). The rooting patterns were related to soil bulk density, maximum plant-available soil water and soil chemistry, and were compared with the rooting pattern of Q. petraea growing on a silty soil on limestone (luvisol) and with that of Q. robur L. growing on clayey loam (stagnic cambisol). All three Q. petraea sites differed in their vertical rooting pattern, as was indicated by significant differences in the parameter which was calculated from the cumulative root fraction (Y) for the cumulative rooting depth (d; Y=1–^d). At the clayey site, a relatively large fraction of roots was found in deeper soil layers (highest value); whereas, at the silty site, the rooting pattern was most superficial (lowest value). No significant difference in the vertical rooting pattern was observed between Q. petraea and Q. robur growing on clayey soil. The soil-area related biomasses of living roots did not differ between the clayey and the sandy site of Q. petraea.Comparisons of the rooting patterns with soil water relations and soil chemistry lead to the hypothesis that under the climatic conditions of Central Europe, the vertical root distribution of Q. petraea is more influenced by the availability of nutrients, especially that of nitrogen, than by the amounts of plant-available soil water.     

Root traits, nutrient uptake, multi-location grain yield and benefit–cost ratio of two lentil (Lens culinaris, Medikus.) varieties

Lentil is a protein-rich pulse, grown mainly in developing countries as a rain-fed crop in nutrient-poor soils. Hence, the importance of root traits for efficient capture of soil nutrients and water can be crucial to its economical yield. Little is known about the lentil root system and even less about its relationship to grain yield. We compared the root system of two Bangladeshi lentil varieties, Barimasur-3 (BM-3) and Barimasur-4 (BM-4), in a pot experiment and related it to their multi-location grain yield in the fields. BM-4 maintained faster root development both at an early growth stage (20days after sowing) and at flowering (60days) compared to BM-3. The roots of BM-4 penetrated the 25cm depth of the soil profile after 19±1days and while those of BM-3 took 24±2days to reach the same depth. The roots of BM-4 were covered with denser (26±3mm^–1) and longer (0.48±0.11) root hairs than BM-3 (density 17±2mm^–1, length 0.32±0.09mm). The differential presence of root hairs increased the effective length of root system of BM-4 by 12 times and that of BM-3 by five times. The lentil varieties did not differ in their ability to induce pH change and acid phosphatase activity in rhizosphere. In the pot experiment, the uptake of macro-nutrients (K, P, Ca, and Mg) as well as micro-nutrients (Fe, Mn, Zn, Cu, B and Mo) by BM-4 was significantly higher, compared to BM-3. The varieties produced the same amount of shoot biomass. At five of six agro-ecological distinct field locations in Bangladesh, BM-4 gave significantly higher (10–20%) grain yield than BM-3. Linked with the higher grain yield, the benefit-cost ratio (BCR) of BM-4 was 3.14 and that of BM-3 were 2.62, indicating that BM-4 provided better return per unit investment, compared to BM-3, supported by the better root morphology and higher nutrient uptake. This may be one of the reasons supporting the popularity and preferred adoption of BM-4 among the Bangladeshi farmers, who grow lentil mainly on nutrient-poor soils. The results indicate the benefits of selection and breeding for superior root traits for better agro-economics.     

The role of light and polar auxin transport in root regeneration from hypocotyls of tomato seedling cuttings

A relationship between light conditions, auxin transport and adventitious root formation by hypocotyls of tomato seedling cuttings was demonstrated. Effective rooting of tomato seedling cuttings was observed under continuous white light (WL) irradiation. However, root formation was reduced in darkness or under red (RL) or blue light (BL). At least 3/4-day-long irradiation treatment with (WL) was necessary to increase the number of roots formed in comparison with control cuttings grown in darkness. Light was most effective if applied during the first half of the 13-day-long rooting period. The role of photoreceptor-dependent light perception in the light-regulation of rooting was tested using tomato photomorphogenic mutants: aurea (au) and high pigment (hp). When exposed to WL both mutants generated fewer roots then their isogenic wild type (WT). In darkness or under BL and RL less roots were formed on all plants and no difference was observed between mutants and WT plants. TIBA (2,3,5-triiodobenzoic acid) inhibited rooting in a dose-dependent manner both in darkness and under WL. However, although rooting was suppressed by 0.75 M TIBA in the dark, 8 M TIBA was necessary to block root formation in continuous WL. Inhibition of rooting by TIBA was most efficient when applied at the initial period of rooting, a 1-day-long treatment with TIBA being sufficient to suppress rooting if given during the first 2 days of culture. Later treatment had much less effect on the root formation.     

Non-invasive imaging of roots with high resolution X-ray micro-tomography

X-ray micro-tomography is a well-established technique for non-invasive imaging and evaluation of heterogeneous materials. An inexpensive X-ray micro-tomography system has been designed and built for the specific purposes of examining root growth and root/soil interactions. The system uses a silver target X-ray source with a focal spot diameter of 80 m, an X-ray image intensifier with a sampling aperture of about 100 m, and a sample with a diameter of 25 mm. Pre-germinated wheat and rape seeds were grown for up to 8–10 days in plastic containers in a sandy loam soil sieved to < 250 m, and imaged with the X-ray system at regular intervals. The quality of 3 D image obtained was good allowing the development and growth of both root axes and some first-order laterals to be observed. The satisfactory discrimination between soil and roots enabled measurements of root diameter (wheat values were 0.48–1.22 mm) in individual tomographic slices and, by tracking from slice to slice, root lengths were also measured. The measurements obtained were generally within 10% of those obtained from destructive samples measured manually and with a flat-bed scanner. Further developments of the system will allow more detailed examination of the root:soil interface.     

Factors controlling long-term changes in soil pools of exchangeable basic cations and stream acid neutralizing capacity in a northern hardwood forest ecosystem

Understanding the factors regulating the concentrations of basic cations in soils and surface waters is critical if rates of recovery are to be predicted in response to decreases in acidic deposition. Using a dynamic simulation model (PnET-BGC), we evaluated the extent to which atmospheric deposition of strong acids and associated leaching by strong anions, atmospheric deposition of basic cations through changes in emissions of particulate matter, and historical forest cutting have influenced soil pools of exchangeable basic cations and the acid-base status of stream water at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Historical deposition of basic cations was reconstructed from regression relationships with particulate matter emissions. Simulation results indicate that the combination of these factors has resulted in changes in the percent soil base saturation, and stream pH and acid neutralizing capacity (ANC) from pre-industrial estimates of 20%, 6.3 and 45 eq L^–1, respectively, to current values of 10%, 5.0 and –5 eq L^–1, respectively. These current values fall within the critical thresholds at which forest vegetation and aquatic biotic are at risk from soil and surface water acidification due to acidic deposition. While the deposition of strong acid anions had the largest impact on the acid-base status of soil and stream water, the reduction in deposition of basic cations associated with reductions in particulate emissions was estimated to have contributed about 27% of the depletion in soil Ca²⁺ exchange pool and 15% of the decreases in stream water concentrations of basic cations. Decline in stream water concentrations of basic cation occurred under both increasing and decreasing exchangeable pools, depending on the process controlling the acid base status of the ecosystem. Model calculations suggest that historical forest cutting has resulted in only slight decreases in soil pools of exchangeable basic cations, and has had a limited effect on stream ANC over the long-term.     

Selection of microorganisms which produce raw-starch degrading enzymes

Summary Microorganisms which produce strong raw-starch degrading enzymes were isolated from soil using a medium containing a unique carbon source, -amylase resistant starch (-RS), which is insoluble in water and hardly digested with Bacillus amyloliquefaciens -amylase. Among the isolates, three strains showing high activities were characterized. Two of them, K-27 (fungus) and K-28 (yeast), produced -amylase and glucoamylase, and the final product from starch was only glucose. The third strain, K-2, was a bacterium and produced -amylase, which produced glucose and malto-oligosaccharides from starch. The enzyme preparation of these strains degraded raw corn starch rapidly.     

Morphoregulatory role of thidiazuron: morphogenesis of root outgrowths in thidiazuron-treated geranium (Pelargonium x hortorum Bailey)

Summary Root outgrowths formed on the root tissue of geranium (Pelargonium x hortorum Bailey cv. Kim and cv. Shone Helena) plants in response to treatment with the phenylurea derivative, thidiazuron (N-phenyl-N-1,2,3-thiadiazol-5-ylurea; TDZ). Treatment with the cytokinin N⁶-benzylaminopurine (BAP) or the auxin -naphthaleneacetic acid (NAA) did not result in stimulation of similar abnormal structures on the root tissue. Significantly more outgrowths developed on roots of plants treated with 10 M and 20 M TDZ than on control plants or those treated with 1 M TDZ for the eight-week treatment period. Some outgrowths produced shoots and plantlets while still attached to roots, and regenerants were easily separated from the root tissue and transferred to soil in the greenhouse where they grew to maturity. Histological observations suggested these outgrowths originated from the vascular cambium region of the root.     

The supramolecular organization of red algal cell membranes and their participation in the biosynthesis and secretion of extracellular polysaccharides: a review

Summary The relationship between the supramolecular organization of red algal cell membranes and the biosynthesis and secretion of the cell wall skeletal and matrix polysaccharides is reviewed. Freeze-fracture studies have revealed that organized macromolecular structures — linear terminal complexes and tetrads — are present on the plasma membrane and on membranes of the endomembrane system. The linear terminal complexes seem to be involved in the biosynthesis, assembly, and orientation of the cellulose microfibrils and the tetrads in the synthesis of the matrix polysaccharides. It is shown how the research on the supramolecular organization of cell membranes has increased the knowledge on the biosynthesis and secretion of the extracellular crystalline and non-crystalline polysaccharides in red algae. In this review, the progress to date is discussed.Dedicated to Prof. Dr. Dr. h.c. Eberhard Schnepf on the occasion of his retirement     

Fine root mass in relation to soil N supply in a cool temperate forest

Soil inorganic nitrogen supply and fine root mass in the top layers of mineral soil (0–5 and 5–10cm) were investigated at upper and lower sites of a cool temperate forest where Fagus crenata and Quercus crispula dominate. At both sites, soil inorganic nitrogen supply was greatest in the 0–5cm layer. The predominant forms of soil inorganic nitrogen supply were NH₄⁺-N at the upper site and NO₃^–-N at the lower site. Fine roots were concentrated in the 0–5cm layer at the upper site, but not at the lower site. The form of supplied soil inorganic nitrogen supply can be important in determining the vertical distribution of fine roots.     

Determination of the cell adhesion specificity of Streptococcus suis with the complete set of monodeoxy analogues of globotriose

Streptococcus suis causes meningitis and other serious infections in pigs and humans, and binds to host cell globotriosylceramide. In order to determine the essential hydroxyls involved in binding, the complete set of monodeoxy derivatives of the receptor trisaccharide Gal1-Gal1-4Glc were tested as inhibitors of bacterial hemagglutination. Removal of the 4-, 6, 2 or 3-hydroxyls abolished inhibitory activity, which indicated that they were critically involved in binding. The same results were obtained using synthetic lipid-linked monodeoxy derivatives of the trisaccharides in a thin-layer overlay assay. The P_N and P_O subtypes of the S. suis adhesin showed similar binding patterns. The hydroxyls of the glucose moiety were not critical for binding, although the adhesin binds better to the trisaccharide Gal1-4Gal1-4Glc than the disaccharide Gal1-4Gal.     

Arbuscular mycorrhizal fungal propagules in a salt marsh

The tolerance of indigenous arbuscular mycorrhizal fungi (AMF) to stressful soil conditions and the relative contribution of spores of these fungi to plant colonization were examined in a Portuguese salt marsh. Glomus geosporum is dominant in this salt marsh. Using tetrazolium as a vital stain, a high proportion of field-collected spores were found to be metabolically active at all sampling dates. Spore germination tests showed that salt marsh spores were not affected by increasing levels of salinity, in contrast to two non-marsh spore isolates, and had a significantly higher ability to germinate under increased levels of salinity (20) than in the absence of or at low salinity (10). Germination of salt marsh spores was not affected by soil water levels above field capacity, in contrast to one of the two non-marsh spore isolates. For the evaluation of infectivity, a bioassay was established with undisturbed soil cores (containing all types of AM fungal propagules) and soil cores containing only spores as AM fungal propagules. Different types of propagules were able to initiate and to expand the root colonization of a native plant species, but spores were slower than mycelium and/or root fragments in colonizing host roots. The AM fungal adaptation shown by this study may explain the maintenance of AMF in salt marshes.