The effects of organic amendments on the interactions between a nematophagous fungus Arthrobotrys oligospora and the root-knot nematode Meloidogyne mayaguensis parasitizing tomato plants

The aim of this study was to examine the effects of amendments with leaf biomass on the development of tomato plants in a soil where root-knot nematodes (Meloidogyne mayaguensis) and/or a nematophagous fungus (Arthrobotrys oligospora, strain ORS 18697) had been inoculated. Six origins of leaf biomass were chosen: Acacia mangium, Acacia holosericea, Eucalyptus camaldulensis, Casuarina equisetifolia, Azadirachta indica and Sorghum vulgare. These leaf biomass types inhibited the development of the aerial parts of the tomato plants. This negative effect was not observed when the fungus was inoculated. On the contrary, plant growth was stimulated. Moreover, the antagonistic activity of Arthrobotrys oligospora was strengthened in the presence of ground leaf powder, especially that from Acacia holosericea. The effects of phenolic compounds on fungal growth and predatory activity and on plant growth are discussed.     

Does apoplastic ascorbic acid enhance manganese tolerance of Vigna unguiculata and Phaseolus vulgaris?

In cowpea (Vigna unguiculata), the development of manganese (Mn) toxicity is considered to be accompanied by the formation of reactive oxygen species, oxidized Mn, and phenoxy radicals in the leaf apoplast. Ascorbic acid (AA) is a common antioxidant in plants, and the oxidation of AA, particularly in the leaf apoplast, contributes to the first line of defence against several biotic and abiotic stress factors. The objective of the present study was to contribute to a better understanding of the role of AA in Mn leaf‐tissue tolerance of cowpea and common bean (Phaseolus vulgaris). Five cowpea cultivars (cvs.) differed greatly in Mn tolerance, which was expressed in differences in numbers of brown spots on leaves and in peroxidase (POD) activity in the apoplastic washing fluid (AWF). In a Mn‐sensitive cv., after 3 d of Mn treatment, brown spots were formed, and POD activities were increased, accompanied by a release of proteins into the apoplast. In the AWF, the concentration of AA and the ratio of AA : (AA+DHA) decreased already after 1 d, and to only 2% after 3 d. In the leaf tissue, the ratio was nearly unaffected, and the total AA+DHA content in the leaf tissue was even increased with advanced expression of Mn toxicity. The application of AA solutions in the range of 5–10 μM via the petiole slightly enhanced Mn tolerance as indicated by the reduction of brown spots (however inconsistently) and POD activity (consistently) in the AWF. Common bean cultivars differing in ozone tolerance, which has been reported to be due to a high AA availability in the leaf apoplast, were studied for their Mn tolerance. Clear differences in Mn tolerance between the cultivars existed, however, these differences were not related to their ozone tolerance. From these results, we conclude that the maintenance of sufficient AA levels in the leaf apoplast contribute to Mn tolerance, but does not fully explain genotypic differences in Mn tolerance in cowpea and common bean.     

Physiology of manganese toxicity and tolerance in Vigna unguiculata (L.) Walp.

In cowpea (Vigna unguiculata (L.) Walp.) tolerance of manganese (Mn) excess depends on genotype, silicon (Si) nutrition, form of nitrogen (N) supply, and leaf age. The physiological mechanisms for improved Mn leaf-tissue tolerance are still poorly understood. On the basis of the density of brown spots per unit of leaf area and the callose content which are sensitive indicators of Mn toxicity, it was confirmed that cultivar (cv.) TVu 1987 was more Mn-tolerant than cv. TVu 91, young leaves were more Mn-tolerant, Si improved Mn tolerance, and NO₃^—-grown plants were more Mn-tolerant than NH₄⁺-grown plants. A close positive relationship existed between the bulk-leaf Mn content and the vacuolar Mn concentration from the same leaves. Since no clear and consistent differences existed between leaf tissues differing in Mn tolerance, the results suggest that accumulation of Mn in the vacuoles and its complexation by organic anions do not play a role in Mn leaf-tissue tolerance in cowpea. A near linear relationship was found between leaf Mn contents and concentrations of free (H₂O-soluble) and exchangeable-bound (BaCl₂-extractable) Mn in the apoplastic washing fluid (AWF) extracted from whole leaves by an infiltration and centrifugation technique. There were no differences in apoplastic Mn concentrations owing to genotype and form of nitrogen nutrition. However, Si decreased the Mn concentration in the AWF. With increasing bulk-leaf Mn contents, concentrations of organic anions in the AWF also increased. The results suggest that complexation of Mn by organic anions in the leaf apoplast contribute to Mn tolerance due to genotype and more clearly due to NO₃-N nutrition. Cell wall-bound peroxidase activity increased with leaf age and was higher in the Mn-sensitive cv. TVu 91 than in cv. TVu 1987. This was in agreement with a higher H₂O₂ production rate in cv. TVu 91. Also, a lower ratio of reduced to oxidized ascorbic acid in the AWF revealed that in Mn-sensitive leaf tissue, the apoplastic reduction capacity was lower than in Mn-tolerant leaf tissue when genotypes and leaves of different age were compared. We interpret our results as strong circumstantial evidence that Mn tolerance depends on the control of the free Mn²⁺concentration and of Mn²⁺-mediated oxidation/reduction reactions in the leaf apoplast.     

Rhizobacterium Bacillus amyloliquefaciens Strain SQRT3-Mediated Induced Systemic Resistance Controls Bacterial Wilt of Tomato

Tomato bacterial wilt caused by Ralstonia solanacearum seriously threats tomato growth in tropical and temperate regions around the world. This study reported an antagonistic bacterial strain, Bacillus amyloliquefaciens strain SQRT3, isolated from the rhizosphere soil of tomato plants, which strongly inhibited in vitro growth of pathogenic R. solanacearum. The suppression of tomato bacterial wilt by strain SQRT3 was demonstrated under greenhouse conditions. Additionally, induced systemic resistance (ISR) in tomato as one of the potential disease suppression mechanisms was investigated in the plants inoculated with the isolated bacterial strain SQRT3. The results showed that strain SQRT3 applied with R. solanacearum by drenching significantly reduced tomato bacterial wilt by 68.1% biocontrol efficiency (BE) and suppressed the R. solanacearum populations in the rhizosphere soil compared to the control only drenched with R. solanacearum. The BE of the isolated bacterial strain SQRT3 against tomato wilt increased to 84.1% by root-dipping. Tomato plants treated with both strain SQRT3 and R. solanacearum showed increases in activities of peroxidase and polyphenol oxidase compared with other treatments. The application of strain SQRT3 reduced membrane lipid peroxidation in tomato leaves. The expressions of marker genes for jasmonic acid-and salicylic acid-dependent signaling pathways were faster and stronger in tomato plants treated with both strain SQRT3 and R. solanacearum than in plants treated with either R. solanacearum or strain SQRT3 alone. Collectively, the findings indicated that strain SQRT3 can effectively control tomato wilt.     

Silicon modulates the metabolism and utilization of phenolic compounds in cucumber (Cucumis sativus L.) grown at excess manganese

Cucumber plants (Cucumis sativus L. cv. Chinese long) were grown in nutrient solution with increasing manganese (Mn) concentrations (0.5, 50, and 100 µM) with (+Si) or without silicon (–Si) supplied as silicic acid at 1.5 mM. High external Mn supply induced both growth inhibition of the whole plant and the appearance of Mn‐toxicity symptoms in the leaves. The application of Si alleviated Mn toxicity by increasing the biomass production. Although the total Mn concentration in the leaves did not differ significantly between +Si and –Si plants, symptoms of Mn toxicity were not observed in Si‐treated plants. The concentrations of phenolic compounds, particularly in the leaf extracts of cucumber plants grown at high external Mn concentrations, differed from those of plants grown without Si. The increased tissue concentrations of phenols (e.g., coniferyl alcohol, coumaric and ferulic acids) were in agreement with enhanced enzymes activities, i.e., peroxidases (PODs) and polyphenol oxidases (PPO) in the tissues of –Si plants. The activities of both enzymes were kept at a lower level in the tissue extracts of +Si plants grown at high external Mn concentrations. These results suggest that Si nutrition modulates the metabolism and utilization of phenolic compounds mainly at the leaf level, most probably as a consequence of the formation of Si‐polyphenol complexes.     

Suppression of rice blast,cabbage black leaf spot,and tomato bacterial wilt diseases by Meyerozyma guilliermondii TA-2 and the nature of protection

This study was conducted to evaluate the efficacy of yeast strain TA-2 for controlling rice blast, cabbage black leaf spot, and tomato bacterial wilt diseases. Microscopic and phylogenetic analyses based on rDNA-internal transcribed region (ITS) and rDNA-D1/D2 sequences indicated that yeast strain TA-2 is Meyerozyma guilliermondii. Pretreatment with TA-2 by soil drenching significantly reduced the severity of black leaf spot disease caused by Alternaria brassicicola and leaf blast disease caused by Magnaporthe oryzae. Symptom development of tomato bacterial wilt caused by Ralstonia solanacearum in both soil drench and needle inoculation tests was significantly reduced in TA-2-pretreated plants under soil drenching. Disease severity and R. solanacearum growth were significantly reduced in tomato plants pretreated with yeast culture, cell suspension, or culture filtrate of TA-2 under soil drenching. TA-2 does not produce antibiotics. The present study indicates that disease suppression is systemic, as the roots were treated with TA-2 and the pathogens were inoculated onto leaves or stems, thereby separating the two spatially. M. guilliermondii TA-2 could become a promising natural antimicrobial agent against rice blast, cabbage black leaf spot, and tomato bacterial wilt diseases and might be useful as an eco-friendly control measure, contributing to sustainable agriculture.     

Effects of interaction of Meloidogyne incognita,Alternaria dauci and Rhizoctonia solani on the growth,chlorophyll, carotenoid and proline contents of carrot in three types of soil

Disease complex of carrot (Daucus carota L.) involving root knot nematode Meloidogyne incognita and two fungi Alternaria dauci and Rhizoctonia solani were studied in three soil types. More plant growth, chlorophyll, carotenoid and proline contents were found in carrot grown in fly ash mix soil than plants grown in sand mix soil and loamy soil. Inoculation of M. incognita, R. solani, and A. dauci reduced plant growth, chlorophyll and carotenoid but increased proline contents. Inoculation of M. incognita 20 days prior to a fungal pathogen caused a greater reduction in plant growth, chlorophyll and carotenoid than fungal pathogen was inoculated prior. Inoculation of A. dauci prior to R. solani or vice versa had a similar effect on plant growth, chlorophyll, and carotenoid. Nematode multiplication and galling was higher in plants grown in sand mix soil followed by loamy soil and fly ash mix soil. Both fungi had adverse effects on galling and nematode multiplication. Blight disease index caused by A. dauci was 3 and crown rot index by R. solani was also recorded 3. These disease indices were 5 when pathogens were inoculated in combinations.     

Effect of magnesium fertilization on yield and leaf composition of tomato plants

Tomato plants were grown for 2 years at 4 different rates of Mg fertilization on a Princeton loamy sand at pH 4.8 with 29 kg exchangeable Mg/ha. Calcareous limestone was used to provide a pH treatment in the second year. Magnesium deficiency symptoms were observed on plants grown on plots having ≤ 38 kg/ha NH₄OAC‐extractable Mg. Application of 56 kg Mg/ha corrected Mg deficiency and produced a significant increase in yield. Application of calcitic limestone also produced significant yield increases, but did not affect the development of Mg deficiency symptoms. Tomato yield was increased 27.9% by Mg application and 17.7% by lime application. Highest tomato yield was obtained with application of 112 kg Mg/ha. Symptoms of Mg deficiency were observed when the Mg concentration in recently mature leaf tissue was in the 0.30 to 0.32% range. Magnesium concentration in leaf tissue increased linearly with increasing Mg rate. Leaf Mg concentration at various growth stages of the tomato plant was variable depending on Mg treatment. Magnesium fertilization rate bad little effect on Ca or K leaf concentrations. Application of Calcltic limestone increased leaf tissue Ca and reduced leaf tissue Mg and Mn concentrations.     

Tomato seedling development is improved by a substrate inoculated with a combination of rhizobacteria and fungi

Chile's seedling production industry has been growing for the last 10 years, and demand has actually reached 1250 million seedlings per year. This system has special relevance due to the high cost of seeds. In addition, there is an increasing demand for substituting synthetic agrochemicals. Therefore, the potential use of plant growth-promoting rhizobacteria (PGPR) in tomato production has been investigated. Before sowing, the micro-organisms provided by Biogram S.A. were inoculated into the substrate diluted in 250 mL/L unchlorinated water. The experiment was laid out in a ‘split-plot’ design with the two plant substrates as main plots and the inoculants as subplots, including six replicates per treatment. Tomato seedlings were grown using two different plant substrates: a mixture of 70% peat and 30% perlite by volume, and a substrate with 20% peat, 20% perlite and 60% compost by volume, both inoculated with Bacillus subtilis or Pseudomonas fluorescens or Bioroot®, which is a commercial product containing B. subtilis, P. fluorescens, Trichoderma harzianum, yeast, algae and Nocardia. For control, uninoculated tomato seedlings were grown on the respective plant substrates. Variance analysis did not identify significant interactions between substrate type (main plots) and inoculation treatment (subplots), P ≤ 0.05. There were significant differences between inoculants (P ≤ 0.05). Means were compared by using the Tukey's multiple range test. Tomato growth in terms of leaf area (cm²/plant) and shoot and root dry weight (g/10 plants) was improved for the seedlings grown on the substrate with 70% peat and 30% perlite, compared to the compost containing an alternative that is valid for both uninoculated perlite peat and all inoculated treatments where perlite peat was outstanding. Inoculation with Bioroot® improved the leaf area, shoot dry weight, root dry weight, radical contact area, volume of roots and root forks compared with the control without inoculation, when both plant substrates were analysed together. Thus, inoculation with Bioroot® can be recommended as an alternative to tomato seedling growers' dependence on synthetic agrochemicals.     

Plant microcosm studies demonstrating bioremediation of cyanide toxicity by Trichoderma and Fusarium spp.

The biodegradation of cyanide by Trichoderma and Fusarium spp. growing in association with plant roots in microcosms was investigated with CN^– at 50 or 100 mg/kg. Pea and wheat seeds germinated and plants grew only when seeds were inoculated with the fungi, probably because the plant/fungal association was capable of promoting cyanide catabolism. Inoculation by fungi also increased plant shoot lengths and the biomass of shoots and root compared with control plants without CN^– and fungi. Such plant/fungal association shows potential as a land remediation system.     

An investigation of the impact of Glomus clarum (mycorrhiza) on the growth of tomato (Lycopersicum esculentum mill.) on both sterilized and non-sterilized soils

Abstract

A greenhouse experiment was carried out to investigate the influence of Glomus clarum (mycorrhiza) on the growth of tomato seedlings grown in both sterilized and non-sterilized soils. Highest growth parameter values were recorded in tomato plants inoculated with mycorrhiza but grown in sterilized soil, followed by those grown in non-sterilized soil but inoculated with mycorrhiza also. Sterilized but non-inoculated tomato plants also had growth and were closely followed by non-sterilized, non-inoculated tomato plants. There was no significant difference in all the treatments when girth of the tomato plants used was measured in this study. Nutrient uptake (N,P,K) was significantly found highest in the inoculated sterilized tomato plants while it was found lowest in the non-sterilized, non-inoculated tomato plants. Generally, mycorrhizal-inoculated tomato plants (whether sterilized or non-sterilized) showed better growth in all the treatments used.     

Drought pretreatment increases the salinity resistance of tomato plants

Agricultural productivity is worldwide subjected to increasing salinity problems. Various strategies are applied to overcome the deleterious effects of salinity on plants. This study was conducted in order to determine whether drought pretreatment of seedlings or seed pretreatment with NaCl increases the long‐term salinity resistance of tomato (Solanum lycopersicum L.) and whether the adaptive response to salinity is accompanied by physiological changes throughout the plant‐growth cycle. When plants were pretreated at the five‐leaf growth stage, the plant dry weight was significantly higher in drought‐pretreated than in non‐pretreated plants after 50 d of salt treatment. The positive effect of drought pretreatment applied at the five‐leaf stage was maintained throughout the entire growth cycle, as fruit yield of drought‐pretreated plants was 40% higher than that of non‐pretreated plants at the end of the harvest period (150 d of 70 mM NaCl treatment). Moreover, the most productive plants maintained lower Na⁺ and Cl^– accumulation in their leaves until the end of the growth cycle, which shows that adaptation is a long‐term response during which the plants adjust their physiology to the environmental conditions. Salt resistance was also improved through seed pretreatment with NaCl. In conclusion, drought pretreatment applied at the five‐leaf stage or seed pretreatment with NaCl provide an alternative way to enhance salt resistance in tomato, and the increase in yield is associated with physiological changes throughout the plant‐growth cycle.     

Effects of indigenous and introduced arbuscular mycorrhizal fungi on the growth of Allium fistulosum under field conditions

ABSTRACT

Enhanced phosphorus (P) uptake from the soil and increased plant growth related to arbuscular mycorrhizal (AM) fungi in pot culture, using sterilized soil, are well-known phenomena. However, these enhancements are not widely observed under field conditions because field sterilization is difficult. The aim of this study was to investigate the effects of AM fungi on P uptake and the growth of Allium fistulosum in non-fumigated and fumigated fields, under different levels of P availability. Plants were inoculated with the AM fungus Glomus R-10 and grown in fumigated soil. For the uninoculated treatment, a sterilized inoculum was applied directly. The field was fumigated using dazomet. Superphosphate was applied to the field at the rates of 0 (P0) or 500 (P500) kg P₂O₅ ha^?1. The inoculated and uninoculated plants were transplanted into the fields and sampled three times to measure AM fungal colonization, shoot P concentration, and shoot dry weight of the plants. At the transplanting stage, AM fungal colonization was observed in the inoculated plants (>70%) but not in the uninoculated plants. At the third sampling, irrespective of P treatment, AM fungal colonization was observed both in the uninoculated and inoculated plants in the non-fumigated field, and there was no difference in shoot P content and shoot dry weight between the inoculated and uninoculated plants. AM fungal colonization in the fumigated field was higher in the inoculated than uninoculated plants, irrespective of P treatment; shoot P content and shoot dry weight were both higher in the inoculated plants than in the uninoculated plants with P0. These results suggest that the responses of A. fistulosum to AM fungal inoculation under the low-P and fumigated conditions are similar to those observed in sterilized pot culture conditions.     

Manganese toxicity in tomato plants: Effects on cation uptake and distribution

Two experiments are described in which tomato plants (Lycopersicon esculentum L. var Ailsa Craig) were grown in water culture supplied with 10–300 μM Mn. Toxicity symptoms associated with a yield reduction were observed only in treatments in excess of 50 μM Mn indicating that this species is relatively tolerant of high Mn supply. Dark brown/black spots appeared first in the cotyledons. Similar symptoms were observed in the leaves, progressively from the oldest leaf. Manganese concentration in the shoot tissues ranged from 286 to 4240 μg. g^‐1 dry weight. The high Mn concentration values found in the shoot tissues of the toxic plants indicate that Mn was highly mobile in the xylem as confirmed by xylem sap analysis.

The concentrations of both Ca and Mg were lower in the smaller Mn toxic plants. Not only was uptake of Ca and Mg retarded but so also was the distribution of Ca and Mg to the younger tissues as illustrated by measurements of Ca and Mg concentrations along a leaf age sequence. This is in accord with the cation‐anion balance of the xylem exudates collected from decapitated plants.

Higher cation exchange capacity (CEC) was found in the leaf tissues of toxic plants particularly in the older leaves but similar values of C.E.C were recorded for the younger leaf tissues of both control and toxic plants.     

Arbuscular mycorrhizal fungi influence tomato competition with bahiagrass

A strip-tillage production system for tomatoes (Lycopersicon esculentum Mill.) is impacted by nutrient competition from bahiagrass (Paspalum notatum Flügge). Tomato and bahiagrass differ in mycorrhizal responsiveness and our objective was to evaluate the influence of arbuscular mycorrhizal (AM) fungi on the competitive pressure of bahiagrass on growth of tomato. The first experiment evaluated the effect of bahiagrass competition, soil pasteurization, and AM fungal inoculation on tomato growth, P content, and root colonization in a low-P soil. Tomato grown alone was very responsive to mycorrhizal colonization - shoot dry mass of inoculated plants was up to 243% greater than that of noninoculated plants. Tomato grown with bahiagrass had reduced root and shoot growth across all treatments compared with tomato grown alone, but there was an increase in shoot mass following AM fungal inoculation across both pasteurized and nonpasteurized treatments resulting in a >50% increase in shoot dry mass of tomato compared to noninoculated controls. A second experiment was conducted to test bahiagrass competition, soil pasteurization, AM fungal inoculation, and P amendment on tomato growth in a moderate-P soil. With bahiagrass competition and no P addition, inoculation increased root mass by 115% and shoot mass by 133% in pasteurized soil; however, with the application of 32 mg P kg^-1 the trend was reversed and inoculated plants were smaller than noninoculated controls. We conclude that the role of mycorrhizae in plant competition for nutrients is markedly impacted by soil nutrient status and reduced P application may allow tomatoes to take advantage of their inherent responsiveness to mycorrhizae in a low to moderate soil-P environment.     

Effect of manganese-reducing rhizosphere bacteria on the growth of Gaeumannomyces graminis var. tritici and on manganese uptake by wheat (Triticum aestivum L.)

Summary Five bacterial strains capable of Mn reduction were isolated from the rhizosphere of plants growing in different South Australian soils. They differed in their Mn-reducing capacity. The antagonism of these strains compared to the imported strain 2–79 (from the United States) against Gaeumannomyces graminis var. tritici was tested in agar and in a soil sandwich experiment at different Mn²⁺ concentrations in the soil. In addition, wheat seeds were coated with the different strains and with MnSO₄ or with MnSO₄ only in order to investigate their effect on plant growth and Mn uptake. With one exception, all strains inhibited the growth of G. graminis in agar, but to different degrees. In contrast, only two strains significantly inhibited the growth of the fungus in the soil. The hyphal density was decreased more than the hyphal length. The Mn²⁺ concentration in the soil also had a marked effect on fungal growth; low Mn concentrations slightly increased while high Mn concentrations strongly decreased the fungal growth. Seed treatment with MnSO₄ only (+Mn) increased Mn uptake above that of the control (no seed treatment). Only the weakest Mn reducer on agar significantly increased plant growth and Mn uptake from soil in comparison with the Mn treatment. One strain was tested as seed coating without adding MnSO₄; it increased the plant growth to an extent similar to the Mn treatment. Increasing the Mn uptake by plants may be one of the growth-promoting effects exerted by rhizosphere bacteria.     

Enhancement of alkalinity tolerance in two cucumber genotypes inoculated with an arbuscular mycorrhizal biofertilizer containing Glomus intraradices

The aim of the present study was to determine whether arbuscular mycorrhizal (AM) inoculation with a biofertilizer containing clays as granular carriers, leek root pieces and Glomus intraradices spores could improve alkalinity tolerance of two cucumber genotypes, and to study the changes induced by AM at agronomical and physiological level. A greenhouse experiment was carried out to determine yield, growth, fruit quality, net photosynthesis (A_CO2), electrolyte leakage, and mineral composition of two cucumber (Cucumis sativus L.) genotypes (hybrid “Ekron” or open-pollinated variety “Marketmore”) with inoculated and noninoculated arbuscular mycorrhizal biofertilizer. Plants were supplied with nutrient solutions at two pH values (6.0 or 8.1). The high pH nutrient solution had the same basic composition with an additional 10 mM NaHCO₃ and 0.5 g l⁻¹ CaCO₃. The percentage root colonization was higher in “Marketmore” (21.8%) than “Ekron” (12.7%). Total and marketable yield and total biomass were significantly higher by 189%, 213%, and 77%, respectively, with Ekron in comparison to those recorded with Marketmore. The highest crop performance with Ekron in comparison to Marketmore was due to the improved nutritional status (higher N, P, K, Ca, Mg, Fe, Mn, and B), higher leaf area, and net photosynthesis. Increasing the concentration of NaHCO₃ from 0 to 10 mM in the nutrient solution significantly decreased yield, plant growth, A_CO2, N, P, Fe, Cu, Zn, Mn, and B concentration in leaf tissue, whereas the electrolyte leakage increased. The inoculated plants under alkaline conditions had higher total, marketable yield, and total biomass than noninoculated plant. Mycorrhizal cucumber plants grown under alkaline conditions had a higher macronutrient concentration in leaf tissue compared to noninoculated plants. The highest yield and biomass production in inoculated plants seems to be related to the capacity of maintaining higher net A_CO2 and to a better nutritional status (high P, K, Mg, Fe, Zn, and Mn and low Na accumulation) in response to bicarbonate stress with respect to −AM plants.     

Azospirillum brasilense mitigates water stress imposed by a vascular disease by increasing xylem vessel area and stem hydraulic conductivity in tomato

Water stress, with its negative consequences on plant growth and survival, can be mitigated by Azospirillum brasilense inoculation. In tomato, A. brasilense delays wilting caused by a vascular pathogen, Clavibacter michiganensis subsp. michiganensis, by yet unknown mechanisms. We studied morphological, anatomical and physiological changes induced by A. brasilense in tomato that relate to water stress tolerance, which could explain the deferral in symptom expression. For this purpose, tomato seeds were treated or not with A. brasilense BNM65, and 5 weeks later plants were challenged with C. michiganensis subsp. michiganensis or mock inoculated with water. There was a large growth promotion associated to Azospirillum: treated plants had higher total biomass and leaf area. In relation to water stress tolerance, Azospirillum treated plants had larger xylem vessel area, higher stem specific hydraulic conductivity, thicker stems, and lower shoot/root dry matter and specific leaf area. These changes were opposite to those induced by C. michiganensis subsp. michiganensis. We conclude that A. brasilense favoured a better adjustment of plant-water relations by several mechanisms, and thus, transitorily alleviated symptoms expression of a vascular disease.     

Physiological Responses of Calcium-Efficient and Calcium-Inefficient Tomato Genotypes to Differential Bacterial Wilt Conditions

Bacterial wilt (BW) disease, caused by Ralstonia solanacearum, can severely limit tomato (Solanum lycopersicum) production in southern United States. Tomato genotypes display variable responses to BW disease. The physiological mechanism underlying BW resistance is not well understood. In this study, experiments were conducted to compare two tomato genotypes, PI117566 [calcium (CA)–efficient] and PI109315 (Ca-inefficient), for wilting, shoot growth, final fresh weight, and shoot Ca concentrations when inoculated with R. solanacearum. The inoculation concentration of R. solanacearum varied from 0 to 10⁸ colony forming units/ml. Genotype PI109315 appear to be more BW-resistant compared with genotype PI117566 under sufficient Ca conditions. Furthermore, we found that genotype PI109315 had greater shoot growth and final fresh weight than genotype PI117566. These findings implied that Ca-efficient tomato genotypes may not play a key role in BW resistance of tomato under sufficient Ca conditions because Ca efficiency showed no effect on the suppression of BW.     

Manganese toxicity alleviated by mycorrhizae in soybean

Abstract

Soybean (Glycine max (L.) Merr.) plants nodulated with Bradyrhizobium japonicum. Nitragin strain 61A118, were grown with or without the vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. Gerd.) Gerd. and Trappe in pot cultures in soil high (40.4 μg/g) in available Mn. Leaves of the nonVAM plants showed severe symptoms of Mn toxicity and had toxic (314 μg/g) concentrations of Mn in the foliage. NonVAM plants had significantly lower dry weights and nodule mass than VAM plants. Concentrations of Mn in the VAM plants were significantly (P<0.05) lower than in the nonVAM plants, and there were no symptoms of Mn toxicity. Both VAM and nonVAM plants had a significant negative correlation between shoot dry mass and leaf Mn concentration. Since levels of Mn increased with increasing VAM‐fungal colonization, we conclude that it was not the VAM condition per se which alleviated Mn toxicity. We suggest that the significantly higher levels of Mn in the leaves (P<0.05) and the roots (P<0.001) of nonVAM plants was due to increased uptake of Mn by the nonVAM plants. This exudation, which are generally observed in nonVAM plants, and to the role of such exudates in solubilizing MnO₂ and chelating the resulting Mn²? for facilitated absorption.