Alex Seguel

TILLAGE EFFECT ON SOIL ORGANIC MATTER, MYCORRHIZAL HYPHAE AND AGGREGATES IN A MEDITERRANEAN AGROECOSYSTEM

Arbuscular mycorrhizal fungi (AMF) and their product glomalin (GRSP) play a decisive role in the soil aggregation, affecting the carbon (C) dynamics in agroecosystems. Tillage affects the AMF activity and GRSP content, influencing the stability and the soil C forms as well. The aim of this study was to compare the effect of no tillage (NT) and conventional tillage (CT) on: i) arbuscular mycorrhizal hyphal length and GRSP content; ii) the nature of soil organic matter by means of physical fractionation (free particulate organic matter (fPOM); occluded particulate organic matter (oPOM) and mineral-associated soil organic matter (Mineral)), as well as chemical fractionation (fulvic acid, humic acid and humin), and iii) the relationships between AMF parameters, soil carbon and water stable aggregates (WSA) in a Mollisol of Central Chile managed for 6 years under NT and CT using a wheat-corn rotation. Higher values in the AMF hyphal length, GRSP and WSA in NT compared with CT were observed. Significant relationships were found between GRSP and WSA (r = 0.66, p < 0.01) and total mycelium and GRSP (r = 0.58, p< 0.05). The total carbon increased 44% under NT compared with CT. The chemical fractionation showed percentage greater than 95% for humim in both treatments. Physical fractionation indicates that the higher part of the SOC (89.4 - 95.1%) was associated with the mineral fraction.

Effect of Compost Application on Some Properties of a Volcanic Soil from Central South Chile

Soil compost application is a common soil management practice used by small farmers of Central-South Chile that produces positive effects on soil properties and also promotes presence and activity of arbuscular mycorrhizal fungi (AMF). This fungi form symbiosis with plant roots improving plant nutrition, as well as producing glomalin, a glycoprotein that has been associated with soil aggregation stability. Therefore, the aim of this study was to evaluate, in an Ultisol from Central-South Chile, the effect of different doses of compost on some soil characteristics at the end of the third year of a crop sequence including wheat (Triticum aestivum L.), bean (Phaseolus vulgaris L.), and grassland (Lolium multiflorum Lam. associated with Trifolium repens L.). Studied soil characteristics included chemical (pH, available-P, organic C), biological (C and N biomass, AMF spore number, root colonization percentage, mycelium length, and glomalin content), as well as physical parameters (water holding capacity [WHC], and water stable aggregates [WSA]). Results showed that, in general, compost application increased soil

Role of arbuscular mycorrhizal symbiosis in phosphorus-uptake efficiency and aluminium tolerance in barley growing in acid soils

Abstract. Arbuscular mycorrhizal fungi (AMF) play an important role in protecting plant growth against such stresses as phytotoxic aluminium (Al) in soil. To understand some of the AMF interactions that relate to amelioration of Al phytotoxicity and phosphorus (P)-uptake efficiency in barley (Hordeum vulgare L.), this study examined the effect of soil Al levels and mycorrhizal symbiosis on plant response, including root colonisation, AMF propagules and glomalin production. A greenhouse experiment was conducted using two native barley cultivars, Sebastián and Aurora, grown in an acidic soil at two Al-saturation levels (80% Al-sat, unlimed soil; 7% Al-sat, limed soil) with and without AMF propagules. Root dry weight, total and colonised root lengths, and root P and Al contents were determined at 60 and 150 days after sowing. AMF spore density, total hyphal length, glomalin-related soil protein (GRSP) and Al bound to GRSP (Al-GRSP) were analysed at final harvest. AM root colonisation was not inhibited in limed soil, mycorrhizal propagule numbers increased at high Al levels, and Al-GRSP ranged from 5.6% to 8.3% of the total GRSP weight. These values also increased in unlimed soil, particularly those associated with cv. Aurora. Root Al concentration correlated inversely with AMF spores (r = –0.85, P < 0.001) and Al-GRSP (r = –0.72, P < 0.01), but only in plants growing in limed soil. Conversely, the AMF treatments in which Al was present showed a greater relationship between total root length and both root Al (r = –0.72, P < 0.01) and root P (r = 0.66, P < 0.01) concentrations. Sebastián showed a greater response to lime, whereas Aurora responded better to mycorrhizal presence. The relative growth rate of roots, P uptake efficiency and mycorrhizal parameters such as root colonisation, spores, hyphae and GRSP showed Aurora to be more Al-tolerant than Sebastián. In conclusion, the greater rate of increase of AM propagules, GRSP and Al-GRSP associated with cv. Aurora supports the hypothesis that AMF play an important role in the Al-tolerance capacity and P-uptake efficiency of H. vulgare growing in soils with high Al levels.

Arbuscular Mycorrhizal symbiosis in four Al-tolerant wheat genotypes grown in an acidic Andisol

Arbuscular mycorrhizal (AM) fungi play an important role in protecting host plant against phytotoxic aluminum (Al) in soil. The aim of this work was to analyze the effect of AM fungi native from acid soil on the growth of four Al-tolerant wheat (Triticum aestivum L.) genotypes. A greenhouse experiment was conducted using three near isogenic Chilean wheat genotypes (‘Crac’, ‘Invento’ and ‘Otto’) and one of recognized Al-tolerance (‘Atlas 66’) which were grown in an acid Andisol with 34% Al-saturation. The plant dry biomass and root colonization were determined at six early growth stages and AM spore density, glomalin (as GRSP) and acid phosphatase (P-ase) activity were analyzed at two stages; i) 11 days after sowing -DAS-, and at 60 DAS. Results showed that in all genotypes AM root colonization was not inhibited in spite of high soil Al saturation in the soil and a significant root colonization degree was observed at the first phenological stage mainly in the native wheat genotypes. Also, ‘Crac’ and ‘Invento’ genotypes showed the highest densities of AM spores and GRSP production. All wheat cultivars increased the P-ase activity overtime. Root biomass correlated positive and significantly with root colonization (r=0.71; P<0.001) and inversely with AM spores (r=-0.61; P<0.001). ‘Atlas 66’ showed a high adaptability to grow in acid conditions but produced the lesser amounts of AM propagules, which suggest that this genotype would show Al-tolerance mechanisms not fully associated to AM symbiosis as the Chilean wheat cultivars do. In conclusion, the higher early root colonization, AM spores and GRSP production associated to native wheat genotypes could indicate that AM symbiosis play a principal role in the Al tolerance capacity of T. aestivum developed in those soils with high Al levels and fungal native populations adapted to this conditions.

Phosphorus acquisition by three wheat cultivars contrasting in aluminium tolerance growing in an aluminium-rich volcanic soil

Abstract. Phosphorus (P) deficiency and aluminium (Al) phytotoxicity are major limitations for crop yield in acid soils. To ameliorate such limitations, agricultural management includes application of lime and P fertilisers, and the use of Al-tolerant plant genotypes. The mechanisms of Al tolerance and P efficiency may be closely related through strategies that decrease the toxicity of the Al3+ ion and increase P availability in soils. However, the effects of soils with high Al saturation on P acquisition by wheat have been little studied under field conditions. The aim of this work was to study Al–P interactions on wheat genotypes of contrasting Al tolerance when grown under field conditions in a volcanic soil with high Al saturation (32%) and low pH (5.0). A field-plot experiment was performed with winter wheat genotypes, two Al-tolerant (TCRB14 and TINB14) and one Al-sensitive (STKI14), with application of 0, 44 and 88 kg P ha–1. At the end of tillering and after physiological maturity (90 and 210 days after sowing), plants were harvested and yield and P and Al concentrations in shoots and roots were measured. Soil acid phosphatase, root arbuscular mycorrhizal (AM) colonisation, AM spore number and soil glomalin were determined. Shoot and root production and P uptake were higher in Al-tolerant genotypes than the sensitive genotype. In addition, root AM colonisation and soil acid phosphatase activity were also higher in tolerant genotypes. By contrast, Al concentration in shoots and roots was higher in the sensitive genotype with a concomitant decrease in P concentration. Grain yield of Al-tolerant genotypes was higher than of the Al-sensitive genotype with and without P fertiliser. Overall, the Al-tolerant genotypes were more effective at P acquisition from soil as well as from P fertiliser added, suggesting that plant traits such as Al tolerance, P efficiency, and AM colonisation potential co-operate in overcoming adverse acid soil conditions.

Arbuscular Mycorrhizal Fungi Improve Tolerance of Agricultural Plants to Cope Abiotic Stress Conditions

Abiotic stresses have strong impact on agriculture, decreasing the stability of agroecosystems worldwide, due mainly to water and nutrient limitations and the presence of toxic elements. Several studies have demonstrated that soil microorganisms can improve plant growth, even more when plants are under stressful conditions, being probably the most important are the arbuscular mycorrhizal fungi (AMF). This kind of fungi forms symbiosis with approximately 80% of plant species, including the majority of agricultural plants, and is present in all terrestrial ecosystems. Via its extraradical mycelium, the AMF can improve the absorption of water and nutrients of their host plants under stress conditions, as well as contribute to cope with the presence of toxic elements such as phytotoxic aluminum and other toxic metal(loid)s, increasing plant growth and crop production. Moreover, several studies have determined that AMF strains isolated from agroecosystems affected by different abiotic limiting conditions enhance the growth of plants than those isolated from soils without such limiting condition. In this chapter we describe the main ways by which AMF contribute to the plant tolerance to cope the abovementioned abiotic stresses. Moreover, the physiological, biochemical, and molecular bases that explain the responses mediated by AMF in host plants are covered. Finally, biotechnological prospects of AMF present under stress conditions and their potential use as bio-inoculants are presented.

Contribution of inoculation with arbuscular mycorrhizal fungi to the bioremediation of a copper contaminated soil using Oenothera picensis

The Bradford-reactive soil protein (BRSP) fraction includes glomalin, a glycoprotein produced by arbuscular mycorrhizal (AM) fungi that is able to bind some metals, such as copper (Cu), which could promote the bioremediation of Cu-polluted soils. This study aimed to analyze the Cu-binding capacity of BRSP in Oenothera picensis that was inoculated or not inoculated with AM fungi. O. picensis plants were established in a Cu contaminated sterilized soil and treated with the following: i) uninoculated (-M); ii) inoculated with native AM fungal propagules (+M); or iii) inoculated with a Claroideoglomus claroideum (CC) strain isolated from non-contaminated soil. In each case, five Cu levels were applied to the soil (basal level 497.3 mg Cu kg-1): 0 (T1); 75 (T2); 150 (T3); 225 (T4); and 300 mg Cu kg-1 (T5). A high BRSP accumulation in AM inoculated treatments, especially with CC, was observed. A higher Cu-bound-to-BRSP content was found with increasing Cu concentrations, representing up to 20-22% of the total Cu in the soil. Moreover, a higher root Cu concentration in +M was observed. These results suggest a high Cu binding capacity by BRSP, which is a relevant aspect to consider in the design of bioremediation programs together with the selection of endemic metallophytes and AM fungal strains, which are able to produce glomalin at high quantities.

Phosphorus acquisition efficiency related to root traits: Is mycorrhizal symbiosis a key factor to wheat and barley cropping?

Wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) are major crops cultivated around the world, thus playing a crucial role on human diet. Remarkably, the growing human population requires a significant increase in agricultural production in order to feed everybody. In this context, phosphorus (P) management is a key factor as it is component of organic molecules such as nucleic acids, ATP and phospholipids, and it is the most abundant macronutrient in biomass after nitrogen (N), although being one of the scarcest elements in the lithosphere. In general, P fertilization has low efficiency, as only a fraction of the applied P is acquired by roots, leaving a substantial amount to be accumulated in soil as not readily available P. Breeding for P-efficient cultivars is a relatively low cost alternative and can be done through two mechanisms: i) improving P use efficiency (PUE), and/or ii) P acquisition efficiency (PAE). PUE is related to the internal allocation/mobilization of P, and is usually represented by the amount of P accumulated per biomass. PAE relies on roots ability to acquire P from the soil, and is commonly expressed as the relative difference of P acquired under low and high P availability conditions. In this review, plant adaptations related to improved PAE are described, with emphasis on arbuscular mycorrhizal (AM) symbiosis, which is generally accepted to enhance plant P acquisition. A state of the art (1980–2018) of AM growth responses and P uptake in wheat and barley is made to discuss about the commonly accepted growth promoting effect and P increased uptake by AM fungi and the contrasting evidence about the generally accepted lack of positive responses in both plant species. Finally, the mechanisms by which AM symbiosis can affect wheat and barley PAE are discussed, highlighting the importance of considering AM functional diversity on future studies and the necessity to improve PAE definition by considering the carbon trading between all the directly related PAE traits and its return to the host plant.