Cm Of Root Research Articles

Citrus rootstocks vs. nematodes: A battle for resistance against Tylenchulus semipenetrans

Tylenchulus semipenetrans, the causal agent of the ‘citrus slow decline’, is considered to be the most devastating nematode pest of citrus, causing considerable crop losses throughout the world. The management of this citrus nematode represents a significant challenge due to its wide host range and its adaptability to various environmental conditions. Nematicides application is the most prevalent control approach, however, their use often is associated with increased production costs and major risks to consumer health and the environment. The objective of this study is to contribute to the development of an alternative and effective method to control this plant-parasite. It intends to evaluate the resistance of the seven commonly used citrus rootstocks in Morocco based on their agronomic, physiological, and biochemical behavior in response to inoculation with T. semipenetrans. Results indicated that T. semipenetrans exhibited a strong reproductive capacity on Sour Orange, Australian sour orange, and volkameriana, showing higher root and soil infestation rates, with respectively 466.3, 250.5, and 273.3 females/10 g of root, and 1062.2, 1030.7, and 1053.2 females per 100 g of soil, compared to the other rootstocks varieties. These rootstocks allowed highest T. semipenetrans reproduction rates in both rhizospheric soil (> 3) and root (> 0.68), they also showed the highest infestation rate with more than 3 nematodes per cm of root. Moreover, these rootstocks showed significant biochemical alterations including a sharp increase in the activity of antioxidant enzymes, catalase (CAT), peroxidase (PO), polyphenol-oxidase (PPO), and in proline and malondialdehyde (MDA) accumulation compared to the control and other tested rootstocks. In contrast, T. semipenetrans reproduced poorly (with less than 135 females/10 g of root), less infested the roots (less than 0.33 females/cm of root), and induced slight biochemical modifications in Carrizo citrange. Thus, Carrizo citrange endowed with high resistance to T. semipenetrans could serve as a suitable rootstock to control this citrus devastating nematode.

EFFECTS OF LEAD AND CADMIUM ON SHOOT LENGTH AND ROOT LENGTH OF CASCABELA THEVETIA (L.) LIPPOLD

Agricultural sustainability and civilization of society enormously depend on land. However, contamination of soil due to heavy metals is a major threat to mankind. Cadmium is one of the most toxic metals in the environment and has noxious effects on vegetative growth. Lead is the second ranked of most widely distributed hazardous substances in the environment and can cause morphological, physiological, and biological dysfunctions in plants. This study investigates the effect of various concentrations of lead and cadmium, i.e. 400mg, 800mg, 1200mg, 1600mg Pb kg-1 and 5mg, 10mg, 15mg, 20mg Cd kg-1, on root length and shoot length of Cascabela thevetia (L.) Lippold. The results states lead treatments have decreased the length of shoot by 19 cm and 11.99 cm of root. Cadmium treatments to the plants have decreased by 20 cm and 11 cm of shoot and root respectively. Higher concentrations of heavy metals in the soil can significantly reduce root lengths and shoot lengths.

Nondestructive Evaluation of Cotton Genotypes for Resistance to Reniform Nematode

Identification of new sources of reniform nematode (Rotylenchulus reniformis Linford and Oliveira) resistance in cotton is critical to expanding host plant resistance to manage this important pathogen. Phenotyping plants in early breeding generations without destructive sampling would be useful for introgression of nematode resistance from exotic germplasm resources; therefore, a rapid, nondestructive method was developed to assess host plant resistance to the reniform nematode based on the number of females infecting the roots. In one set of experiments, the root system was cut off at 0, 1, 2.5, or 5 cm below the soil line and used to assess the number of females infecting this portion of the root system. Resistance could be accurately determined while leaving up to 5 cm of roots with the shoot. In a second set of experiments, the rate of plant recovery and reproductive development was evaluated using a combination of root retention (0, 1, 2.5 cm, or all root) and shoot retention (leaves at top two nodes, leaves at bottom two nodes, no leaves, all leaves) treatments. Plants more rapidly recovered using a treatment combination in which the top leaves and 2.5 cm roots were kept. This combination performed similarly to plants with neither shoots nor roots modified and was harvested 20 days sooner compared to some other treatment combinations.

Clonal propagation of Gmelina arborea Roxb grown in southeastern Mexico

Objective: To evaluate the clonal behavior of Gmelina arborea Roxb from mother plant cuttings in southeastern Mexico. Design/Methodology/Approach: A completely random design of three treatments with four repetitions was established; the treatments were concentrations of IBA (4000, 6000 and 3000 ppm). The following were evaluated: percentage of rooting, days until root formation, type of cutting, number of roots, length of roots and absorbent roots. Analysis of variance and Tukey’s test were performed with Statistix 9.0. Results: In the analysis of variance (ANOVA), significant factors were observed (callus and number of roots), and also, a fluctuation was observed in the percentage of rooting from 84% to 92%. Regarding the number of roots and cm of roots, there were significant differences in the presence of number of roots of the 6000 ppm treatment. Conclusions: The concentration of IBA in the rooting generates good results in different concentrations, depending on the cutting implemented, the factors callus and number of roots; significant records were obtained between treatments, favoring the acceleration of presence of roots.

Growth and establishment of container-grown London planetrees in response to mulch, root-ball treatment and fertilization

We conducted two experiments to evaluate the impact of cultural treatments on growth and establishment of container-grown London planetrees (Platanus × acerifolia’ Bloodgood’). In both experiments, 48 trees grown in 100 l (#25) black plastic containers were assigned at random to one of three root-ball treatments prior to planting; no treatment (Control), outer 3 cm of roots removed around entire root-ball (Shave), or outer circling roots disentangled from the root-ball (Tease). In Experiment 1, half of the trees were fertilized with 400 g of controlled release fertilizer (15-9-12; N-P2O5-K2O) at planting and the remainder of the trees were not fertilized. In Experiment 2, half of the trees were mulched with an 8 cm deep × 2 m diameter ring of coarse ground pine bark at planting and the remainder of the trees were not mulched. In Experiment 1, fertilization at planting increased SPAD chlorophyll content on two of four measurement dates but did not affect cumulative height or caliper growth after two years. After two growing seasons, root-ball treatments (shaving or teasing) increased root growth outside the original root-ball compared to control trees. Both root-ball treatments also reduced circling roots. In Experiment 2, mulching at planting increased soil moisture and cumulative tree height and diameter growth. Shaving increased new root growth and both root-ball treatments improved root architecture and reduced circling roots. Overall, the study demonstrates that root-ball manipulations can stimulate new root growth and reduce circling roots. Mulch is a valuable aid to conserve soil moisture and increase tree growth. Fertilization at planting provided little benefit in this experiment, which may have been related to a high level of soil fertility at the site or nutrient loading of the trees from nursery culture prior to transplanting.

First Report of Verticillium dahliae Causing Dieback of Highbush Blueberry (Vaccinium corymbosum) in Oregon and Washington

In the spring of 2011, 2013, 2015, and 2016, cultivar Aurora, Blue Ribbon, Bluetta, and Cargo blueberry plants (Vaccinium corymbosum L.) from Oregon and Washington were sent to the Oregon State University Plant Clinic for diagnosis. Symptoms included foliar chlorosis, reddening, blackening of either part or the entire stem followed by stem dieback, vascular discoloration in the lower stems, and occasional wilt. The diseased plants were scattered in the fields, with incidence ranging from less than 5 to 40% of acreage. Isolations were made from surface-disinfected debarked stems and crown tissues to water agar and potato dextrose agar. Verticillium dahliae Kleb. was recovered from stems and crowns of all four cultivars, from which single-spore isolates were derived. Morphology of the conidiophores, conidia, and microsclerotia fit the description for V. dahliae (Klebahn 1913). Two of the isolates (OSU herbarium #OSC# 156080 and #OSC# 156081) were first identified by multiplex PCR assay (Inderbitzin et al. 2013) and subsequently sequenced. Partial DNA sequencing of the internal transcribed spacer (ITS) regions and TEF-1 alpha from both isolates were deposited in GenBank as accession numbers KY039311 and KY039312 (ITS) and KY039313 and KY039314 (TEF-1), and they showed 100% identity to the epitype strain of V. dahliae (ITS accession no. HQ206718, 461/461 and 466/466 bp for the two isolates, respectively; TEF-1 alpha accession no. HQ414624, 579/579 bp for both). Pathogenicity tests were performed by wounding and inoculating three lower stems each of three 2-year-old Bluetta blueberry plants with agar plugs of a 7-day-old single-spore culture of V. dahliae (#OSC# 156080). Three stems of another plant were wounded and inoculated with uncolonized agar plugs. Three months after inoculation, the stems had turned black up to 30 cm above inoculation sites, and the vascular tissue was discolored. Microscopic examination revealed masses of embedded V. dahliae microsclerotia along entire stems. Control plants remained symptomless, and V. dahliae was not recovered. Using the same two V. dahliae isolates as above, 15 cm tall blueberry plants of cultivars Bluetta, Earliblue, Draper, Polaris, and Rubel were inoculated by removing the bottom 1 cm of roots and dipping them in a conidial suspension (5 × 10⁶ conidia/ml) for 10 min, followed by planting in the greenhouse at 27°C. After 10 weeks, all inoculated cultivars showed defoliation, stunting, leaf reddening, vascular discoloration, and blackened stems. V. dahliae morphologically similar to our isolates was consistently recovered from stems of all inoculated cultivars, at least 2.5 cm above the soil line. Control plants remained healthy and did not yield V. dahliae. Verticillium-infected field-grown blueberries were largely from sites previously planted with peppermint or potato, crops susceptible to V. dahliae. Verticillium wilt has previously been reported on lowbush blueberry (V. angustifolium) in the United States and Canada (Brisson et al. 1976), on Vaccinium myrtillus (listed on PlantWise database), and on blueberry (Vaccinium sp.) in Chile (Cisternas and France 2009). None of these reports were confirmed by DNA evidence. To our knowledge, this is the first report of V. dahliae causing disease of highbush blueberry in the United States. Our results strongly suggest that fields with a history of V. dahliae infestation should not be planted with blueberry.

Colonización y estructura de la comunidad de hongos micorrízicos arbusculares en Alsophila firma (Cyatheaceae) en bosque mesófilo de montaña en Veracruz, México

Alsophila firma is a tree fern that is distributed mainly in tropical montane cloud forest (TMCF) and is considered as a threatened species. Arbuscular mycorrrhizal fungi (AMF) have been proposed as an alternative in rescue programs of endangered species. However, our knowledge about diversity of AMF and mycorrhizal status of the species of TMCF is limited. In Mexico TMCF shows different degrees of conservation because of fragmentation and land use change. In this study, we evaluated the level of colonization, richness and abundances of spores of AMF in three fragments with different conservation status: conserved (100 years), secondary vegetation (17 years) and disturbed. For this, soil samples and roots were collected from five individuals of A. firma per site, with at least 100 m away from each other; a total of 100 cm of roots were analysed per site. Root samples showed AMF and occasionally dark septate fungi (DSF) colonizations. For the overall study, 19 species of AMF were recorded: Gigaspora (7), Acaulospora (4), Glomus (4), Funneliformis (2), Sclerocystis (1) and Scutellospora (1). The dominant species in the three sites were Funneliformis geosporum and Acaulospora scrobiculata. The highest diversity (H') and evenness (J') (p < 0.05) were found in the conserved site (H' = 1.7, J' = 0.66), when compared to the secondary vegetation (H' = 1.5, J' = 0.61), and the disturbed site (H' = 0.74, J' = 0.41). Statistical analysis showed that the AMF degree of colonization was significantly higher (p < 0.05) in the conserved site; although, the disturbed site showed low richness and abundances of AMF, the degree of root colonization did not differ statistically (p < 0.05) with the secondary vegetation site. Chao2 (Richness estimation model) showed that the number of analysed samples were sufficient to represent the structure of the AME communities with values > 90%. The present study confirmed that A. firma is a mycorrhizal species that exhibits high levels of colonization even in disturbed sites. We suggest that F. geosporum and A. scrobiculata may have the potential to inoculate the gametophyte and young sporophyte of A. firma, to support restoration programs, because of their abundances and high tolerance to disturbed sites.

Micropropagation and assessment of genetic fidelity of Dendrocalamus strictus (Roxb.) nees using RAPD and ISSR markers.

Dendrocalamus strictus popularly known as ‘Male bamboo’ is a multipurpose bamboo which is extensively utilized in pharmaceutical, paper, agricultural and other industrial implements. In this study, in vitro regeneration of D. strictus through nodal culture has been attempted. Murashige and Skoog’s medium supplemented with 4 mg/l BAP was found to be most effective in shoot regeneration with 3.68 ± 0.37 shoots per explant. The effect of Kn was found to be moderate. These hormones also had considerable effect on the shoot length. The highest shoot length after 6 weeks (3.11 ± 0.41 cm) was noted with 5 mg/l BAP followed by 3.07 ± 0.28 cm with 5 mg/l Kn, while decrease in the shoot length was noted with other treatments. The effect of IBA and NAA individually or in combination at different concentrations on rooting was evaluated. The highest number of root (1.36 ± 0.04) was regenerated on full-strength MS medium supplemented with 3 mg/l NAA, while maximum length of 1.64 ± 0.03 cm of roots was recorded with combination of 1 mg/l IBA and 3 mg/l NAA. Tissue-cultured plants thus obtained were successfully transferred to the soil. The clonal fidelity among the in vitro-regenerated plantlets was assessed by RAPD and ISSR markers. The ten RAPD decamers produced 58 amplicons, while nine ISSR primers generated a total of 66 bands. All the bands generated were monomorphic. These results confirmed the clonal fidelity of the tissue culture-raised D. strictus plantlets and corroborated the fact that nodal culture is perhaps the safest mode for multiplication of true to type plants.

Belowground Biomass ofPhragmites australisin Coastal Marshes

The distribution of below ground biomass within monotypic stands of invasive Phragmites australis (Common Reed) was documented from a series of oligo-, meso-, and polyhaline coastal marshes in New Hampshire. Soil profiles were described, and live biomass was documented growing to a maximum depth of 95 cm for roots and 85 cm for rhizomes. Our data show that invasive P. australis utilizes a greater depth range than native graminoids (90% within the top 70 cm and top 20 cm, respectively). We corroborate prior anecdotal observations and provide further evidence illustrating the potential for this invasive plant to access resources (i.e., water and nutrients) at depths greater than the native species with which it competes.

Resprouting of Echinacea angustifolia Augments Sustainability of Wild Medicinal Plant Populations

Resprouting ofEchinacea angustifoliaAugments Sustainability of Wild Medicinal Plant Populations. Overharvest of wild Echinacea species root has been a significant concern to the herbal industry. Harvesters of wild Echinacea angustifolia showed us that even after harvesting the top 15 to 20 cm of root, some plants resprout. We marked locations of harvested plants at sites in Kansas and Montana and reexamined them two years later to see if they resprouted from remaining root reserves. Approximately 50% of the roots resprouted at both Kansas and Montana sampling sites, despite droughty weather conditions in Montana. The length of root harvested significantly affected the ability of the plant to resprout. Those plants that were more shallowly harvested and had less root length removed were more likely to resprout. These data indicate that echinacea stands can recover over time from intensive harvest if periods of nonharvest occur. Our echinacea harvest study emphasizes that the entire biology of medicinal plants must be considered when evaluating their conservation status.

Elongation rates and endogenous indoleacetic acid levels in roots of pea mutants differing in internode length

Growth of the primary root of 12 genotypes of peas (Pisum sativum) differing in their stem height was recorded for 14 days. The growth rate of roots of wild‐type tall, gibberellin (GA)‐deficient le dwarf or slender mutants (with la crys) was similar (3 cm day−1); that of severely GA‐deficient nana (na‐1) plants was 50% of wild‐type but elongation ceased after 8 days; moderately severe dwarf GA‐deficient lines ls‐1 and lh‐1 had a 15% reduction in elongation rate but displayed no time‐dependent slowing of the growth rate and brassinosteroid‐insensitive and ‐deficient dwarfs lka and lkb showed slightly decreased root elongation. GA (levels reported in Yaxley et al. 2001) is not substantially limiting to root growth until it is severely deficient. The terminal 3 cm of roots of tall plants contained about 25 or 35 ng g−1 fresh weight indole‐3‐acetic acid (IAA), depending on the genetic background, and le‐1 dwarfs were similar. Nana (na‐1) had less than 50% the level of IAA of tall, all the moderately severe dwarfs had reductions of about 30% and the slender plants had about 40% more IAA than the corresponding wild‐type. With the exception of slender plants, IAA level in the root tips correlated with root elongation. Root growth seems to be promoted by IAA within the range of the internal concentrations detected. Nana plants had a reduced amount of IAA and a lower root‐growth rate. Whereas external application of IAA always inhibits root growth, even at very low concentrations, root growth is not similarly inhibited by internal IAA as slender plants had the highest IAA level and growth rate similar to wild‐type, regardless of the shoot GA content.

Phosphorus Influx and Root‐Shoot Relations as Indicators of Phosphorus Efficiency of Different Crops

The large variation in phosphorus acquisition efficiency of different crops provides opportunities for screening crop species that perform well on low phosphorus (P) soil. To explain the differences in P efficiency of winter maize (Zea mays L.), wheat (Triticum aestivum L.), and chickpea (Cicer arietinum L.), a green house pot experiment was conducted by using P‐deficient Typic ustochrept loamy sand soil (0.5 M NaHCO3‐extractable P 4.9 mg kg−1, pH 7.5, and organic carbon 2.7 g kg−1) treated with 0, 30, and 60 mg P kg−1 soil. Under P deficiency conditions, winter maize produced 76% of its maximum shoot dry weight (SDW) with 0.2% P in shoot, whereas chickpea and wheat produced about 30% of their maximum SDW with more than 0.25% P in shoot. Root length (RL) of winter maize, wheat, and chickpea were 83, 48, and 19% of their maximum RL, respectively. Considering relative shoot yield as a measure of efficiency, winter maize was more P efficient than wheat and chickpea. Winter maize had lower RL/SDW ratio than that of wheat, but it was more P efficient because it could maintain 2.2 times higher P influx even under P deficiency conditions. In addition, winter maize had low internal P requirement and 3.3 times higher shoot demand (i.e., higher amount of shoot produced per cm of root per second). Even though chickpea had 1.2 times higher P influx than winter maize, it was less P efficient because of few roots (i.e., less RL per unit SDW). Nutrient uptake model (NST 3.0) calculations satisfactorily predicted P influxes by all the three crops under sufficient P supply conditions (CLi 48 µM), and the calculated values of P influx were 81–99% of the measured values. However, in no‐P treatment (CLi 3.9 µM), under prediction of measured P influx indicated the importance of root exudates and/or mycorrhizae that increase P solubility in the rhizosphere. Sensitivity analysis showed that in low P soils, the initial soil solution P concentration (CLi) was the most sensitive factor controlling P influx in all the three crops.

The early nodulin enod40 is induced by, and also promotes arbuscular mycorrhizal root colonization

Expression of Tenod40, a tobacco (Nicotiana tabacum cv. Xanthi nc) homologue of Msenod40, a gene expressed early during alfalfa (Medicago sativa) nodule development, was significantly and consistently increased (about three-fold) in tobacco roots of plants grown in sand and colonized by the arbuscular mycorrhizal (AM) fungus Glomus intraradices, as compared to non-mycorrhizal controls. In alfalfa (Medicago sativa cv. Gilboa), a similar induction of Msenod40 expression was also detected in mycorrhizal vs. non-mycorrhizal roots. In the same experimental system, the application of the cytokinin 6-benzylaminopurine to non-mycorrhizal roots resulted in a similar increase in Msenod40 expression. A significantly higher incidence of AM fungal colonization (vesicles per cm of root) in M. sativa cv. Gilboa roots was observed following application of luteolin-induced Sinorhizobium meliloti exudates as compared to plants amended with non-induced exudates. Moreover, overexpression of the Mtenod40 gene in Medicago truncatula plants under the control of a 35S constitutive promoter resulted in significantly enhanced colonization as compared to non-transgenic plants. The fact that enod40 gene expression was increased both in a legume and in a non-legume during colonization by the AM fungus, and that colonization levels were higher in enod40 -overexpressing transgenic plants strongly suggests that this gene is involved in the establishment of the fungal partner in its host.

Performance of direct-seeded and transplanted guayule

The commercialization of guayule ( Parthenium argentatum, Gray) depends on economical plant production. Direct seeding has been successful and can reduce establishment costs when compared with seedling transplanting. Most economic and yield analyses have been based on transplants. The performance of direct-seeded and transplanted shrubs grown under the same field conditions has never been compared. The objective of this study was to evaluate various production parameters (biomass, rubber and resin content, rubber and resin yield) of five guayule lines established by direct seeding and transplanting. An experiment was initiated on a Delnorte very gravelly loam on 17 and 18 May 1994 at the Texas Agricultural Experiment Station near Fort Stockton, TX. Guayule selections AZ-R2, AZ-R3, 11 605, CAL-6, and UC-101 were direct-seeded and transplanted on raised beds spaced 1 m apart. Six plants from each line/replication were harvested on 6 March 1996 and 11 March 1997. Each plant was divided into branch (clipped 10 cm above the soil surface) and root (10 cm of lower branches plus approximately 15 cm of roots) sections. Guayule branch biomass was greater than root biomass at both harvests. Direct-seeded branch, root and total biomass was significantly lower than transplanted biomass in 1996. After 34 months at the 1997 harvest, no significant differences were observed between direct-seeded and transplanted shrubs. Resin and rubber content (%), and resin and rubber yield (kg/ha) were significantly greater in transplants versus direct-seeded shrubs in 1996, but not in 1997. Thus, direct seeding would be a viable system for commercially establishing guayule stands. Line AZ-R3 exhibited the least potential of the five lines to overcome the initial growth advantage of the transplants. Lines AZ-R2, 11 605, and CAL-6 are recommended for cultivation in west Texas and southern New Mexico.

Interactions among Pythium aphanidermatum, roots, root mucilage, and microbial agents in hydroponic cucumbers

Epidemics and biological control of Pythium root rot were investigated in greenhouse cucumbers grown in small-scale hydroponic systems with troughs and recirculating nutrient solution. The systems were inoculated with Pythium aphanidermatum only or with the pathogen plus an isolate of Pseudomonas fluorescens or Pseudomonas chlororaphis. In systems with P. aphanidermatum only, growth rate of roots, amount of root mucilage in the nutrient solution, and root discoloration (browning) oscillated in patterns that were generally similar and synchronous. These variables abruptly increased for 6-14 days, starting at about 21 days after transplanting, subsequently declined to low levels at 41-56 days, and then increased again starting at 53-61 days. Logarithm to the base 10 of propagule density of P aphanidermatum usually was several orders higher in the mucilage (2.09-4.90/g) than in the nutrient solution (0.25-2.67/mL). The pathogen was also more frequent in roots with associated mucilage (14-60 colonies/60 cm of root) compared to those lacking mucilage (2-26 colonies/60 cm of root). These findings, combined with positive correlation between mucilage and discolored roots, and microscopic observations of Pythium hyphae in mucilage and discolored roots, indicated that the mucilage supported prolific growth of P. aphanidermatum and served as a food base from which the pathogen invaded roots. The bacterial agents each reduced root mucilage until about 68 days after transplanting, and root discoloration chiefly at 47-61 days. Mucilage breakdown by the agents was an apparent mode of biological control of P. aphanidermatum. Pseudomonas fluorescens reduced mucilage and root discoloration more effectively than P. chlororaphis, but did not significantly promote marketable yield (fresh mass basis). Pseudomonas chlororaphis increased cumulative marketable fruit yield at 47-78 days, but not at 78-127 days. Both agents increased numbers of marketable fruit. The agents did not affect incidence of the pathogen in young roots, which are normally infected by zoospores. Densities of incidental microbes in nutrient solution were high after 69-74 days. It is hypothesized that shifts in assimilate partitioning to roots successively affected root growth, mucilage production, saprotrophic growth of P. aphanidermatum, and root invasion by hyphae of the pathogen. The microbial agents appear to have value for mucilage management and root rot control.

Description of the Colonization of a Gnotobiotic Tomato Rhizosphere by Pseudomonas fluorescens Biocontrol Strain WCS365, Using Scanning Electron Microscopy

To study colonization of the tomato root system, we previously have described a gnotobiotic quartz sand system, in which seedlings inoculated with one or two bacterial strains were allowed to grow. Here we present a scanning electron microscope description of the colonization of the tomato root system by Pseudomonas fluorescens biocontrol strain WCS365, with emphasis on spatial-temporal colonization patterns, based on an improved scanning electron microscopy procedure. Upon inoculation of the germinated seed, proliferation on the seed coat was observed for 2 to 3 days. Within 1 to 3 days, micro-colonies developed, mainly at the root base. Most micro-colonies were localized in junctions between epidermal root cells, whereas others were found in indented parts of the epidermal surface. Downward to the root tip, only single bacterial cells were found. Colonization progressed down the root, initially as single cells. A semi-transparent film appeared to enclose the root surface and micro-colonies present on the root. After 7 days, micro-colonies had developed at positions where only single cells were observed previously and distribution of the bacteria along the root varied from ≈106 CFU per cm of root near the root base to ≈102 to 103 CFU per cm of root near the root tip. Similar colonization patterns were found for the P. fluorescens biocontrol strains CHA0 and F113, and P. putida strain WCS358, as well as for four species that have repeatedly been isolated from tomato roots from a commercial tomato field near Granada, Spain. In contrast, four Rhizobium strains and one Acinetobacter radioresistens strain showed poor colonization and micro-colonies were not observed. Based on the described data, we present a model for colonization of the deeper root parts after seed inoculation by P. fluorescens biocontrol strains, in which single cells occasionally establish on a deeper root section where they sometimes develop into micro-colonies. We hypothesize that micro-colonies are the sites where the intracellular N-acyl-L-homoserine lactone concentration is sufficiently high to cause maximal production of biocontrol factors such as antibiotics and exoenzymes and that micro-colonies explain the relatively high conjugation frequency observed between pseudomonads in the rhizosphere.

Colonization of soybean roots by Bacillus megaterium B 153-2-2

Colonization of soybean seedling roots and hypocotyls by Bacillus megaterium strain B 153-2-2 was studied in the greenhouse using two autoclaved growth media, vermiculite or a mixture of sand, soil, pea gravel and vermiculite (1:1:1:1, v/v), and in the field with natural soil. In the greenhouse B153-2-2 was recovered from hypocotyls of colonized seedlings at 5 cm above the medium line and the entire length of roots at 1 wk after coating the seed with the bacterium. No differences were recorded between the two media in the recovery of B153-2-2 from roots or hypocotyl with time. Populations of B153-2-2 on roots declined toward the root tip and approached a linear relationship with root depth. At 4 wk, populations of B153-2-2 were significantly greater than those at 1 wk on deeper roots. Bacterial cells were found to attach on the root surface of 1-wk-old seedlings. In the field, populations of B153-2-2 were recovered from hypocotyls and from roots at least 20cm deep with the highest populations occurring on the upper 10cm of root. Distributions of populations of B153-2-2 on hypocotyl and roots approached a cubic relationship. In the presence of an introduced fungal pathogen, Rhizoctonia solani isolate 65L-2, populations of B153-2-2 varied, but root colonization by B153-2-2 was not affected. Once introduced either by seed coating or soil treatment, soybean seedling, roots and hypocotyls were colonized by B153-2-2 for 4wk, whether grown in sterile media in the greenhouse or in natural soil in the field.

Relationship between root colonization and initial adsorption ofAzospirillum to plant roots

Four strains ofAzospirillum were ranked according to numbers of cells adsorbed on the roots of seedlings in liquid medium, and the rankings were evaluated for their usefulness in predicting success of colonization of the roots of pot-grown plants.Different rankings were observed on different parts of the roots and on different host plants. Rhizosphere colonization results for rice were similar to those for clover and showed little difference between bacterial strains. The population densities were approximately equal to those of the most dense strains in the wheat rhizosphere, whereas the highest concentrations in the root interior of clover and rice were only about one-tenth of those in wheat.Rankings of initial adsorptive ability on various parts of the roots showed potential for predicting the best strains for colonizing the root interiors of wheat and clover. On wheat, the two strains (Cd and SpBr14) which showed best initial adsorption to the root cap were best at colonizing the endorhizosphere of pot-grown plants. For rice, strains Cd and SpBr14 gave lowest and highest values, respectively, both for adsorption to the terminal 2 cm of roots and for subsequent colonization of the root interior. Data on initial adsorption were of no value in predicting the relative success of strains in colonizing the root surface of any host plants or the interior of clover roots.

The effect of soil compaction on wheat during early tillering

summaryWheat (Triticum aestivum L. cv. Eradu) was grown in the field or glasshouse on a deep loamy sand which had either a compact soil layer (compact soil) between 10 and 55 cm depth or a deep‐tilled profile (loosened soil). The pattern of assimilate transport to the roots was investigated. Previously reported growth rates and solute concentrations of the seminal root axes were coupled with rates of O2 uptake measured in the field to give estimates of dry weight import into the ethanol‐insoluble (structural), ethanol‐soluble (osmotic) and respiratory pools. The calculated daily import of dry weight was 28% less in mechanically impeded than unrestricted apices. However, the dry weight required to produce 1 cm of root was twice as high for axes in compact soil. In these roots, a greater proportion of imported substrates went into ethanol‐insoluble matter, probably cell walls. Non‐respired 14C lost from root systems into the rhizosphere was measured in the glasshouse and found to be less than 4% of the 14C fixed 72 h earlier, even in compact soil.The partitioning of 14C‐labelled assimilate in plants from the two treatments was compared. The proportion of 14C in the ethanol‐soluble fraction was hardly affected by tissue age or soil strength. Furthermore, 62–82% of the soluble 14C was in the neutral (sugar) fraction in the roots. However, the 14C concentration (dry‐weight basis) in apices of seminal root axes was substantially greater when the roots grew in loosened soil. The greater sink strength of seminal root? growing in loosened soil was reflected in a 3‐fold larger incorporation of 14C into each gram of new ethanol‐insoluble material formed. In contrast, concentrations of 14C were similar in mature root tissues, nodal roots and shoots taken from both soil treatments.

Respiration of valine by higher plants

A range of tissues were incubated in [U- 14C]valine for 6 hr. The 14CO 2 production, as percentages of uptake, were: apical 1 -cm of roots of Pisum sativum, 18.6; cotyledons of germinating seeds of Pisum sativum, 7.3; florets of Brassica oleracea, 9.7; leaves of Ananas comosus, 6.1; pulp of fruit of Musa cavendishii, 1.8. The detailed distribution of 14C in the roots of Pisum sativum showed that 44% of the absorbed [ 14C]valine had been metabolized to compounds other than valine (mainly CO 2, acids and amino acids) but only 4% appeared as [ 14C]leucine. It is suggested that a range of plants have a significant ability to respire valine.