Metal Complexation in Xylem Fluid : I. CHEMICAL COMPOSITION OF TOMATO AND SOYBEAN STEM EXUDATE

Xylem fluid was analyzed for numerous solutes to characterize chemically the sap as a medium for forming and transporting metal complexes. The stem exudate was collected hourly for 8 hours from topped 31-day-old soybean (Glycine max L. Merr.) and 46-day-old tomato (Lycopersicon esculentum Mill.) plants grown in normal (0.5 micromolar) and Za-phytotoxic nutrient solutions. Soybean plants were grown in the normal and high-Zn solutions for 24 days; tomato plants were grown for 32 days. The exudate was analyzed for seven organic acids, 22 amino acids, eight inorganic solutes, apparent ionic strength, and pH. Significant changes in many solutes occurred over the 8-hour sampling period. These fluctuations depended on plant species, individual solute, and Zn treatment, and demonstrated that extrapolation of xylem-fluid analyses to whole-plant xylem sap is valid only for sap samples collected shortly after topping a plant. Exudate pH decreased over the 8-hour period for both species; exudate ionic strength increased for tomato and decreased for soybean. At the normal-Zn treatment (0 to 1 hour), the highest acid micromolar concentrations in soybean exudate were: asparagine, 2,583; citric, 1,706; malic, 890; and malonic, 264. Under the same conditions, the highest acid micromolar concentrations in tomato exudate were: maleic, 1,206; malic, 628; glutamine, 522; citric, 301; and asparagine, 242. Cysteine and methionine were above detection limits only in soybean exudate. Zinc phytotoxicity caused significant changes in many solutes. The analyses reported here provide a comprehensive data base for further studies on metal-complex equilibria in xylem fluid.     

Meloidogyne incognita Inoculum Source Affects Host Suitability and Growth of Yellow Nutsedge and Chile Pepper

Meloidogyne incognita (Mi) reproduction and host plant responses in chile pepper (Capsicum annuum) and yellow nutsedge (Cyperus esculentus = YNS) to three sources of inoculum obtained by rearing a single Mi population on chile, YNS, and tomato were evaluated in two factorial greenhouse experiments. The interactive effects of Mi inoculum source and crop-weed competition were determined. In the absence of YNS competition, chile growth was reduced less by Mi inoculum from chile than by inoculum from YNS or tomato. When YNS was present, chile root weight was not affected and shoot weight increased with Mi initial inoculation, regardless of inoculum source. Chile plants inoculated with Mi from tomato exhibited double the nematode reproduction observed with inoculum from chile or YNS. With chile present, Mi reproduction on YNS was nearly three times greater with inoculum from tomato, but reproduction was similar among inoculum sources when chile was absent. Reductions in YNS root mass due to competition from chile failed to reduce the total number of Mi eggs produced on YNS plants. Differences in total Mi reproduction among inoculum sources were not attributable to differences in root growth or plant competition. This study illustrates the influence of Mi-YNS interactions and previous hosts on severity of Mi infection.     

Biology of Meloidogyne platani Hirschmann Parasitic on Sycamore, Platanus occidentalis

The development of Meloidogyne platani on sycamore was followed for 40 days (22-28 C). Juveniles penetrated the feeder roots behind the root cap and invaded the vascular cylinder within 3 days after inoculation. All subsequent development of the nematodes and host effects occurred only within the stele. The second juvenile molt and sex differentiation occurred by the 17th day. Young females were observed by the 26th day. Eggs were observed inside the roots by the 35th day and were exposed to the surface of galls by the 40th day. In pathogenicity studies, a significant negative correlation was shown to exist between fresh shoot and root weights and inoculum density. Besides sycamore, white ash was the only hardwood species tested to become infected. Of the herbacious plants tested, tobacco was heavily galled, tomato and watermelon moderately galled, and pepper only slightly galled. Egg production was moderate on tobacco, slight on tomato and watermelon, and absent on pepper.     

The Effects of Root-knot Nematode Infection and Mi-mediated Nematode Resistance in Tomato on Plant Fitness

The Mi-1.2 resistance gene in tomato (Solanum lycopersicum) confers resistance against several species of root-knot nematodes (Meloidogyne spp.). This study examined the impact of M. javanica on the reproductive fitness of near-isogenic tomato cultivars with and without Mi-1.2 under field and greenhouse conditions. Surprisingly, neither nematode inoculation or host plant resistance impacted the yield of mature fruits in field microplots (inoculum=8,000 eggs/plant), or fruit or seed production in a follow-up greenhouse bioassay conducted with a higher inoculum level (20,000 eggs/plant). However, under heavy nematode pressure (200,000 eggs/plant), greenhouse-grown plants carrying Mi-1.2 had more than ten-fold greater fruit production than susceptible plants and nearly forty-fold greater estimated lifetime seed production, confirming prior reports of the benefits of Mi-1.2. In all cases Mi-mediated resistance significantly reduced nematode reproduction. These results indicated that tomato can utilize tolerance mechanisms to compensate for moderate levels of nematode infection, but that the Mi-1.2 resistance gene confers a dramatic fitness benefit under heavy nematode pressure. No significant cost of resistance was detected in the absence of nematode infection.     

The concentration of xylem sap constituents in root exudate, and in sap from intact, transpiring castor bean plants (Ricinus communis L.)

Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.     

Changes to the proteome and targeted metabolites of xylem sap in Brassica oleracea in response to salt stress

Root-to-shoot signalling via xylem sap is an important mechanism by which plants respond to stress. This signalling could be mediated by alteration in the concentrations of inorganic and/or organic molecules. The effect of salt stress on the contents of xylem sap in Brassica olarecea has been analysed by mass spectrometry in order to quantify these changes. Subcellular location of arabinogalactan proteins (AGPs) by immunogold labelling and peroxidase isozymes was also analysed by isoelectrofocusing. The xylem sap metabolome analysis demonstrated the presence of many organic compounds such as sugars, organic acids and amino acids. Of these, amino acid concentrations, particularly that of glutamine, the major amino acid in the sap, were substantially reduced by salt stress. The xylem sap proteome analysis demonstrated the accumulation of enzymes involved in xylem differentiation and lignification, such as cystein proteinases, acid peroxidases, and a putative hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyl transferase under salt stress. The peroxidase isozyme pattern showed that salt stress induced a high accumulation of an acid isoform. These results suggest that xylem differentiation and lignification is induced by salt stress. The combination of different methods to analyse the xylem sap composition provides new insights into mechanisms in plant development and signalling under salt stress.     

Ultrastructural Changes Caused by Fusarium oxysporum f. sp. lycopersici in Meloidogyne javanica Induced Giant Cells in Fusarium Resistant and Susceptible Tomato Cultivars

Tomato (Lycopersicon esculentum Mill.) seedlings, susceptible (cv. Pearson A-I Improved) and resistant (cv. Pearson Improved) to race 1 Fusarium oxysporum f. sp. lycopersici (Sacc.) Snyd &Hans., were inoculated with Meloidogyne javanica (Trueb) Chitwood second-stage juveniles and 3 weeks later with race 1 F. oxysporum f. sp. lycopersici spores. One week after fungal inoculation, no fungus was visible in root tissue of the tomato cultivars and the giant cells were normal. Two weeks after fungal inoculation, abundant hyphae were visible in xylem tissues of Fusarium-susceptible but not of Fusarium-resistant plants. In susceptible plants, giant cell degeneration occurred, characterized by membrane and organelle disruption. In addition, where hyphae were in direct contact with the giant cell, dissolution of the giant cell wall occurred. Three weeks after fungal inoculation, fungal hyphae and spores were visible inside xylem tissues and giant cells in Fusarium-susceptible plants and in xylem tissue of the resistant plants. In susceptible and resistant plants, giant cell degeneration was apparent. Giant cell walls were completely broken down in Fusarium-susceptible tomato plants. In both cultivars infected by Fusarium, giant cell nuclei became spherical and dark inclusions occurred within the chromatin material which condensed adjacent to the fragmented nuclear membrane. No such ultrastructural changes were seen in the giant cells of control plants inoculated with nematode alone. Giant cell deterioration in both cultivars is probably caused by toxic fungal metabolites.     

Flagella-driven chemotaxis towards exudate components is an important trait for tomato root colonization by Pseudomonas fluorescens

Motility is a major trait for competitive tomato root-tip colonization by Pseudomonas fluorescens. To test the hypothesis that this role of motility is based on chemotaxis toward exudate components, cheA mutants that were defective in flagella-driven chemotaxis but retained motility were constructed in four P. fluorescens strains. After inoculation of seedlings with a 1:1 mixture of wild-type and nonmotile mutants all mutants had a strongly reduced competitive root colonizing ability after 7 days of plant growth, both in a gnotobiotic sand system as well as in nonsterile potting soil. The differences were significant on all root parts and increased from root base to root tip. Significant differences at the root tip could already be detected after 2 to 3 days. These experiments show that chemotaxis is an important competitive colonization trait. The best competitive root-tip colonizer, strain WCS365, was tested for chemotaxis toward tomato root exudate and its major identified components. A chemotactic response was detected toward root exudate, some organic acids, and some amino acids from this exudate but not toward its sugars. Comparison of the minimal concentrations required for a chemotactic response with concentrations estimated for exudates suggested that malic acid and citric acid are among major chemo-attractants for P. fluorescens WCS365 cells in the tomato rhizosphere.     

Biochemical Changes in Terminal Root Galls Caused by an Ectoparasitic Nematode,Longidorus africanus: Amino Acids

The amino acids of terminal root galls caused by Longidorus africanus on bur marigold (Bidens tripartita L.) and grapevine (Vitis vinifera L.) were studied. The galled roots of bur marigold contained 73% more cell-wall protein and 184% more free amino acids. The main changes among the free amino acids of the galled tissue were a large increase (1900%) in proline and a decrease in aspartic acid (56%) compared with the respective check tissue. Hydroxyproline decreased in the wall protein fraction from 5.6% in the healthy tissue to 3.6% in the infected tissue.Percent of hydroxyproline in total amino acids of the wall protein fraction of grapevine roots decreased from 0.7% in the healthy tissue to 0.3% in the galled tissue, and total proteins of this fraction decreased from 9.5 mg to 4.5 rag, respectively. Total protein in the protoplasmic fraction also decreased from 3.0 mg in healthy to 1.0 mg in infected roots. No change was noticed in total proteins in the free amino acids fraction but free proline decreased 40% in the infected roots.The relationship of these differences to the specific reactions of the hosts to nematode feeding is discussed.     

Changes in the composition of xylem sap during development of the spadix of skunk cabbage (Symplocarpus foetidus)

The spadix of skunk cabbage, Symplocarpus foetidus, is thermogenic and maintains an internal temperature of around 20 degrees C even when the ambient air temperature drops below freezing. This homeothermic heat production is observed only during the stigma stage, and thereafter ceases at the male stage when pollen is shed. To clarify the regulatory mechanism by which the stigma stage-specific heat production occurs in the spadix, sugars, organic acids, and amino acids in xylem sap were analyzed and compared with those of post-thermogenic plants. Interestingly, no significant difference was observed in the total volume of xylem sap per fresh weight of the spadix between thermogenic (31.2+/-24.7 microl h(-1) g(-1)) and post-thermogenic (50.5+/-30.4 microl h(-1) g(-1)) plants. However, concentrations of sugars (sucrose, glucose, and fructose), organic acids (malate and succinate), and amino acids (Asp, Asn, Glu, Gln, Gly, and Ala) in xylem sap decreased remarkably in post-thermogenic plants. Our results indicate that the composition of the xylem sap differs during the development of the spadix of S. foetidus.     

Interaction of Vesicular-arbuscular Mycorrhizal Fungi and Phosphorus with Meloidogyne incognita on Tomato

The influence of two vesicular-arbuscular mycorrhizal fungi and phosphorus (P) nutrition on penetration, development, and reproduction by Meloidogyne incognita on Walter tomato was studied in the greenhouse. Inoculation with either Gigaspora margarita or Glomus mosseae 2 wk prior to nematode inoculation did not alter infection by M. incognita compared with nonmycorrhizal plants, regardless of soil P level (either 3 μg [low P] or 30 μg [high P] available P/g soil). At a given soil P level, nematode penetration and reproduction did not differ in mycorrhizal and nonmycorrhizal plants. However, plants grown in high P soil had greater root weights, increased nematode penetration and egg production per plant, and decreased colonization by mycorrhizal fungi, compared with plants grown in low P soil. The number of eggs per female nematode on mycorrhizal and nonmycorrhizal plants was not influenced by P treatment. Tomato plants with split root systems grown in double-compartment containers which had either low P soil in both sides or high P in one side and low P in the other, were inoculated at transplanting with G. margarita and 2 wk later one-half of the split root system of each plant was inoculated with M. incognita larvae. Although the mycoorhizal fungus increased the inorganic P content of the root to a level comparable to that in plants grown in high P soil, nematode penetration and reproduction were not altered. In a third series of experiments, the rate of nematode development was not influenced by either the presence of G. margarita or high soil P, compared with control plants grown in low P soil. These data indicate that supplemental P (30 μ/g soil) alters root-knot nematode infection of tomato more than G. mosseae and G. margarita.     

Nitrogen Nutrition and Xylem Sap Composition of Peanut (Arachis hypogaea L. cv Virginia Bunch)

The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanut (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methyleneglutamine, were identified as the major components of xylem exudate collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4-methyleneglutamine. Pulse-feeding nodulated roots with ¹⁵N₂ gas showed asparagine to be the principal nitrogen product exported from N₂-fixing nodules. Maintaining root systems in an N₂-deficient (argon:oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-Methyleneglutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methyleneglutamine in xylem exudate did not appear to be associated with either N₂ fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from study of changes in amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of ¹⁵N-nitrate, and a ¹⁵N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N₂ or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagine:nitrate fell, while the ratio of nitrate:total amino acid rose as plants derived less of their organic nitrogen from N₂. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N₂ and nitrate are being used in peanuts.     

Induction of xylem sap methylglycine by a drought and rewatering treatment and its inhibitory effects on the growth and development of plant organs

In higher plants, the xylem vessels functionally connect the roots with the above-ground organs. The xylem sap transports various organic compounds, such as proteins and amino acids. We examined drought and rewatering-inducible changes in the amino acid composition of root xylem sap collected from Cucurbita maxima roots. The major free amino acids in C . maxima root xylem sap were methylglycine (MeGly; sarcosine) and glutamine (Gln), but MeGly was not detected in the xylem sap of cucumber. MeGly is an intermediate compound in the metabolism of trimethylglycine (TMG; betaine), but its physiological effects in plants are unknown. Drought and rewatering treatment resulted in an increase in the concentration of MeGly in root xylem sap to 2.5 m M . After flowering, the MeGly concentration in the xylem sap dropped significantly, whereas the concentration of Gln decreased only after fruit ripening. One milli molar MeGly inhibited the formation of adventitious roots and their elongation in C . maxima , but glycine, dimethylglycine, or TMG had no effect. Similar effects and the inhibition of stem elongation were observed in shoot cuttings of cucumber and Phaseolus angularis . These observations seem to imply a possible involvement of xylem sap MeGly in the physiological responses of C . maxima plants to drought stress.     

Reproduction of Meloidogyne javanica on Plant Roots Genetically Transformed by Agrobacterium rhizogenes

Reproduction of Meloidogyne javanica was compared on several Agrobacterium rhizogenes-transformed root cultures under monoxenic conditions. M. javanica reproduced on all transformed roots tested; however, more females and eggs were obtained on potato and South Australian Early Dwarf Red tomato than on bindweed, Tropic tomato, lima bean, or carrot. Roots that grew at moderate rates into the agar and produced many secondary roots supported the highest reproduction. Numbers of females produced in cultures of transformed potato roots increased with increasing nematode inoculum levels, whether inoculum was dispersed eggs or juveniles. Females appeared smaller, produced fewer eggs, and were found in coalesced galls at the higher inoculum levels. The ratio between the final and initial population decreased sharply as the juvenile inoculum increased. The second-stage juvenile was preferred to dispersed eggs or egg masses for inoculation of tissue culture systems because quantity and viability of inoculum were easily assessed. Meloidogyne javanica reared on transformed root cultures were able to complete their life cycles on new transformed root cultures or greenhouse tomato plants.     

Changes in the Composition of Xylem Sap during Development of the Spadix of Skunk Cabbage (Symplocarpus foetidus)

The spadix of skunk cabbage, Symplocarpus foetidus, is thermogenic and maintains an internal temperature of around 20 °C even when the ambient air temperature drops below freezing. This homeothermic heat production is observed only during the stigma stage, and thereafter ceases at the male stage when pollen is shed. To clarify the regulatory mechanism by which the stigma stage-specific heat production occurs in the spadix, sugars, organic acids, and amino acids in xylem sap were analyzed and compared with those of post-thermogenic plants. Interestingly, no significant difference was observed in the total volume of xylem sap per fresh weight of the spadix between thermogenic (31.2±24.7 μl h^?1g^?1) and post-thermogenic (50.5±30.4 μl h^?1g^?1) plants. However, concentrations of sugars (sucrose, glucose, and fructose), organic acids (malate and succinate), and amino acids (Asp, Asn, Glu, Gln, Gly, and Ala) in xylem sap decreased remarkably in post-thermogenic plants. Our results indicate that the composition of the xylem sap differs during the development of the spadix of S. foetidus.     

Influence of Pratylenchus penetrans on Plant Growth and Water Relations in Potato

Plants of potato (Solanum tuberosum) cultivars Katahdin and Superior were inoculated with 0, 1,500, or 15,000 Pratylenchus penetrans. Transpiration, measured in the greenhouse with a porometer after 56 days of growth, was not significantly different among nematode inoculum levels or between cultivars. The rate of xylem exudation from decapitated root systems of Katahdin plants inoculated with 1,500 or 15,000 P. penetrans and Superior plants inoculated with 15,000 P. penetrans was lower than from noninoculated plants. Root weight of Katahdin and Superior was not affected by P. penetrans inoculum level. Transpiration of plants inoculated with 0, 500, 5,000 or 50,000 P. penetrans was recorded weekly from 14 to 56 days after planting. No consistent effects of nematode inoculum density on transpiration rate were observed. Root hydraulic conductivity was lower in Katahdin plants inoculated with 266 P. penetrans per plant and in Chippewa with 5,081 per plant than in noninoculated plants. Nematodes reduced leaf area of Superior, Chippewa, and Katahdin and root dry weight of Chippewa but had no effect on growth of Hudson, Onaway, or Russet Burbank plants. Assessing nematode effects on root hydraulic conductivity may provide a measure of the tolerance of potato cultivars to nematodes.     

Carbon Partitioning in Soybean Infected with Meloidogyne incognita and M. javanica

Seven-day-old seedlings of two cultivars (Cristalina and UFV ITM1) of Glycine max were inoculated with 0, 3,000, 9,000, or 27,000 eggs of Meloidogyne incognita race 3 or M. javanica and maintained in a greenhouse. Thirty days later, plants were exposed to ¹⁴CO₂ for 4 hours. Twenty hours after ¹⁴CO₂ exposure, the root fresh weight, leaf dry weight, nematode eggs per gram of root, total and specific radioactivity of carbohydrates in roots, and root carbohydrate content were evaluated. Meloidogyne javanica produced more eggs than M. incognita on both varieties. A general increase in root weight and a decrease in leaf weight with increased inoculum levels were observed. Gall tissue appeared to account for most of the root mass increase in seedlings infected with M. javanica. For both nematodes there was an increase of total radioactivity in the root system with increased levels of nematodes, and this was positively related to the number of eggs per gram fresh weight and to the root fresh weight, but negatively related to leaf dry weight. In most cases, specific radioactivities of sucrose and reducing sugars were also increased with increased inoculum levels. Highest specific radioactivities were observed with reducing sugars. Although significant changes were not observed in endogenous levels of carbohydrates, sucrose content was higher than reducing sugars. The data show that nematodes are strong metabolic sinks and significantly change the carbon distribution pattern in infected soybean plants. Carbon partitioning in plants infected with nematodes may vary with the nematode genotype.     

Comparison of Methods for Assessing Resistance to Meloidogyne arenaria in Peanut

Use of resistant cultivars is a desirable approach to manage the peanut root-knot nematode (Meloidogyne arenaria). To incorporate resistance into commercially acceptable cultivars requires reliable, efficient screening methods. To optimize the resistance screening protocol, a series of greenhouse tests were done using seven genotypes with three levels of resistance to M. arenaria. The three resistance levels could be separated based on gall indices as early as two weeks after inoculation (WAI) using 8,000 eggs of M. arenaria per plant, while four or more weeks were needed when 1,000–6,000 eggs/plant were used. High inoculum densities (over 8,000 eggs/plant) were needed to separate the three resistance levels based on eggs per gram of root within eight WAI. A gall index based on percentage of galled roots could separate the three resistance levels at lower inoculum levels and earlier harvest dates than other assessment methods. The use of eggs vs. second-stage juveniles (J2) as inoculum provided similar results; however, it took three to five more days to collect J2 than to collect eggs from roots. Plant age affected gall index and nematode reproduction on peanut, especially on the susceptible genotypes AT201 and D098. The genotypes were separated into their correct resistance classes when inoculated 10 to 30 days after planting, but were not separated correctly when inoculated on day 40.     

Biological Relationship of Meloidogyne hapla Populations to Alfalfa Cultivars

Greenhouse and growth chamber studies were established to determine if there are pathological and physiological differences among Meloidogyne hapla populations from California (CA), Nevada (NV), Utah (UT), and Wyoming (WY) on alfalfa cultivars classified as resistant or susceptible to root-knot nematodes. In the greenhouse, plant survival was not consistent with resistance classifications. While all highly resistant Nevada Synthetic germplasm (Nev Syn XX) plants survived inoculation with all nematode populations, two cultivars classified as moderately resistant (''Chief'' and ''Kingstar'') survived (P ≤ 0.05) inoculation with M. hapla populations better than did ''Lobo'' cultivar, which is classified as resistant. Plant growth of Nev Syn XX was suppressed by only the CA population, whereas growth of the other alfalfa cultivars classified as M. hapla resistant or moderately resistant was suppressed by all nematode populations. Excluding Nev Syn XX, all alfalfa cultivars were severely galled and susceptible to all nematode populations. Except for Nev Syn XX, reproduction did not differ among the nematode populations on alfalfa cultivars. Nev Syn XX was not as favorable a host to CA as were the other cultivars; but, it was a good host (reproductive factor [Rf] = 37). Temperature affected plant resistance; the UT and WY populations were more pathogenic at 15-25 C, and CA was more pathogenic at 30 C. Nev Syn XX was susceptible to all nematode populations, except for CA, at only 30 C, and all other alfalfa cultivars were susceptible to all nematode populations at all temperatures.     

Low-level herbivory by root-knot nematodes (Meloidogyne incognita) modifies root hair morphology and rhizodeposition in host plants (Hordeum vulgare)

Low amounts of root infestation by plant parasitic nematodes are suggested to increase nutrient supply and in turn enhance microbial activity and net mineralization rate in the rhizosphere. These effects are generally related to “leakage” of plant-derived metabolites from damaged roots. Besides leakage, the present study examines other nematode–host interactions such as alterations in root exudation and morphology, which were almost not considered yet. This includes undamaged root parts in order to assess systemic plant response. The root-knot nematode Meloidogyne incognita (Kofoid and White 1919; Chitwood 1949) and barley (Hordeum vulgare L. cv. Europa) was used as model system. Host plants were grown in mini-rhizotrons inoculated with 0, 2,000, 4,000 or 8,000 M. incognita for 4 weeks. Root morphology, rhizodeposition (sugars, carboxylates, amino acids), and rhizosphere microbial communities (PLFAs) were assessed. In treatments with 4,000 nematodes, shoot biomass, total N and P content increased by the end of the experiment. Generally, an enhanced release of plant metabolites (sugars, carboxylates, amino acids) from the apical root zone occurred 1 week after inoculation with 4,000 and 8,000 M. incognita, indicating root leakage. Low levels of root herbivory stimulated root hair elongation in both infected and uninfected roots. These systemic changes in root morphology likely contributed to the increased sugar exudation in uninfected roots in all nematode treatments at 3 weeks after inoculation. Root-knots formed a separate microhabitat within the root-system. They were characterised by decreased rhizodeposition and increased fungal to bacterial ratio in the adhering rhizosphere soil. The present study provides the first evidence that, apart from leakage, nematode root herbivory at background levels induces local and systemic effects on root morphology and exudation, which in turn may affect plant performance.