Co-managing soil and plant pathogens: effects of organic amendments on soil fertility and fungal pathogen survival

Backgrounds and aims

Interactions between plants can be both positive and negative, denoting facilitation and competition. Although facilitative effects of having legume neighbours (focus on yield productivity) are well studied, a better mechanistic understanding of how legumes interact with non-legumes in terms of root distribution is needed. We tested the effects of neighbour identity, its spatial location, as well as the effects of plant order of arrival on above and belowground traits and root distribution.

Methods

We performed a rhizotron experiment (4 weeks duration) in which we grew maize alone, with only a legume, only another grass, or with both species and tracked roots of the plant species using green and red fluorescent markers.

Results

Maize grew differently when it had a neighbour, with reduced development when growing with wheat compared to alone. Growing with a legume generally equated to the same outcome as not having a neighbour. Roots grew towards the legume species and away from the wheat. Order of arrival affected aboveground traits to a certain extent, but its effects on maize roots were dependent on spatial location.

Conclusions

Our study provides evidence of facilitation, showing the importance of the identity of the neighbours, together with their spatial location, and how order of arrival can modulate the outcome of these initial interactions.

     

Root responses to nitrogen pulse frequency under different nitrogen amounts

Responses of morphology and biomass allocation of roots to frequency of nitrogen (N) pulse potentially influence the fitness of plants, but such responses may be determined by root size. We grew 12 plant species of three functional groups (grasses, forbs, and legumes) under two N pulse frequencies (high vs. low supply frequency) and two N amounts (high vs. low supply amount). Compared to low-amount N supply, high-amount N supply stimulated biomass accumulation and root growth by either increasing the thickness and length of roots or decreasing the root mass fraction. Compared to low-frequency N supply, high-frequency N supply improved biomass accumulation and root growth in forbs or grasses, but not in legumes. Furthermore, the magnitude of the response to N frequency was significantly negatively correlated with root size at the species scale, but this was only true when the N amount was high. We conclude that root responses to N frequency are related to plant functional types, and non-legume species is more sensitive to N frequency than legume species. Our results also suggest that root size is a determinant of root responses to N frequency when N supply amount is high.     

Effects of physical connection and genetic identity of neighbouring ramets on root-placement patterns in two clonal species

Root-placement patterns were examined in the clonal species Glechoma hederacea and Fragaria vesca when grown with different types of neighbours. Three different patterns were predicted as consequences of different types of interactions between roots: the avoidance pattern if root growth decreases in the presence of neighbouring roots; the intrusive pattern if root growth increases towards neighbouring roots; and the unresponsive pattern if root growth is unaffected by neighbouring roots. Experiments were conducted in which physical connection between ramets, and the genetic identity of neighbouring ramets, were manipulated. The patterns of distribution of entire root systems and elongation rates of individual roots were measured. Root systems and individual roots of G. hederacea avoided contact with roots of neighbouring ramets, irrespective of connection to the neighbour and its genetic or specific identity. In contrast, F. vesca roots grew equally towards and away from intraspecific ramet neighbours and their elongation was stimulated by contact with roots of G. hederacea ramets. These results demonstrate that root-placement patterns of plants grown with different types of neighbours vary between species, and suggest that factors additional to resource depletion could be involved in their development.     

Aggregative Root Placement: A Feature During Interspecific Competition in Inland Sand-Dune Habitats

Segregation of roots is frequently observed in competing root systems. However, recently, intensified root growth in response to a neighbouring plant has been described in pot experiments [Gersani M, Brown J S, O'Brien E E, Maina G M and Abramsky Z 2001. J. Ecol. 89, 660–669]. This paper examines whether intense root growth towards a neighbour (aggregation) plays a role in competitive interactions between plant species from open nutrient-poor mid-European sand ecosystems. In a controlled field-competition experiment, root distribution patterns of intra- and interspecific pairs as well as single control plants of Corynephorus canescens, Festuca psammophila, Hieracium pilosella, Hypochoeris radicata and Conyza canadensis were investigated after one growing season. Under intraspecific competition plants tended to segregate their root systems, while under interspecific competition most species tended to aggregate roots towards their neighbours even at the expense of root development at the opposite competition-free side of the target. Preference of a root aggregation strategy over the occupation of competition-free soil in interspecific competition emphasizes the importance of contesting between individuals in relation to mere resource acquisition. It is suggested that in the presence of a competitor the plants might use root aggregation as a defensive reaction to maintain a strong competitive response and exclusive access to the resources of already occupied soil volumes.     

Effect of neighbour presence and soil volume on the growth of Andropogon gerardii Vitman

Background: Theory predicts that plants can reduce their fitness in the presence of neighbours by allocating resources to root growth, in order to pre-empt resource capture. A number of studies that have tested this idea have done so by using experiments where neighbour presence is confounded with soil volume.

Aims : To avoid confounding effects of neighbour presence and soil volume we adjusted these variables independently from one another.

Methods: We grew Andropogon gerardii with and without neighbours, holding soil volume available to each plant constant, and compared plant performance with a treatment where both neighbour presence and soil volume were varied. We also grew plants with a quarter of the soil volume but four times the nutrient concentration to determine if changes in plant growth in response to soil volume are caused by access different levels of soil resources.

Results: We found no evidence that plants adjust root growth to the presence of neighbour roots alone. We did, however, find a significant reduction in plant growth when soil volume was reduced. The reduction was overcome by increasing nutrient concentrations in the growth media.

Conclusions: Our results suggest the effects of soil volume on plant growth are mainly due to changes in nutrient availability.     

Spatial root distribution of plants growing in vertical media for use in living walls

Background and Aims

For plants growing in living walls, the growth potential is correlated to the roots ability to utilize resources in all parts of the growing medium and thereby to the spatial root distribution. The aim of the study was to test how spatial root distribution was affected by growing medium, planting position and competition from other plants.

Methods

Five species (Campanula poscharskyana cv. ‘Stella’, Fragaria vesca cv. ‘Småland’, Geranium sanguineum cv. ‘Max Frei’, Sesleria heufleriana and Veronica officinalis cv. ‘Allgrün’) were grown in three growing media (coir and two of rockwool) in transparent boxes under greenhouse conditions. Root frequency was registered and the activity of individual root systems was studied via ¹⁵N uptake and plant dry weight was measured.

Results

Plants in coir had stronger root growth in all parts of the medium than plants in rockwool. Upwards root growth was limited for plants in the middle or lower parts of the medium and ¹⁵N measurements confirmed that only plants in the bottom of the box had active roots in the bottom of the medium. The species differed in root architecture and spatial root distribution.

Conclusions

The choice of growing medium, plant species and planting position is important for a living wall as it affects the spatial root growth of the plants.     

Metallophytes—a view from the rhizosphere

Abundance of Fabaceae declines in representation through post-fire-succession in fynbos vegetation of the Cape Floristic Region (CFR). This reduction in legume occurrence coincides with a known decline in post-fire soil P availability. It was hypothesized that the disappearance of legume species during post-fire succession is due to an inability to acquire P effectively from sparingly soluble sources. P-acquisition strategies and response to P supply were compared between legume (Aspalathus, Cyclopia, Indigofera, Podalyria) and non-legume (Elegia, Leucadendron, Protea) genera when supplied with 1 or 10 mg P kg^?1 dry sand. Each genus consisted of a seeder (non-persistent) and resprouter (persistent) species. Non-legumes showed a greater investment in below-ground biomass, more root clusters, with higher concentrations of carboxylates exuded by cluster roots and carboxylates that were better suited to the mobilization of sparingly soluble P compared to legumes. The growth response to increased P supply was 53% higher in legumes than in non-legumes. The lack of a growth response to an elevated P supply in the non-legumes was attributed to N-limitation. Legume resprouters had a higher investment in cluster-root biomass and a lower capacity to down-regulate P-uptake than the seeders. Therefore the inability to acquire sufficient P from low concentration and sparingly soluble soil P-sources may contribute to the lack of indigenous legume persistence in fynbos vegetation of the CFR.     

Dark septate endophytes colonizing the roots of ‘non-mycorrhizal’ plants in a mine tailing pond and in a relatively undisturbed environment,Southwest China

Dark septate endophytes (DSEs), one of the most common fungal colonizers of roots, are considered to overlap in function with mycorrhizal fungi. However, there is little knowledge on the distribution and identity of DSEs in ‘non-mycorrhizal’ plants. In the current study, colonization and diversity of DSEs colonizing the roots of eight typically ‘non-mycorrhizal’ families were assessed. In total, 120 root samples of 31 plant species were all colonized by DSEs. Intensity of DSE colonization varied greatly among different plant species, with a range of 0.56–47.56%, 8.13% on average. Cladosporium, Cyphellophora and Phialophora were the dominant genera, with a relative abundance of more than 60% over a total of 90 isolates. Our results showed that diverse DSE species colonized the roots of ‘non-mycorrhizal’ plants, especially they were more common in degraded mine tailings than in the undisturbed site, but their integral roles to the functional roots are in need of further experimental demonstration.     

Factors determining plant–neighbour interactions on different spatial scales in young species-rich grassland communities

In naturally colonised species-rich grassland communities, we examined the properties of a plant’s aboveground neighbourhood that affect its performance (aboveground biomass). To this end a range of neighbourhood parameters were measured: number, biomass and species richness of the neighbours, number and biomass of the conspecific neighbours, and light availability at the base of the target plant. We also determined at which neighbourhood size the strongest target plant–neighbour interactions occurred, and whether conspecific neighbours affected competitively stronger or weaker target species differently. Target plant performance varied with target identity, and was significantly affected by light availability and the number of neighbouring plants (neighbourhood density). Depending on the target species, there was also an effect of total neighbour biomass on plant performance. The target plants were most strongly affected by their neighbours within a 3-cm distance, which could account for 78% of the variance in target biomass. Number or biomass of the conspecific neighbours did not contribute to the explanation of target performance in any of the target species. Whereas in an 8-cm neighbourhood the amount of light penetration was the strongest predictor of target performance, the number of neighbours was more important in a 3-cm neighbourhood. These experimental results might be useful to extend existing neighbourhood competition models for one or two species to multi-species competition models.     

Organ regulated expression of the Parasponia andersonii haemoglobin gene in transgenic tobacco plants

Summary Plant haemoglobin genes are known to occur in legume and non-legume families and in both nodulating (e.g. Parasponia andersonii) and non-nodulating species (e.g. Trema tomentosa). Their presence in non-nodulating plants raises the possibility that haemoglobins might serve a function in non-symbiotic tissues distinct from their role in the nitrogen-fixing root nodules induced by micro-organisms. We report here that a P. andersonii haemoglobin promoter can regulate expression of either the P. andersonii haemoglobin gene, or a hybrid construct with the bacterial chloramphenicol acetyltransferase gene (cat), in the nonsymbiotic plant, Nicotiana tabacum. Expression is predominantly in the roots, implying that haemoglobins might have a function in roots of non-nodulated plants. We have also observed a low level of haemoglobin protein in non-nodulated P. andersonii roots, but not leaves, supporting this assertion. The expression in transgenic plants will allow further characterization of the promoter sequences essential for the organ-specific expression of haemoglobins in nonsymbiotic tissues.     

Bi-directional transfer of nitrogen between alfalfa and bromegrass: Short and long term evidence

Transfer of N from legumes to associated non-legumes has been demonstrated under a wide range of conditions. Because legumes are able to derive their N requirements from N₂ fixation, legumes can serve, through the transfer of N, as a source of N for accompanying non-legumes. Studies, therefore, are often limited to the transfer of N from the legume to the non-legume. However, legumes preferentially rely on available soil N as their source of N. To determine whether N can be transferred from a non-legume to a legume, two greenhouse experiments were conducted. In the short-term N-transfer experiment, a portion of the foliage of meadow bromegrass (Bromus riparius Rhem.) or alfalfa (Medicago sativa L.) was immersed in a highly labelled ¹⁵N-solution and following a 64 h incubation, the roots and leaves of the associated alfalfa and bromegrass were analyzed for ¹⁵N. In the long-term N transfer experiment, alfalfa and bromegrass were grown in an ¹⁵N-labelled nutrient solution and transplanted in pots with unlabelled bromegrass and alfalfa plants. Plants were harvested at 50 and 79 d after transplanting and analyzed for ¹⁵N content. Whether alfalfa or bromegrass were the donor plants in the short-term experiment, roots and leaves of all neighbouring alfalfa and bromegrass plants were enriched with ¹⁵N. Similarly, when alfalfa or bromegrass was labelled in the long-term experiment, the roots and shoots of neighbouring alfalfa and bromegrass plants became enriched with ¹⁵N. These two studies conclusively show that within a short period of time, N is transferred from both the N₂-fixing legume to the associated non-legume and also from the non-legume to the N₂-fixing legume. The occurrence of a bi-directional N transfer between N₂-fixing and non-N₂-fixing plants should be taken into consideration when the intensity of N cycling and the directional flow of N in pastures and natural ecosystems are investigated.     

Glutamine synthetase polypeptides in the roots of 55 legume species in relation to their climatic origin and the partitioning of nitrate assimilation

Glutamine synthetase (GS) exists as two main isoforms in plants, a cytosolic form (GSI) and a chloroplast or plastidie form (GS2). Fifty-five species of legume, representing a phylogenetically diverse group of tropical and temperate species, were screened by western blotting for the presence of GS2 in their roots. A remarkably strong correlation was found between the climatic origin of the species and the presence or absence of a GS2-like polypeptide in the root. Root GS2 was found in all 31 temperate species examined (30 papilionoids, one caesalpinoid), but was not detected in any of the 17 tropical papilionoid species. It was also absent in the roots of four out of seven tropical non-papil-ionoid species. The ‘in vivo’ NR activities of roots, stems and leaves of 46 of the legume species were analysed to establish their major site of nitrate reduction, and the ratio of nitrate: reduced N in the xylem sap was determined for some species, but no clear correlation between possession of a root GS2 and a preference for root nitrate assimilation was found. We discuss the possibility that expression of GS2 in the root was part of a more extensive physiological adaptation to root nitrate assimilation that evolved in temperate species to suit the alkaline, nitrate rich soils found in the centres of origin in temperate latitudes.     

How plant diversity and legumes affect nitrogen dynamics in experimental grassland communities

Positive relationships between species richness and ecosystem processes such as productivity or nitrogen cycling can be the result of a number of mechanisms. We examined how species richness, biomass, and legume presence, diversity, and abundance explained nitrogen dynamics in experimental grassland plots in northern Sweden. Nitrogen concentrations and '¹⁵N values were measured in plants grown in 28 mixtures (58 plots) including 1, 2, 4, 8 or 12 local grassland species over four years. Values for '¹⁵N declined over time for all three functional groups (grasses, legumes, and non-leguminous forbs), suggesting greater reliance on N fixed by legumes over time by all species. Above ground percent nitrogen (%N) also declined over time but root %N and total N did not. Path analysis of above ground data suggested that two main factors affected %N and the size of the N pool. First, higher plant diversity (species richness) increased total N through increased biomass in the plot. Although in the first two years of the experiment this was the result of a greater probability of inclusion of at least one legume, in the last two years diversity had a significant effect on biomass beyond this effect. Second, percent legumes planted in the plots had a strong effect on above ground %N and '¹⁵N, but a much smaller effect on above ground biomass. In contrast, greater plant diversity affected N in roots both by increasing biomass and by decreasing %N (after controlling for effects mediated by root biomass and legume biomass). Increased legume biomass resulted in higher %N and lower '¹⁵N for both non-legume forbs and grasses in the first year, but only for grasses in the third year. We conclude that a sampling effect (greater probability of including a legume) contributed towards greater biomass and total N in high-diversity communities early on in the experiment, but that over time this effect weakened and other positive effects of diversity became more important.     

Short-term learning and the searching accuracy of egg-laying butterflies

Searching constraints influence foraging patterns in three species of Colias butterflies. The activity of egg-laying females is partitioned into periods of searching for oviposition plants and periods of visiting flowers for nectar. Eggs are laid most frequently upon legume host-plants, although females frequently land upon non-legume species, particularly those bearing a superficial resemblance to suitable hosts. The average frequency of these landing ‘errors’ decreases over the course of an egglaying flight, implying thet females learn to identify host-plants more accurately on the basis of recent experience. Landing accuracy is low after periods of nectar-feeding, which suggests a trade-off between the two searching modalities. Within oviposition sequences, females that visit a narrow range of host specis make fewer landing errors than females visiting a broader host-plant set. Two interpretations of these data are presented: (1) that partitioning searching time into discrete modes may enhance overaccuracy in invertebrates, as has been demonstrated previously for vertebrates. Searching dynamics of this type may explain some discrepancies between the predictions made by simple optimal diet models and the occurrence of ‘switching’ behaviours in foragers.     

Root architecture of Arabidopsis is affected by competition with neighbouring plants

How roots detect and respond to the presence of neighbors is relevant to understand plant belowground interactions. The aim of the present work was to evaluate the effect of the presence of neighboring plants and the limited availability of phosphorus on root architecture. A target plant of Arabidopsis thaliana (Ler or Col) was surrounded by combinations of two individuals (Ler and Col), and subjected to different growth conditions (levels of activated charcoal (AC) and phosphorus). Both accessions consistently concentrated their roots towards the competition zone shared with a neighbor of the same accession, avoiding the side shared with the other accession. All these competition strategies disappeared when plants were limited by phosphorus or when activated charcoal was added to the growth media. Plants produced consistently fewer but longer lateral roots when activated charcoal was added to the growth media irrespective of the neighbors. Our results indicate a direct role of secondary metabolites present in the root exudates and phosphorus availability in the response of presence and identity of neighboring roots.     

Using coloured roots to study root interaction and competition in intercropped legumes and non-legumes

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems. The comprehension of such phenomena places big methodological challenges, and still needs clarification. The objectives of this work were (i) to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system; (ii) to verify the importance of initial root growth on the successive root development of mixture component plants; (iii) to test if the root interaction in the shallow layers has consequences for deep root growth and (iv) to compare the effect of intraspecific and interspecific competition on root development and biomass growth.Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet, a variety of Beta vulgaris L. with clear red roots. Within the rhizotrons, the three species were grown either without competitors, with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition. The use of mixtures where one component has clearly coloured roots, together with several scalar measurements of root depth and proliferation, allowed the measurement of the root system of each species when grown in the mixtures.Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems. The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension. Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil. The root system of the red beet showed much faster and deeper growth than that of the legumes, and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer. Intraspecific competition was well tolerated by the legumes, but it was limiting for the highly competitive red beet. The outcome of root interaction between neighbour plants was confirmed to be species-specific as it changed according to the intensity of the competitive effect/response of each species of the mixture: both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.     

Lead,zinc and iron (Fe2+) tolerances in wetland plants and relation to root anatomy and spatial pattern of ROL

Metal (Pb, Zn and Fe²⁺) tolerances, root anatomy and profile of radial oxygen loss (ROL) along the root (i.e., spatial pattern of ROL) were studied in 10 emergent wetland plants. The species studied could be classified into three groups. Group I included Alternanthera philoxeroides, Beckmannia syzigachne, Oenanthe javanica and Polypogon fugax, with high ROL along the whole length of root (‘partial barrier’ to ROL). Group II included Cyperus flabelliformis, Cyperus malaccensis, Juncus effusus, Leersia hexandra and Panicum paludosum, ROL of which was remarkably high just behind the root apex, but decreased significantly at relatively basal regions (‘tight barrier’ to ROL). Group III consisted of only Neyraudia reynaudiana, with extremely low ROL along the length of root. The results indicated that metal tolerance in wetland plants was related to root anatomy and spatial pattern of ROL. Co-evolution of metal (Fe and Zn) tolerance and flood tolerance possibly developed in wetland plants since species showing a ‘tight barrier’ to ROL (a common trait of flood-tolerant species) in basal root zones had higher Fe and Zn tolerances than those showing a ‘partial barrier’. Root anatomy such as lignin and suberin deposition contributed to a ‘tight barrier’ in root and conferred to exclusion ability in tolerant species.     

Structures and functions of adventitious roots in species of the genus Philodendron Schott (Araceae)

We discuss here the anatomical variations of the arrangements and compositions of stele types observed in different roots types in four populations of the three species of Philodendron as probable adaptations to their habitats. Terrestrial individuals of P. corcovadense have cylindrical steles while rupicolous individuals have lobate steles with dispersed internal cortical parenchyma. The Philodendron species sampled showed polyarch structures. The crampon roots of P. oblongum and anchor roots of P. cordatum show medullated protosteles, with the former species having a reduced pith with sclerified parenchyma cells while the latter has a wide pith and parenchyma cells with only slightly thickened walls. The feeder roots of P. cordatum also show a medullated protostele—although a central vessel is present until approximately 60 cm from the apex that later disappears, forming a parenchymatous pith. We conclude that the different root types reflect adaptations of the subgenera Philodendron and Meconostigma to their different habits and habitats, such as in P. corcovadense, where the roots of rupicolous individuals have lobate steles while the roots of the terrestrial plants have cylindrical steles.