Plant-bacteria interactions with special emphasis on the kallar grass association

Kallar grass is a highly salt-tolerant grass grown as a pioneer plant on alkaline, salt-affected soils in Pakistan. Nitrogen-fixing bacteria and kallar grass were found to be in close association, which was even root-zone specific: rhizoplane and endorhizosphere were colonized by two different populations. Among theAzospirillum isolates originating from the root surface, some were of a new species, now namedA. halopraeferens. To study plant-bacterium interactions, this natural kallar grass association was chosen. The possible role of bacterial chemotaxis and oxygen tolerance are discussed.     

Attachment,colonization and proliferation ofAzospirillum brasilense andEnterobacter spp. on root surface of grasses

Root colonization studies, employing immunofluorescence and using locally isolated strains, showed thatEnterbacter sp. QH7 andEnterobacter agglomerans AX12 attached more readily to the roots of most plants compared withAzospirillum brasilense JM82. Heat treatment of either root or inoculum significantly decreased the adsorption of bacteria to the root surface. Kallar grass and rice root exudates sustained the growth ofA. brasilense JM82,Enterobacter sp. QH7 andE. agglomerans AX12 in Hoagland and Fahraeus medium. All the strains colonized kallar grass and rice roots in an axenic culture system. However, in studies involving mixed cultures,A. brasilense JM82 was inhibited byEnterobacter sp. QH7 in kallar grass rhizosphere and the simultaneous presence ofEnterobacter sp. QH7 andE. agglomerans AX12 suppressed the growth ofA. brasilense JM82 in rice rhizosphere. The bacterial colonization pattern changed from dispersed to aggregated within 3 days of inoculation. The colonization sites corresponded mainly to the areas where root mucigel was present. The area around the point of emergence of lateral roots usually showed maximum colonization.     

水稻根系通气组织与根系泌氧及根际硝化作用的关系

通过根箱土培试验研究了不同产量籼稻品种中旱22(ZH,高产品种)及禾盛10号(HS,低产品种)苗期根系生长、通气组织发育、根系径向泌氧量(radial oxygen loss,ROL)以及根表和根际土壤硝化强度差异。结果表明,除水稻播种40 d时二者根数量和根干重无显著差异外,ZH根直径、根数量和根干重均显著高于HS,二者差异尤其表现在根系生物量差异。两个水稻品种在距根尖20 mm处均可见辐射状通气组织,ZH皮层薄壁细胞已经完全崩溃形成连接中柱和外皮层的纵向气腔,而HS皮层薄壁细胞未发生完全离解,但仍能观察到明显的连接中柱和外皮层的纵向气腔的形成。同时ZH外皮层厚壁细胞体积较小,排列紧密,细胞壁增厚程度大;而HS外皮层厚壁细胞体积相对较大,排列疏松,细胞壁增厚程度相对较小。表明高产品种通气组织发育比低产品种更加完善,表现为ZH根孔隙度(porosity of root,POR)显著高于HS,且高产品种对水稻根系ROL的屏蔽作用较低产品种更强,为根系提供更充足的氧气供应,促进根系生长。除了水稻播种后40 d时ZH和HS单根ROL无显著差异外(P<0.05),ZH单株、单位重量以及单根ROL均显著高于HS(P<0.01)。两个水稻品种硝化强度均表现为根际土壤显著高于根表土壤 (P<0.01),前者大约是后者的3-6倍。两个品种根表土壤硝化强度无显著差异,而ZH根际土壤硝化强度均显著高于HS。相关性分析结果表明水稻根际土壤硝化强度和整株水稻ROL呈极显著正相关关系(r=0.803,P<0.01),和水稻POR也呈现极显著正相关关系(r=0.808,P<0.01),同时和根系直径、数量和干重均呈极显著正相关关系(P<0.01)。而根表土壤硝化强度和以上指标均无相关关系。由于硝化作用是好氧过程,因此高产品种由于根系发达,通气组织发育好,相应ROL也较大,造成根际土壤氧气含量高,从而可能导致根际土壤硝化强度显著高于低产品种。     

Gas Transport through the Root–shoot Transition Zone of Rice Tillers

Rice plants (Oryza sativa L.) are mainly cultivated in flooded paddy fields and are thus dependent on oxygen transport through the plant to maintain aerobic root metabolism. This gas transport is effectuated through the aerenchyma of roots and shoots. However, the efficiency of gas transport through the root–shoot transition zone is disputed and there are indications that the root–shoot transition zone may represent one of the largest resistances for gas transport. Therefore, we present gas conductance measurements of the root–shoot transition of individual rice tillers measured using SF₆. SF₆ was detected with a highly advanced laser based photoacoustic detection scheme allowing sensitive, high resolution measurements. In conjunction with these measurements, various plant morphological parameters were quantified. These measurements indeed indicate that the conductance at the root–shoot transition may be much smaller than the conductance of root and shoot aerenchyma within the rice plant. Conductance was strongly correlated to tiller transverse area. After elimination of tiller area from the conductance equation, the resulting permeance coefficient was still correlated to tiller area, but negatively and related to the process of radial tiller expansion. In addition, a decrease in the permeance coefficient was also observed for increasing distance from the plant centre. No correlation was found with tiller type or age of the mother tiller. Incorporation of estimates of the conductance of the root–shoot transition zone coupled to plant morphological parameters will allow considerable improvement of understanding and models on gas transport through plants.     

涝渍对3个树种生长、组织孔隙度和渗漏氧的影响

为了了解落羽杉(Taxodium distichum)、乌桕(Sapium sebiferum)和美国山核桃(Carya illinoensis)等树种的耐涝机制, 采用盆栽模拟涝渍环境的试验方法, 设置了淹水、渍水和对照3个处理, 测定了一年生落羽杉、乌桕和美国山核桃实生苗的生长、组织孔隙度、根氧消耗等指标。结果表明, 涝渍处理抑制了落羽杉、乌桕和美国山核桃的生物量和生物量增量(渍水处理下落羽杉的生长得到了促进), 增加了3树种的根冠比, 从生物量和生物量增量下降幅度来评价, 落羽杉的耐涝性最强, 其次为美国山核桃。淹水和渍水处理下, 落羽杉、乌桕和美国山核桃的根、茎和叶中的组织孔隙度显著增加, 且随着处理时间的延长, 各器官的组织孔隙度有增加的趋势, 3个树种中, 落羽杉的根、茎和叶中的组织孔隙度均较其他2个树种高。淹水和渍水处理下, 移除茎明显增加了落羽杉、美国山核桃和乌桕的根的氧消耗, 表明涝渍处理增强了O₂在3个树种体内的运输并通过根系扩散到涝渍土壤中的能力, 并且随着处理时间的延长, 3个树种体内运输O₂并扩散到土壤中的能力有逐渐增强的趋势。因此, 涝渍环境总体上抑制了落羽杉、乌桕和美国山核桃等树种的生长, 但各树种为了适应这种生长环境, 形成了大量的通气组织, 从而导致各器官组织孔隙度的增加, 增强了O₂通过植物体运输到根系并扩散到土壤中的能力, 解决了根系及根际缺氧的矛盾。     

不同渗氧能力水稻品种对砷的耐性和积累

水稻是目前世界上(尤其是东南亚)最主要的粮食作物之一,也是砷(As)通过食物链进入人体的主要途径。日益加剧的土壤砷污染,严重影响了稻米的产量和品质,进而威胁着人体健康。通过温室实验,研究CNT 87059-3、玉香油占和巴西陆稻3种不同渗氧能力的水稻品种在不同砷浓度处理下的生长情况和砷积累特征,结果表明:(1)渗氧能力强的玉香油占砷耐性指数最高,砷处理浓度为2 mg/L时耐性指数高达0.71,而CNT 87059-3的耐性指数为0.55,巴西陆稻仅有0.17;(2)随着砷处理浓度的升高,3种水稻品种的生物量呈现下降趋势,但渗氧能力强的玉香油占较其它两品种生物量的下降幅度小;(3)在不同砷浓度处理下水稻地下部分的砷含量有显著性差异(P0.001),且同种砷浓度处理下不同水稻品种的地下部分砷含量也存在显著性差异(P0.01),渗氧能力较强的水稻品种与渗氧能力较弱的品种相比能显著降低砷在根部(地下部分)的积累。水稻渗氧能力与其砷耐性和砷积累有显著相关性,渗氧能力越强,水稻的砷耐性越强,砷的积累量越少。因此,通过筛选渗氧能力强的水稻品种,有望降低污染农田水稻的砷含量和健康风险。     

Root-Zone-Specific Oxygen Tolerance of Azospirillum spp. and Diazotrophic Rods Closely Associated with Kallar Grass

The effect of oxygen on N₂-dependent growth of two Azospirillum strains and two diazotrophic rods closely associated with roots of Kallar grass (Leptochloa fusca) was studied. To enable precise comparison, bacteria were grown in dissolved-oxygen-controlled batch and continuous cultures. Steady states were obtained from about 1 to 30 μM O₂, some of them being carbon limited. All strains needed a minimum amount of oxygen for N₂-dependent growth. Nitrogen contents between 10 and 13% of cell dry weight were observed. The response of steady-state cultures to increasing O₂ concentrations suggested that carbon limitation shifted to internal nitrogen limitation when N₂ fixation became so low that the bacteria could no longer meet their requirements for fixed nitrogen. For Azospirillum lipoferum Rp5, increase of the dilution rate resulted in decreased N₂ fixation in steady-state cultures with internal nitrogen limitation. Oxygen tolerance was found to be strain specific in A. lipoferum with strain Sp59b as a reference organism. Oxygen tolerance of strains from Kallar grass was found to be root zone specific. A. halopraeferens Au 4 and A. lipoferum Rp5, predominating on the rhizoplane of Kallar grass, and strains H6a2 and BH72, predominating in the endorhizosphere, differed in their oxygen tolerance profiles. Strains H6a2 and BH72 still grew and fixed nitrogen in steady-state cultures at O₂ concentrations exceeding those which absolutely inhibited nitrogen fixation of both Azospirillum strains. It is proposed that root-zone-specific oxygen tolerance reflects an adaptation of the isolates to the microenvironments provided by the host plant.     

Stimulation of ethylene production and gas-space (aerenchyma) formation in adventitious roots of Zea mays L. by small partial pressures of oxygen

Adventitious roots of two to four-weekold intact plants of Zea mays L. (cv. LG11) were shorter but less dense after extending into stagnant, non-aerated nutrient solution than into solution continuously aerated with air. Dissolved oxygen in the non-aerated solutions decreased from 21 kPa to 3–9 kPa within 24 h. When oxygen partial pressures similar to those found in non-aerated solutions (3, 5 and 12 kPa) were applied for 7 d to root systems growing in vigorously bubbled solutions, the volume of gas-space in the cortex (aerenchyma) was increased several fold. This stimulation of aerenchyma was associated with faster ethylene production by 45-mm-long apical root segments. When ethylene production by roots exposed to 5 kPa oxygen was inhibited by aminoethoxyvinylglycine (AVG) dissolved in the nutrient solution, aerenchyma formation was also retarded. The effect of AVG was reversible by concomitant applications of 1-aminocyclopropane-1-carboxylic acid, an immediate precursor of ethylene. Addition of silver nitrate, an inhibitor of ethylene action, to the nutrient solution also prevented the development of aerenchyma in roots given 5 kPa oxygen. Treating roots with only 1 kPa oxygen stimulated ethylene production but failed to promote gas-space formation. These severely oxygen-deficient roots seemed insensitive to the ethylene produced since a supplement of exogeneous ethylene that promoted aerenchyma development in nutrient solution aerated with air (21 kPa oxygen) failed to do so in nutrient solution supplied with 1 kPa oxygen. Both ethylene production and aerenchyma formation were almost completely halted when roots were exposed to nutrient solutions devoid of oxygen. Thus both processes require oxygen and are stimulated by oxygen-deficient surroundings in the 3-to 12-kPa range of oxygen partial pressures when compared with rates observed in air (21 kPa oxygen).Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - AVG aminoethoxyvinylglycine     

Soil carbon dioxide venting through rice roots

The growth of rice in submerged soils depends on its ability to form continuous gas channels—aerenchyma—through which oxygen (O₂) diffuses from the shoots to aerate the roots. Less well understood is the extent to which aerenchyma permits venting of respiratory carbon dioxide (CO₂) in the opposite direction. Large, potentially toxic concentrations of dissolved CO₂ develop in submerged rice soils. We show using X‐ray computed tomography and image‐based mathematical modelling that CO₂ venting through rice roots is far greater than thought hitherto. We found rates of venting equivalent to a third of the daily CO₂ fixation in photosynthesis. Without this venting through the roots, the concentrations of CO₂ and associated bicarbonate (HCO₃^?) in root cells would have been well above levels known to be toxic to roots. Removal of CO₂ and hence carbonic acid (H₂CO₃) from the soil was sufficient to increase the pH in the rhizosphere close to the roots by 0.7 units, which is sufficient to solubilize or immobilize various nutrients and toxicants. A sensitivity analysis of the model showed that such changes are expected for a wide range of plant and soil conditions.     

Oxygen loss from Spartina alterniflora and its relationship to salt marsh oxygen balance

Spartina alterniflora has been reported to lose significant amounts of oxygen to its rhizosphere with potentially important effects on salt-marsh biogeochemical cycling and plant productivity. The potential significance of this oxidative pathway was evaluated using laboratory split-chamber experiments to quantify oxygen loss from intact root systems under a wide variety of pre-treatment and incubation conditions including antibiotics to inhibit microbial respiration. The aerenchyma system of S. alterniflora was found to transport O₂, N₂, Ar, and CH₄ from above-ground sources to its below-ground roots and rhizomes. While non-respiratory gases were observed to move from the lacunae to water bathing the root systems, net O₂ loss did not occur; instead oxygen present outside of the roots/rhizomes was consumed. Net oxygen loss was found when resistance to gas transport was reduced in the lacunae-rhizosphere pathway by placing the root systems in a gas phase and when plant respiration was significantly reduced. Root system respiration appeared to be the major variable in the plant oxygen balance. When root and rhizome respiration was inhibited using poisons or lowered by cooling, the oxygen deficit was greatly reduced and oxygen loss was indicated. The effect of seasonal temperature changes on root system oxygen deficit presents a possible explanation as to why Spartina produces root systems with respiration rates that cannot be supported by gas transport. Overall, while oxygen loss from individual plant roots is likely, integrating measured root system oxygen loss with geochemical data indicates that the mass amount of oxygen lost from S. alterniflora root systems is small compared to the total oxygen balance of vegetated salt marsh sediments.     

Close Association of Azospirillum and Diazotrophic Rods with Different Root Zones of Kallar Grass

The populations of diazotrophic and nondiazotrophic bacteria were estimated in the endorhizosphere and on the rhizoplane of Kallar grass (Leptochloa fusca) and in nonrhizosphere soil. Microaerophilic diazotrophs were counted by the most-probable-number method, using two semisolid malate media, one of them adapted to the saline-sodic Kallar grass soil. Plate counts of aerobic heterotrophic bacteria were done on nutrient agar. The dominating N₂-fixing bacteria were differentiated by morphological, serological, and physiological criteria. Isolates, which could not be assigned to a known species, were shown to fix nitrogen unequivocally by ¹⁵N₂ incorporation. On the rhizoplane we found 2.0 × 10⁷ diazotrophs per g (dry weight) of root, which consisted in equal numbers of Azospirillum lipoferum and Azospirillum-like bacteria showing characteristics different from those of known Azospirillum species. Surface sterilization by NaOCI treatment effectively reduced the rhizoplane population, so that bacteria released by homogenization of roots could be regarded as endorhizosphere bacteria. Azospirillum spp. were not detected in the endorhizosphere, but diazotrophic, motile, straight rods producing a yellow pigment occurred with 7.3 × 10⁷ cells per g (dry weight) of root in the root interior. In nonrhizosphere soil we found 3.1 × 10⁴ nitrogen-fixing bacteria per g. Diazotrophs were preferentially enriched in the Kallar grass rhizosphere. In nonrhizosphere soil they made up 0.2% of the total aerobic heterotrophic microflora, on the rhizoplane they made up 7.1%, and in the endorhizosphere they made up 85%. Owing to high numbers in and on roots and their preferential enrichment, we concluded that diazotrophs are in close association with Kallar grass. They formed entirely different populations on the rhizoplane and in the endorhizosphere.     

Biological Dinitrogen Fixation in Gramineae and Palm Trees

Biological nitrogen fixation (BNF) in the Gramineae family has been well documented, but a complete understanding of this issue is needed to turn the research into a practical approach. The literature has a long and diverse list of diazotrophic bacteria found colonizing several plant tissues, such as roots, stems, leaves, and trash as well as the rhizosphere. However, only a limited amount of research has focussed on existing associations of N₂-fixing microorganisms with grasses or cereal, especially for BNF inputs and ecological studies under field conditions. The recent discovery of the endophytic diazotroph bacteria such as Acetobacter diazotrophicus, Herbaspirillum spp. and Azoarcus spp. colonizing the interior of sugarcane, rice, Kallar grass (Leptochloa fusca (L.) Kunth), respectively, and other species of grasses as well as cereals has led to a considerable interest in exploring these novel associations. There is a general consensus that plant genotype is a key factor to higher contributions of BNF together with the selection of more efficient bacterial strains. This review summarizes the present data on this field and introduces the discovery of a new group of diazotrophic bacteria colonizing palm trees and therefore opening a future perspective for using these plants, especially African oil palm, to replace diesel as a fuel.     

Similarity and diversity in adventitious root anatomy as related to root aeration among a range of wetland and dryland grass species

Assessments of the anatomy, porosity and profiles of radial O₂ loss from adventitious roots of 10 species in the Poaceae (from four subfamilies) and two species in the Cyperaceae identified a combination of features characteristic of species that inhabit wetland environments. These include a strong barrier to radial O₂ loss in the basal regions of the adventitious roots and extensive aerenchyma formation when grown not only in stagnant but also in aerated nutrient solution. Adventitious root porosity was greater for plants grown in stagnant compared with aerated solution, for all 10 species in the Poaceae. The ‘wetland root’ archetype was best developed in Oryza sativa and the two species of the Cyperaceae, in which the stele contributed less than 5% of the root cross‐sectional area, the cells of the inner cortex were packed in a cuboidal arrangement, and aerenchyma was up to 35–52%. Variations of this root structure, in which the proportional and absolute area of stele was greater, with hexagonal arrangements of cells in the inner cortex and varying in the extent of aerenchyma formation, were present in the other wetland species from the subfamilies Pooideae, Panicoideae and Arundinoideae. Of particular interest were Vetiveria zizanoides and V. filipes, wetland grass species from the tribe Andropogoneae (the same tribe as sorghum, maize and sugarcane), that had a variant of the root anatomy found in rice. The results are promising with regard to enhancing these traits in waterlogging intolerant crops.     

Ethylene‐dependent aerenchyma formation in adventitious roots is regulated differently in rice and maize

In roots of gramineous plants, lysigenous aerenchyma is created by the death and lysis of cortical cells. Rice (Oryza sativa) constitutively forms aerenchyma under aerobic conditions, and its formation is further induced under oxygen‐deficient conditions. However, maize (Zea mays) develops aerenchyma only under oxygen‐deficient conditions. Ethylene is involved in lysigenous aerenchyma formation. Here, we investigated how ethylene‐dependent aerenchyma formation is differently regulated between rice and maize. For this purpose, in rice, we used the reduced culm number1 (rcn1) mutant, in which ethylene biosynthesis is suppressed. Ethylene is converted from 1‐aminocyclopropane‐1‐carboxylic acid (ACC) by the action of ACC oxidase (ACO). We found that OsACO5 was highly expressed in the wild type, but not in rcn1, under aerobic conditions, suggesting that OsACO5 contributes to aerenchyma formation in aerated rice roots. By contrast, the ACO genes in maize roots were weakly expressed under aerobic conditions, and thus ACC treatment did not effectively induce ethylene production or aerenchyma formation, unlike in rice. Aerenchyma formation in rice roots after the initiation of oxygen‐deficient conditions was faster and greater than that in maize. These results suggest that the difference in aerenchyma formation in rice and maize is due to their different mechanisms for regulating ethylene biosynthesis.     

Root-induced changes in radial oxygen loss, rhizosphere oxygen profile, and nitrification of two rice cultivars in Chinese red soil regions

Aims

To evaluate the external and internal morphological differences of roots that might influence rice root radial oxygen loss (ROL) and the corresponding rhizosphere nitrification activity, growth characteristics and nitrogen nutrition of rice.

Methods

The root ROL and rhizosphere oxygen profile were determined using a miniaturised Clark-type oxygen microelectrode system, and the rhizosphere nitrification activity was studied with a short-term nitrification activity assay.

Results

The rice biomass, nitrogen accumulation and nitrogen use efficiency (NUE) of ZH (high yield) were significantly higher than those of HS (low yield). The root biomass, number, diameter and porosity of ZH were also much greater than those of HS. The inner and surface oxygen concentrations of the root of ZH were significantly higher than those of HS. The order of paddy soil oxygen penetration depth was ZH?>?HS?>?CK, and the order of the oxygen concentrations detected in the water layer and rhizosphere soil was the same. The rhizosphere nitrification activity and nitrate concentration of ZH were significantly higher than those of HS.

Conclusions

More porous and thicker roots improved the individual root ROL, and more adventitious root numbers enhanced the entire plant ROL and correspondingly improved the rhizosphere nitrification activity, which might influence the growth and nitrogen nutrition of rice.     

The relationship between rhizosphere nitrification and nitrogen-use efficiency in rice plants

Two different rice cultivars, Yangdao 6 [Indica rice cultivar with high nitrogen-use efficiency (NUE)] and Nongken 57 (Japonica rice cultivar with low NUE) were used to study the relationship between NUE and nitrification activity in the rice seedling rhizosphere soil using a rhizobox with three compartments, and a soil-slicing method. The roots of both rice cultivars developed aerenchyma tissue [expressed as percentage porosity of root (POR)], but Yangdao 6 showed better development than Nongken 57. This root morphology change results in more radial oxygen loss (ROL) into the rhizosphere. Leaf glutamine synthetase activity (GSA) and nitrate (NO3-) reductase activity (NRA) of Yangdao 6 were significantly higher than those of Nongken 57, while there was no significant difference in root NRA between the cultivars. The nitrification activities were maximal in rhizosphere soil, followed by those in the bulk soil and the root surface for both cultivars. The rhizosphere nitrification activity, NO3- concentration and abundance of ammonia-oxidizing bacteria (AOB) associated with Yangdao 6 were always higher than those of Nongken 57. Therefore, we conclude that the greater N uptake by Yangdao 6 when compared to Nongken 57 can be mainly attributed to the bigger capacity for nitrification in Yangdao 6.     

The physiological mechanism of enhanced oxidizing capacity of rice (Oryza sativa L.) roots induced by phosphorus deficiency

Plants show various responses to phosphorus (P) deficiency. Root oxidizing capacity enhancement is one of adaptive mechanisms for rice (Oryza sativa L.) to P deficiency. However, it remains unclear how P deficiency enhances the root oxidizing capacity. In this study, rice seedlings were treated in P-deficient nutrient solution for different periods. Variations of reactive oxygen species (ROS), antioxidant enzyme activity, root lignin content, root porosity, root oxygen release, total oxidative substances and root structural changes in rice roots in response to P-sufficient and P-deficient treatments were investigated. Results indicated that P deficiency induced the production of H₂O₂ and O ₂ ^·? in roots significantly, which reached their maximum after 1- to 2-day P-deficient treatment. Interestingly, the endogenous total oxidative substances kept stable in rice roots. P deficiency increased the activities of peroxidase and superoxide dismutase by 89.5 and 51.8 % after 4-day P-deficient treatment, respectively. Moreover, one-day P deficiency elevated lignin accumulation. Root porosity of rice seedling under 2-day P-deficient treatment was 19.8 % higher than that under P-sufficient treatment. P deficiency also enhanced the release of both O₂ and total oxidative substances after 1- to 4-day P deficiency. In addition, results from electronic microscopy indicated that the thickness of root cell wall tended to increase after 2-day P-deficient treatment. Taken together, our results suggested that P-deficiency-induced enhancement of root oxidizing capacity in rice roots was probably associated with ROS production, antioxidant enzyme activity increment in root tissues, and the release of O₂ and oxidative substances from root inside to rhizosphere.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

Formation and function of secondary aerenchyma in hypocotyl, roots and nodules of soybean (Glycine max) under flooded conditions

Flooding is a major problem in many areas of the world and soybean is susceptible to the stress. Understanding the morphological mechanisms of flooding tolerance is important for developing flood-tolerant genotypes. We investigated secondary aerenchyma formation and function in soybean (Glycine max) seedlings grown under flooded conditions. Secondary aerenchyma, a white and spongy tissue, was formed in the hypocotyl, tap root, adventitious roots and root nodules after 3 weeks of flooding. Under irrigated conditions aerenchyma development was either absent or rare and phellem was formed in the hypocotyl, tap root, adventitious roots and root nodules. Secondary meristem partially appeared at the outer parts of the interfascicular cambium and girdled the stele, and then cells differentiated to construct secondary aerenchyma in the flooded hypocotyl. These morphological changes proceeded for 4 days after the initiation of the flooding. After 14 days of treatment, porosity exceeded 30% in flooded hypocotyl with well-developed secondary aerenchyma, while it was below 10% in hypocotyl of irrigated plants that had no aerenchyma. When Vaseline was applied to the hypocotyl of plants from a flooded treatment to prevent the entry of atmospheric oxygen into secondary aerenchyma, plant growth, especially that of roots, was sharply inhibited. Thus secondary aerenchyma might be an adaptive response to flooding.     

Biocontrol of Fusarium Root Rot in the Common Bean (Phaseolus vulgaris L.) by using Symbiotic Glomus mosseae and Rhizobium leguminosarum

Abstract The interaction between VA mycorrhiza Glomus mosseae (Gm), root rodulating symbiont Rhizobium leguminosarum (Rl), and root rot pathogen Fusarium solani (Fs) on the common bean (Phaseolus vulgaris) in relation to plant growth, nutrient uptake, disease severity, rhizosphere microbial biomass, and nutrient availability was investigated. Mycorrhizal plants yielded significantly greater plant biomass and mobilized more N and P uptake as compared to nonmycorrhizal plants or those infected with Fs. However, the mycorrhizal root colonizing ability, in presence of Fs, was reduced by 27%, whereas Rl enhanced it by 37%. The inoculation of Gm, besides decreasing propagule number of Fs in the rhizosphere, decreased pathogenic root rot by 34 to 77%. However, in the presence of Rl, Gm-inoculated plants were more tolerant of the fungal root pathogen. The Gm + Rl inoculated plants not only had maximum plant biomass and root nodulation, but also exhibited higher microbial biomass, alkaline phosphatase activity, and available phosphorus in their rhizosphere. Rl, alone or in association with Gm, caused the maximum increase in mineral nitrogen (NH₄ ⁺ and NO₃ ⁻) content in soil. These results indicate that Gm has a vital role in inhibiting the root pathogen from invasion, more so in the presence of R. leguminosarum. Received: 26 February 1996; Revised: 12 July 1996