Vertical Growth and Mycorrhizal Infection of Woody Plant Roots as Potential Limits to the Restoration of Woodlands on Landfills

We assessed the vertical growth and mycorrhizal infection of woody plant roots on a closed landfill, using tree and shrub clusters that had been previously installed in patches of increasing size to establish protocols for woodland restoration. The density of the fine roots of shrubs, which had poor-to-moderate mycorrhizal infection, decreased strongly with increasing depth. Oak ( Quercus ) seedlings planted within and outside patches were assessed for ectomycorrhizal infection. Oak root systems were mycorrhizal, but root-tip proliferation was improved and ectomycorrhizal composition was influenced by woody debris in the mineral soil. Most surviving oaks were found within patches, but all seedlings showed poor growth: most taproots were deflected horizontally above the boundary between surface soil and subsoil layers (−15 cm). Abrupt decreases in pH between surface and subsurface horizons (6.9 versus 5.3), together with poor drainage and aeration of the latter soil, were probably responsible for poor root growth. Root growth of greenhouse-grown pine and maple seedlings was similarly restricted in pots packed with topsoil over subsoil material. Our results suggest that many current specifications for the cover of closed landfills will not permit restoration of native woody plant communities because of physical limitations to root growth and infectivity. The structure of the engineered soil must address basic plant growth requirements as well as traditional concerns of drainage and barrier protection.     

Defining Plant Resistance to Phytophthora cinnamomi: A Standardized Approach to Assessment

Phytophthora cinnamomi is a soil‐borne plant pathogen that causes devastating disease in agricultural and natural systems worldwide. While a small number of species survive infection by the pathogen without producing disease symptoms, the nature of resistance, especially under controlled conditions, remains poorly understood. At present, there are no standardized criteria by which resistance or susceptibility to P. cinnamomi can be assessed, and we have used five parameters consisting of plant fresh weight, root growth, lesion length, relative chlorophyll content of leaves and pathogen colonization of roots to analyse responses to the pathogen. The parameters were tested using two plant species, Zea mays and Lupinus angustifolius, through a time course study of the interactions and resistance and susceptibility defined 7 days after inoculation. A scoring system was devised to enable differentiation of these responses. In the resistant interaction with Z. mays, there was no significant difference in fresh weight, root length and relative chlorophyll content in inoculated compared with control plants. Both lesion size and pathogen colonization of root tissues were limited to the site of inoculation. Following inoculation L. angustifolius showed a significant reduction in plant fresh weight and relative leaf chlorophyll content, cessation of root growth and increased lesion lengths and pathogen colonization. We propose that this technique provides a standardized method for plant–P. cinnamomi interactions that could be widely used to differentiate resistant from susceptible species.     

Suppression of Rhizoctonia solani diseases of sugar beet by antagonistic and plant growth-promoting yeasts

AIMS: Isolates of Candida valida, Rhodotorula glutinis and Trichosporon asahii from the rhizosphere of sugar beet in Egypt were examined for their ability to colonize roots, to promote plant growth and to protect sugar beet from Rhizoctonia solani AG-2-2 diseases, under glasshouse conditions. METHODS AND RESULTS: Root colonization abilities of the three yeast species were tested using the root colonization plate assay and the sand-tube method. In the root colonization plate assay, C. valida and T. asahii colonized 95% of roots after 6 days, whilst Rhod. glutinis colonized 90% of roots after 8 days. Root-colonization abilities of the three yeast species tested by the sand-tube method showed that roots and soils attached to roots of sugar beet seedlings were colonized to different degrees. Population densities showed that the three yeast species were found at all depths of the rhizosphere soil adhering to taproots up to 10 cm, but population densities were significantly (P < 0.05) greater in the first 4 cm of the root system compared with other root depths. The three yeast species, applied individually or in combination, significantly (P < 0.05) promoted plant growth and reduced damping off, crown and root rots of sugar beet in glasshouse trials. The combination of the three yeasts (which were not inhibitory to each other) resulted in significantly (P < 0.05) better biocontrol of diseases and plant growth promotion than plants exposed to individual species. CONCLUSIONS: Isolates of C. valida, Rhod. glutinis and T. asahii were capable of colonizing sugar beet roots, promoting growth of sugar beet and protecting the seedlings and mature plants from R. solani diseases. This is the first successful attempt to use yeasts as biocontrol agents against R. solani which causes root diseases. SIGNIFICANCE AND IMPACT OF THE STUDY: Yeasts were shown to provide significant protection to sugar beet roots against R. solani, a serious soil-borne root pathogen. Yeasts also have the potential to be used as biological fertilizers.     

Early growth of Brazilian pine (Araucaria angustifolia[Bertol.] Kuntze) in response to soil compaction and drought

Soil compaction leads to changes in soil physical properties such as density, penetration resistance and porosity, and, by consequence, affects root and plant growth. The initial growth of Brazilian pine is considered as being more affected by soil physical than chemical conditions, and the presence of a well-developed tap root system has been associated with this fact. A greenhouse experiment was conducted in order to evaluate the impact of soil compaction on the growth of Brazilian pine seedlings and on their susceptibility to a simulated drought period. In the first phase of the experiment, the effects of three levels of soil compaction on root morphology and plant growth were examined. Soil cylinders were artificially compacted in PVC tubes. Pre-germinated seeds were planted, and 147 days later 10 plants from each treatment were harvested for analysis. Higher values of soil density were associated with a shorter and thicker tap root. Growth of lateral roots and shoots remained unaffected at this stage. In the second phase, half of the plants (12) in each compaction treatment were drought-stressed by withholding water for a period of 77 days. Increased soil compaction again resulted in reduced length and increased diameter of the main tap root. This time, the effects were also extended to the lateral roots. Shoot extension growth and overall plant mass, however, increased with soil compaction. This greater mass accumulation in plants growing under increased soil compaction may be attributed to a more intimate contact between roots and soil particles. Drought stress reduced both root and shoot growth, but root mass was more negatively affected by drought stress in plants growing under high levels of soil compaction. Future investigations on the effects of soil compaction on the initial growth of Brazilian pine should include a wider range of compaction levels to better establish the relationship between soil physical parameters and plant growth.     

Mechanical resistance by an ectorganic soil layer on root development of seedling Pinus sylvestris

We investigated early root development of Pinus sylvestris seedlings in relation to bulk density and natural particle layering in an ectorganic soil layer from a bracken (Pteridium aquilinum) stand. Responses in root development to two levels of bulk density (0.07 and 0.15 g/cm³) in mixed bracken substrate were compared with effects in peat of similar bulk densities, and in sand of three different bulk densities (0.37, 0.52, and 0.67 g/cm³). The effect on root growth of the natural horizontal layering of the organic particles was examined by comparing intact with mixed ectorganic bracken soil profiles of similar bulk densities (resp. 0.09 and 0.07 g/cm³).Root length growth was significantly reduced in the organic and sandy substrates of high bulk density. Root diameter was not affected by bulk density in the organic substrate, but increased with higher bulk density in sand. Preservation of horizontal layering in the intact ectorganic profile significantly reduced root length compared with mixed substrate of similar bulk density.Roots growing in high bulk density, and intact, organic substrate showed increased twisting, which resulted in a smaller depth reached by the root relative to total root length produced. In sand, root twisting did not change with increased bulk density. It is suggested that roots growing through organic substrate follow a path of least resistance. This implies that organic particle size and orientation are more important in determining root development than bulk density.This study points out that the natural layering of organic particles presents another constraint on the establishment of plant species in sites with a well-developed ectorganic soil layer. Disturbance of this layer may therefore enhance establishment of seedlings by reducing the mechanical resistance of the ectorganic soil profile to developing seedling roots.     

Methodological approaches for studying apoplastic redox activity: 2. Regulation of peroxidase activity

This work is devoted to the study of mechanisms of substrate regulation of extracellular peroxidase (ECPOX) activity at a distant stress (wounding) signal transmission from aboveground organ (leaf) of wheat (Triticum aestivum L., cv. Kazanskaya Yubileinaya) seedlings to the roots. Along with the high dianizidine peroxidase activity, the extracellular solution manifested 3,4-dihydrooxi-L-phenylalanine peroxidase, ascorbate peroxidase, and catalase activities. Dianizidine peroxidases were represented by several isoforms and had broad substrate specificity. It was found that ECPOX was released from the roots into the growing solution and its activity in the solution increased with root growth. Excision of the apical leaf parts in seedlings induced a sharp activation of root ECPOX in the growing solution. The interaction between ECPOX substrates at oxidation in two- and three-component systems is demonstrated. The role of ECPOX in the control of ROS balance in the plant cell apoplast might be determined by competitive and complementary interactions between different peroxidase substrates. Such substrate-substrate regulation of peroxidase activities may be important for stress-induced oxidative burst in plant cells.     

Growth and morphology of eucalypt seedling-roots, in relation to soil strength arising from compaction

Information on the response of root growth and morphology to soil strength is useful for testing suitability of existing and new tillage methods and/or for selecting plants suitable for a specific site with or without tillage. Although there is extensive published information on the root growth-soil strength relationships for annual agricultural plants, such information is scarce for woody, perennial tree species. The purpose of this study is to examine growth and morphology of the root systems of 17-day-old eucalypt seedlings with respect to variation in soil strength. Soil strength in this study was varied by compaction of a well-aggregated clay soil to bulk densities of 0.7–1.0 Mg m^-3 whilst maintaining adequate water availability and aeration for plant growth. Lengths and tip-diameters of primary and lateral roots were measured on the excavated root systems of seedlings.With increase in bulk density and also soil strength (expressed as penetrometer resistance), total length of primary and lateral roots decreased. There were 71 and 31% reduction in the lengths of primary and lateral roots respectively with an increase in penetrometer resistance from 0.4 to 4.2 MPa. This indicated primary roots to be more sensitive to high soil strength than the lateral roots. Average length of lateral roots and diameters of both primary and lateral root tips increased with an increase in soil strength as well. There was greater abundance of lateral roots (no. of lateral roots per unit length of primary root) and root hairs with increased soil strength. The observed root behaviour to variable soil strength is discussed in the context of compensatory growth of roots and overall growth of plants.     

Root System Development of Oak Seedlings Analysed using an Architectural Model. Effects of Competition with Grass

A dynamic 3D model of root system development was adapted to young sessile oak seedlings, in order to evaluate the effects of grass competition on seedling root system development. The model is based on a root typology and the implementation of a series of developmental processes (axial and radial growth, branching, reiteration, decay and abscission). Parameters describing the different processes are estimated for each root type. Young oak seedlings were grown for 4 years in bare soil or with grass competition and were periodically excavated for root system observation and measurements (topology of the root system, length and diameter of all roots with a diameter greater than 0.3 mm). In the fourth year, 40 cm×20 cm×20 cm soil monoliths were excavated for fine root measurement (root density and root length). Root spatial development was analysed on a sub-sample of roots selected on four seedlings. The model was a guideline that provided a complete and consistent set of parameters to represent root system development. It gave a comprehensive view of the root systems and made it possible to quantify the effects of competition on the different root growth processes. The same root typology was used to describe the seedlings in bare soil and in grass. Five root types were defined, from large tap roots to fine roots. Root system size was considerably reduced by grass competition. Branching density was not affected but the branch roots were always smaller for the seedlings grown in competition. Reiteration capacity was also reduced by competition. Cross sectional areas before and after branching were linearly related with a scaling coefficient close to 1, as predicted by the pipe model theory. This relationship was not affected by grass competition.     

Plant Growth Substances Produced by Micro-organisms of Soil and Rhizosphere

S ummary : Micro-organisms isolated from rhizospheres and rhizoplanes of wheat plants, and from root-free soil, produced growth regulating substances with the properties of gibberellins and indolyl-3-acetic acid (IAA). Substances inhibiting extensions of pea plant internodes and lettuce hypocotyls were also produced, especially by bacteria from the root region of seedlings 6 days old. Bacteria producing growth promoting substances were most abundant on roots of older plants. Seedlings grown aseptically with added gibberellic acid (GA₃) and IAA, or grown with a soil inoculum, developed similarly and differed in their morphology from those grown aseptically without additives.     

The influence of soil drought and partial waterlogging on water relations of Gmelina arborea seedlings

Summary Stomatal conductance of unstrossed, soil drought, and previously drought (predrought) Gmelina arborea seedlings increased in the morning and decreased before or immediately after midday. In the unstressed and predrought seedlings, leaf water potential decreased with increases in transpiration. In soil drought seedlings, there was some evidence of decreased hydraulic conductivity from soil to the plant, as indicated by the shape in the slope of the water potential/transpiration relationship. Root growth of drought plants was greater than in their unstressed counterparts at the lowest soil segment of a pot. The partial recovery of predrought seedlings was attributed to this subtantial root growth in the lowest soil segment.In the second experiment, Gmelina arborea seedlings were partially waterlogged, by flooding the polyethylene bag to half its length, for a period of 23 days. Waterlogging induced stomatal closure and reduction in leaf water potential but there was some evidence of tolerance to waterlogging towards the end of treatment. Root growth, shoot and root dry weights were slightly reduced below those of controls. After 9 days of waterlogging, adventitious roots began to form which correlated with depletion of soluble sugars in the shoot but with an increase in the roots.It is suggested that the tolerance of Gmelina plants to either soil drought or waterlogging may partly be due to partitioning of the soluble sugars from shoot to roots for production of roots and formation of adventitious roots respectively which are likely to enhance the flow of water from the soils to the plant. Therefore the plant response is very similar under conditions of increased deficits and surplus of soil water.     

Biomechanical Response of the Root System in Tomato Seedlings under Wind Disturbance

Wind disturbance as a green method can effectively prevent the overgrowth of tomato seedlings, and its mechanism may be related to root system mechanics. This study characterized the biophysical mechanical properties of taproot and lateral roots of tomato seedlings at five seedling ages and seedling substrates with three different moisture content. The corresponding root system-substrate finite element (FE) model was then developed and validated. The study showed that seedling age significantly affected the biomechanical properties of the taproot and lateral roots of the seedlings and that moisture content significantly affected the biomechanical properties of the seedling substrate (p < 0.05). The established FE model was sensitive to wind speed, substrate moisture content, strong seedling index, and seedling age and was robust. The multiple linear regression equations obtained could predict the maximum stress and strain of the root system of tomato seedlings in the wind field. The strong seedling index had the greatest impact on the biomechanical response of the seedling root system during wind disturbance, followed by wind speed. In contrast, seedling age had no significant effect on the biomechanical response of the root system during wind disturbance. In the simulation, no mechanical damage was observed on the tissue of the seedling root system, but there were some strain behaviors. Based on the plant stress resistance, wind disturbance may affect the growth and development of the root system in the later growth stage. In this study, finite element and statistical analysis methods were combined to provide an effective approach for in-depth analysis of the biomechanical mechanisms of wind disturbances that inhibit tomato seedlings’ growth from the root system’s perspective.     

Plant exudates promote PCB degradation by a rhodococcal rhizobacteria

Rhodococcus erythropolis U23A is a polychlorinated biphenyl (PCB)-degrading bacterium isolated from the rhizosphere of plants grown on a PCB-contaminated soil. Strain U23A bphA exhibited 99% identity with bphA1 of Rhodococcus globerulus P6. We grew Arabidopsis thaliana in a hydroponic axenic system, collected, and concentrated the plant secondary metabolite-containing root exudates. Strain U23A exhibited a chemotactic response toward these root exudates. In a root colonizing assay, the number of cells of strain U23A associated to the plant roots (5.7?×?10? CFU g?1) was greater than the number remaining in the surrounding sand (4.5?×?10? CFU g?1). Furthermore, the exudates could support the growth of strain U23A. In a resting cell suspension assay, cells grown in a minimal medium containing Arabidopsis root exudates as sole growth substrate were able to metabolize 2,3,4'- and 2,3',4-trichlorobiphenyl. However, no significant degradation of any of congeners was observed for control cells grown on Luria-Bertani medium. Although strain U23A was unable to grow on any of the flavonoids identified in root exudates, biphenyl-induced cells metabolized flavanone, one of the major root exudate components. In addition, when used as co-substrate with sodium acetate, flavanone was as efficient as biphenyl to induce the biphenyl catabolic pathway of strain U23A. Together, these data provide supporting evidence that some rhodococci can live in soil in close association with plant roots and that root exudates can support their growth and trigger their PCB-degrading ability. This suggests that, like the flagellated Gram-negative bacteria, non-flagellated rhodococci may also play a key role in the degradation of persistent pollutants.     

Response of spring rape (Brassica napus var. oleifera L.) to inoculation with plant growth promoting rhizobacteria containing 1-aminocyclopropane-1-carboxylate deaminase depends on nutrient status of the plant

Responses of rape (Brassica napus var. oleifera L.) to inoculation with plant growth promoting rhizobacteria, Pseudomonas putida Am2, Pseudomonas putida Bm3, Alcaligenes xylosoxidans Cm4, and Pseudomonas sp. Dp2, containing 1-aminocyclopropane-l-carboxylate (ACC) deaminase were studied using growth pouch and soil cultures. In growth pouch culture, the bacteria significantly increased root elongation of phosphorus-sufficient seedlings, whereas root elongation of phosphorus-deficient seedlings was not affected or was even inhibited by the bacteria. Bacterial stimulation of root elongation of phosphorus-sufficient seedlings was eliminated in the presence of a high ammonia concentration (1 mM) in the nutrient solution. Bacterial effects on root elongation of potassium-deficient and potassium-sufficient seedlings were similar. The bacteria also decreased inorganic phosphate content in shoots of potassium- and phosphorus-sufficient seedlings, reduced ethylene production by phosphorus-sufficient seedlings, and inhibited development of root hairs. The effects of treatment with Ag+, a chemical inhibitor of plant ethylene production, on root elongation, ethylene evolution, and root hair formation were similar to bacterial treatments. The number of bacteria on the roots of phosphorus-deficient seedlings was not limited by phosphorus deficiency. In pot experiments with soil culture, inoculation of seeds with bacteria and treatment with aminoethoxyvinylglycine, an inhibitor of ethylene biosynthesis in plants, increased root and (or) shoot biomass of rape plants. Stimulation of plant growth caused by the bacteria was often associated with a decrease in the content of nutrients, such as P, K, S, Mo, and Ba, in shoots, depending on the strain used. The results obtained show that the growth-promoting effects of ACC-utilizing rhizobacteria depend significantly on the nutrient status of the plant.     

Root-Associated Bacteria Contribute to Mineral Weathering and to Mineral Nutrition in Trees: a Budgeting Analysis

The principal nutrient source for forest trees derives from the weathering of soil minerals which results from water circulation and from plant and microbial activity. The main objectives of this work were to quantify the respective effects of plant- and root-associated bacteria on mineral weathering and their consequences on tree seedling growth and nutrition. That is why we carried out two column experiments with a quartz-biotite substrate. The columns were planted with or without pine seedlings and inoculated or not with three ectomycorrhizosphere bacterial strains to quantify biotite weathering and pine growth and to determine how bacteria improve pine growth. We showed that the pine roots significantly increased biotite weathering by a factor of 1.3 for magnesium and 1.7 for potassium. We also demonstrated that the inoculation of Burkholderia glathei PML1(12) significantly increased biotite weathering by a factor of 1.4 for magnesium and 1.5 for potassium in comparison with the pine alone. In addition, we observed a significant positive effect of B. glathei PMB1(7) and PML1(12) on pine growth and on root morphology (number of lateral roots and root hairs). We demonstrated that PML1(12) improved pine growth when the seedlings were supplied with a nutrient solution which did not contain the nutrients present in the biotite. No improvement of pine growth was observed when the seedlings were supplied with all the nutrients necessary for pine growth. We therefore propose that the growth-promoting effect of B. glathei PML1(12) mainly resulted from the improved plant nutrition via increased mineral weathering.     

Root-associated bacteria contribute to mineral weathering and to mineral nutrition in trees: a budgeting analysis

The principal nutrient source for forest trees derives from the weathering of soil minerals which results from water circulation and from plant and microbial activity. The main objectives of this work were to quantify the respective effects of plant- and root-associated bacteria on mineral weathering and their consequences on tree seedling growth and nutrition. That is why we carried out two column experiments with a quartz-biotite substrate. The columns were planted with or without pine seedlings and inoculated or not with three ectomycorrhizosphere bacterial strains to quantify biotite weathering and pine growth and to determine how bacteria improve pine growth. We showed that the pine roots significantly increased biotite weathering by a factor of 1.3 for magnesium and 1.7 for potassium. We also demonstrated that the inoculation of Burkholderia glathei PML1(12) significantly increased biotite weathering by a factor of 1.4 for magnesium and 1.5 for potassium in comparison with the pine alone. In addition, we observed a significant positive effect of B. glathei PMB1(7) and PML1(12) on pine growth and on root morphology (number of lateral roots and root hairs). We demonstrated that PML1(12) improved pine growth when the seedlings were supplied with a nutrient solution which did not contain the nutrients present in the biotite. No improvement of pine growth was observed when the seedlings were supplied with all the nutrients necessary for pine growth. We therefore propose that the growth-promoting effect of B. glathei PML1(12) mainly resulted from the improved plant nutrition via increased mineral weathering.     

Growth dynamics of mechanically impeded lupin roots: does altered morphology induce hypoxia?

BACKGROUND AND AIMS: Root axes elongate slowly and swell radially under mechanical impedance. However, temporal and spatial changes to impeded root apices have only been described qualitatively. This paper aims (a) to quantify morphological changes to root apices and (b) assess whether these changes pre-dispose young root tissues to hypoxia. METHODS: Lupin (Lupinus angustifolius) seedlings were grown into coarse sand that was pressurized through a diaphragm to generate mechanical impedance on growing root axes. In situ observations yielded growth rates and root response to hypoxia. Roots were then removed to assess morphology, cell lengths and local growth velocities. Oxygen uptake into excised segments was measured. KEY RESULTS: An applied pressure of 15 kPa slowed root extension by 75% after 10-20 h while the same axes thickened by about 50%. The most terminal 2-3 mm of axes did not respond morphologically to impedance, in spite of the slower flux of cells out of this region. The basal boundary of root extension encroached to within 4 mm of the apex (cf. 10 mm in unimpeded roots), while radial swelling extended 10 mm behind the apex in impeded roots. Oxygen demand by segments of these short, thick, impeded roots was significantly different from segments of unimpeded roots when the zones of elongation in each treatment were compared. Specifically, impeded roots consumed O2 faster and O2 consumption was more likely to be O2-limited over a substantial proportion of the elongation zone, making these roots more susceptible to O2 deficit. Impeded roots used more O2 per unit growth (measured as either unit of elongation or unit of volumetric expansion) than unimpeded roots. Extension of impeded roots in situ was O2-limited at sub-atmospheric O2 levels (21% O2), while unimpeded roots were only limited below 11% O2. CONCLUSIONS: The shift in the zone of extension towards the apex in impeded roots coincided with greater vulnerability to hypoxia even after soil was removed. Roots still encased in impeded soil are likely to suffer from marked O2 deficits.     

Soil Anti-Scouribility Enhanced by Plant Roots

The magnitude of soil anti-scouribility depends on the physical condition of the soil. Plant roots can greatly enhance soil stability and anti-erodibility. A scouring experiment of undisturbed soil was conducted to investigate the effects of roots on soil anti-scouribility and its distribution in the soil profile. At the end of each erosion test, plant roots were collected from soil samples and root surface area was calculated by means of a computer image analysis system (CIAS). Root surface area density (RSAD), the surface area of the roots per unit of soil volume, was related to soil anti-scouribility. More than 83% of root surface area was concentrated in the 0 - 30 cm soil layer. Soil anti-scouribility increased with an increase in RSAD and the value of intensified soil anti-scouribility (△AS) can be expressed by exponential equations, depending on the plant species. These equations were △AS = 9.578 6 RSAD0.8321 (R2 = 0.951) for afforested Pinus tabulaeformis Cart., △AS = 7.808 7 RSAD0.7894 (R2 = 0.974) for afforested Robinia pseudoacacia L., and △AS = 9.256 6 RSAD0.8707 (R2 = 0.899) for Bothriochloa ischemum L.     

Root Regeneration and Plant Water Status of Pinus radiata D. Don Seedlings Transplanted to Different Soil Temperatures

The regeneration and configuration of new roots on transplanted8 month old Pinus radiata D. Don seedlings were measured inrelation to soil temperature and time after transplanting. Theeffects of root wrenching and nutrition on root regenerationwere compared. Low soil temperature adversely affected the initiationand elongation of new roots. The branching and morphology ofthe new roots were also influenced by soil temperature. Theprocesses of initiation and elongation are greatly retardedbelow a critical root temperature between 11 and 14 ?C. Wrenchingand fertilization stimulated new root growth at low temperatures.There is a close interrelationship between the constituent partsof the network which forms the new root system. The first-orderlaterals on transplants are critical as a basic framework forextension and production of second-order laterals which formthe major part of the regenerating root system. Needle water potential was closely related to the new root growth.The water stress experienced by the transplants for severalweeks after planting was due primarily to the suppressive effectof low soil temperature on root regeneration. Wrenched plantswere less water-stressed than unwrenched plants. Field measurements confirmed the finding from controlled experimentsthat, in southern Australia, the low soil temperature in theplanting season imposes a major restriction on early root regenerationwhich in turn inflicts water stress in transplants. This needsto be considered during the planning of planting and fertilization.     

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.