The importance of integration and scale in the arbuscular mycorrhizal symbiosis

The arbuscular mycorrhizal (AM) fungus contributes to system processes and functions at various hierarchical organizational levels, through their establishment of linkages and feedbacks between whole-plants and nutrient cycles. Even though these fungal mediated feedbacks and linkages involve lower-organizational level processes (e.g. photo-assimilate partitioning, interfacial assimilate uptake and transport mechanisms, intraradical versus extraradical fungal growth), they influence higher-organizational scales that affect community and ecosystem behavior (e.g. whole-plant photosynthesis, biodiversity, nutrient and carbon cycling, soil structure). Hence, incorporating AM fungi into research directed at understanding many of the diverse environmental issues confronting society will require knowledge of how these fungi respond to or initiate changes in vegetation dynamics, soil fertility or both. Within the last few years, the rapid advancement in the development of analytical tools has increased the resolution by which we are able to quantify the mycorrhizal symbiosis. It is important that these tools are applied within a conceptual framework that is temporally and spatially relevant to fungus and host. Unfortunately, many of the studies being conducted on the mycorrhizal symbiosis at lower organizational scales are concerned with questions directed solely at understanding fungus or host without awareness of what the plant physiologist or ecologist needs for integrating the mycorrhizal association into larger organizational scales or process levels. We show by using the flow of C from plant-to-fungus-to-soil, that through thoughtful integration, we have the ability to bridge different organizational scales. Thus, an essential need of mycorrhizal research is not only to better integrate the various disciplines of mycorrhizal research, but also to identify those relevant links and scales needing further investigation for understanding the larger-organizational level responses. The U.S. Government's right to retain a non-exclusive, royalty-free licence in and to any copyright is acknowledged. This revised version was published online in June 2006 with corrections to the Cover Date.     

根区交替控制灌溉条件下玉米根系吸水规律

为了阐明根区交替控制灌溉(CRDAI)条件下玉米根系吸水规律,通过田间试验,在沟灌垄植模式下采用根区交替控制灌溉研究玉米根区不同点位(沟位、坡位和垄位)的根长密度(RLD)及根系吸水动态。研究表明,根区土壤水分的干湿交替引起玉米RLD的空间动态变化,在垄位两侧不对称分布,并存在层间差异;土壤水分和RLD是根区交替控制灌溉下根系吸水速率的主要限制因素。在同一土层,根系吸水贡献率以垄位最大,沟位最低;玉米营养生长阶段,10—30 cm土层的根系吸水速率最大;玉米生殖生长阶段,20—70 cm为根系吸水速率最大的土层,根系吸水贡献率为43.21%—55.48%。研究阐明了交替控制灌溉下根系吸水与土壤水分、RLD间相互作用的动态规律,对控制灌溉下水分调控机理研究具有理论意义。     

Effects of plant-soil feedback on tree seedling growth under arid conditions

Aims Plants are able to influence their growing environment by changing biotic and abiotic soil conditions. These soil conditions in turn can influence plant growth conditions, which is called plant–soil feedback. Plant–soil feedback is known to be operative in a wide variety of ecosystems ranging from temperate grasslands to tropical rain forests. However, little is known about how it operates in arid environments. We examined the role of plant–soil feedbacks on tree seedling growth in relation to water availability as occurring in arid ecosystems along the west coast of South America.Methods In a two-phased greenhouse experiment, we compared plant–soil feedback effects under three water levels (no water, 10% gravimetric moisture and 15% gravimetric moisture). We used sterilized soil inoculated with soil collected from northwest Peru (Prosopis pallida forests) and from two sites in north-central Chile (Prosopis chilensis forest and scrublands without P. chilensis).Important findings Plant–soil feedbacks differed between plant species and soil origins, but water availability did not influence the feedback effects. Plant–soil feedbacks differed in direction and strength in the three soil origins studied. Plant–soil feedbacks of plants grown in Peruvian forest soil were negative for leaf biomass and positive for root length. In contrast, feedbacks were neutral for plants growing in Chilean scrubland soil and positive for leaf biomass for those growing in Chilean forest soil. Our results show that under arid conditions, effects of plant–soil feedback depend upon context. Moreover, the results suggest that plant–soil feedback can influence trade-offs between root growth and leaf biomass investment and as such that feedback interactions between plants and soil biota can make plants either more tolerant or vulnerable to droughts. Based on dissecting plant–soil feedbacks into aboveground and belowground tissue responses, we conclude that plant–soil feedback can enhance plant colonization in some arid ecosystems by promoting root growth.     

Carbon and Nitrogen Decoupling Under an 11-Year Drought in the Shortgrass Steppe

The frequency and magnitude of drought is expected to increase in the US Great Plains under future climate regimes. Although semiarid systems are considered highly resistant to water limitation, novel drought events could alter linkages among biogeochemical processes, and result in new feedbacks that influence the timescale of ecosystem recovery. We examined changes in carbon and nitrogen cycling in the last 2 years of an 11-year drought manipulation in the shortgrass steppe, and under the first 2 years of recovery from drought. We measured plant production, plant tissue chemistry, soil trace gas flux, and soil inorganic nitrogen dynamics to test the extent that this magnitude of drought altered carbon and nitrogen fluxes and how these changes affected post-drought dynamics. We found that soil inorganic nitrogen was up to five times higher under severe drought than under control conditions, but that this nitrogen may not have been accessible to plants and microbial communities during drought due to diffusion limitations. Drought plots had higher N₂O flux when they received equal rainfall pulses, showing that this accumulated N may be vulnerable to loss. In addition, plants in drought plots had higher tissue nitrogen for 2 years following drought. These results show that decadal-length droughts that may occur under future precipitation regimes are likely to alter ecosystem properties through interactions among precipitation, vegetation, and N cycling. Shifts in plant N, vulnerability of nitrogen to loss, and rainfall use efficiency that we observed are likely to affect the recovery time of semiarid systems subject to droughts of this magnitude.     

Understanding water deficit stress-induced changes in the basic metabolism of higher plants – biotechnologically and sustainably improving agriculture and the ecoenvironment in arid regions of the globe

Water is vital for plant growth, development and productivity. Permanent or temporary water deficit stress limits the growth and distribution of natural and artificial vegetation and the performance of cultivated plants (crops) more than any other environmental factor. Productive and sustainable agriculture necessitates growing plants (crops) in arid and semiarid regions with less input of precious resources such as fresh water. For a better understanding and rapid improvement of soil–water stress tolerance in these regions, especially in the water-wind eroded crossing region, it is very important to link physiological and biochemical studies to molecular work in genetically tractable model plants and important native plants, and further extending them to practical ecological restoration and efficient crop production. Although basic studies and practices aimed at improving soil water stress resistance and plant water use efficiency have been carried out for many years, the mechanisms involved at different scales are still not clear. Further understanding and manipulating soil–plant water relationships and soil–water stress tolerance at the scales of ecology, physiology and molecular biology can significantly improve plant productivity and environmental quality. Currently, post-genomics and metabolomics are very important in exploring anti-drought gene resources in various life forms, but modern agriculturally sustainable development must be combined with plant physiological measures in the field, on the basis of which post-genomics and metabolomics have further practical prospects. In this review, we discuss physiological and molecular insights and effects in basic plant metabolism, drought tolerance strategies under drought conditions in higher plants for sustainable agriculture and ecoenvironments in arid and semiarid areas of the world. We conclude that biological measures are the bases for the solutions to the issues relating to the different types of sustainable development.     

Modeling soil–root water transport and competition for single and mixed crops

A knowledge of above and below ground plant interactions for water is essential to understand the performance of intercropped systems. In this work, root water potential dynamics and water uptake partitioning were compared between single crops and intercrops, using a simulation model. Four root maps having 498, 364, 431 and 431 soil-root contacts were used. In the first and second cases, single crops with deep and surface roots were considered, whereas in the third and fourth cases, roots of two mixed crops were simultaneously considered with different row spacing (40 cm and 60 cm). Two soils corresponding to a clay and a silty clay loam were used in the calculations. A total maximum evapotranspiration of 6 mm d^-1 for both single or mixed crops was considered, for the mixed crops however, two transpiration distributions between the crops were analyzed (3:3 mm d^-1, or 4:2 mm d^-1 for each crop, respectively). The model was based on a previous theoretical framework applied to single or intercropped plants having spatially distributed roots in a two-dimensional domain. Although water stress occurred more rapidly in the loam than in the clay, due to the rapid decrease of the soil water reserve in the loam, the role of the root arrangement appeared to be crucial for water availability. Interactions between the distribution of transpiration among mixed crops and the architecture of the root systems which were in competition led to water movements from zones with one plant to another, or vice versa, which corresponded to specific competition or facilitation effects. Decreasing the distances between roots may increase competition for water, although it may determine greater water potential gradients in the soil that increase lateral or vertical water fluxes in the soil profile. The effects of the root competition on water uptake were quite complicated, depending on both environmental conditions, soil hydrodynamic properties, and time scales. Although some biological adaptive mechanisms were disregarded in the analysis, the physically 2-D based model may be considered as a tool to study the exploitation of environmental heterogeneity at microsite scales.     

The influence of the mycorrhiza Glomus etunicatum on drought acclimation in safflower and wheat

A study was done to determine the effects of vesicular‐arbuscular mycorrhizal (VAM) colonization on drought acclimation of host plants. Safflower ( Carthamus tinctorius L. cv. S555) and wheat ( Triticum aestivum L. cv. Anza) were grown under environmentally controlled conditions with or without the VAM fungus, Glomus etunicatum Becker and Gerd., and were either acclimated (by pre‐exposing plants to a 10–11 day drought period) or unacclimated to drought. Plants from all treatments were then exposed to drought for 9 days, and plant water status and root water uptake were measured. To minimize interactions between drought and P uptake, growth periods were adjusted so that acclimated and unacclimated plants were similar in size when measurements were made. When wheat was acclimated to drought, osmotic adjustment occurred (leaf solute potentials of leaf tissue rehydrated to full turgor were approximately 0.5 MPa lower in acclimated than unacclimated plants); in safflower, osmotic adjustment was minimal when plants were acclimated. Consequently, acclimated wheat plants were able to tolerate drought better than unacclimated plants, and maintained higher leaf water potentials and relative water contents as soil water was depleted. For both safflower and wheat, acclimated plants had higher water use efficiency, and therefore produced more biomass when water availability was limited, than unacclimated plants. However, mycorrhizal colonization did not affect osmotic adjustment, plant water status, water use efficiency or water uptake in either plant species, and therefore had no effect on drought acclimation or resistance.     

Modeling soil water movement with water uptake by roots

Soil water movement with root water uptake is a key process for plant growth and transport of water and chemicals in the soil-plant system. In this study, a root water extraction model was developed to incorporate the effect of soil water deficit and plant root distributions on plant transpiration of annual crops. For several annual crops, normalized root density distribution functions were established to characterize the relative distributions of root density at different growth stages. The ratio of actual to potential cumulative transpiration was used to determine plant leaf area index under water stress from measurements of plant leaf area index at optimal soil water condition. The root water uptake model was implemented in a numerical model. The numerical model was applied to simulate soil water movement with root water uptake and simulation results were compared with field experimental data. The simulated soil matric potential, soil water content and cumulative evapotranspiration had reasonable agreement with the measured data. Potentially the numerical model implemented with the root water extraction model is a useful tool to study various problems related to flow transport with plant water uptake in variably saturated soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Quantification of water uptake by arbuscular mycorrhizal hyphae and its significance for leaf growth, water relations, and gas exchange of barley subjected to drought stress

Arbuscular mycorrhizal fungi alleviate drought stress in their host plants via the direct uptake and transfer of water and nutrients through the fungal hyphae to the host plants. To quantify the contribution of the hyphae to plant water uptake, a new split-root hyphae system was designed and employed on barley grown in loamy soil inoculated with Glomus intraradices under well-watered and drought conditions in a growth chamber with a 14-h light period and a constant temperature (15 degrees C; day/night). Drought conditions were initiated 21 days after sowing, with a total of eight 7-day drying cycles applied. Leaf water relations, net photosynthesis rates, and stomatal conductance were measured at the end of each drying cycle. Plants were harvested 90 days after sowing. Compared to the control treatment, the leaf elongation rate and the dry weight of the shoots and roots were reduced in all plants under drought conditions. However, drought resistance was comparatively increased in the mycorrhizal host plants, which suffered smaller decreases in leaf elongation, net photosynthetic rate, stomatal conductance, and turgor pressure compared to the non-mycorrhizal plants. Quantification of the contribution of the arbuscular mycorrhizal hyphae to root water uptake showed that, compared to the non-mycorrhizal treatment, 4 % of water in the hyphal compartment was transferred to the root compartment through the arbuscular mycorrhizal hyphae under drought conditions. This indicates that there is indeed transport of water by the arbuscular mycorrhizal hyphae under drought conditions. Although only a small amount of water transport from the hyphal compartment was detected, the much higher hyphal density found in the root compartment than in the hyphal compartment suggests that a larger amount of water uptake by the arbuscular mycorrhizal hyphae may occur in the root compartment.     

Water deficits and hydraulic limits to leaf water supply

Many aspects of plant water use -- particularly in response to soil drought -- may have as their basis the alteration of hydraulic conductance from soil to canopy. The regulation of plant water potential (Psi) by stomatal control and leaf area adjustment may be necessary to maximize water uptake on the one hand, while avoiding loss of hydraulic contact with the soil water on the other. Modelling the changes in hydraulic conductance with pressure gradients in the continuum allows the prediction of water use as a function of soil environment and plant architectural and xylem traits. Large differences in water use between species can be attributed in part to differences in their 'hydraulic equipment' that is presumably optimized for drawing water from a particular temporal and spatial niche in the soil environment. A number of studies have identified hydraulic limits as the cause of partial or complete foliar dieback in response to drought. The interactions between root:shoot ratio, rooting depth, xylem properties, and soil properties in influencing the limits to canopy water supply can be used to predict which combinations should optimize water use in a given circumstance. The hydraulic approach can improve our understanding of the coupling of canopy processes to soil environment, and the adaptive significance of stomatal behaviour.     

Native and non‐native grasses generate common types of plant–soil feedbacks by altering soil nutrients and microbial communities

Soil conditioning occurs when plants alter features of their soil environment. When these alterations affect subsequent plant growth, it is a plant soil feedback. Plant–soil feedbacks are an important and understudied aspect of aboveground–belowground linkages in plant ecology that influence plant coexistence, invasion and restoration. Here, we examine plant–soil feedback dynamics of seven co‐occurring native and non‐native grass species to address the questions of how plants modify their soil environment, do those modifications inhibit or favor their own species relative to other species, and do non‐natives exhibit different plant–soil feedback dynamics than natives. We used a two‐phase design, wherein a first generation of plants was grown to induce species‐specific changes in the soil and a second generation of plants was used as a bioassay to determine the effects of those changes. We also used path‐analysis to examine the potential chain of effects of the first generation on soil nutrients and soil microbial composition and on bioassay plant performance. Our findings show species‐specific (rather than consistent within groups of natives and non‐natives) soil conditioning effects on both soil nutrients and the soil microbial community by plants. Additionally, native species produced plant–soil feedback types that benefit other species more than themselves and non‐native invasive species tended to produce plant–soil feedback types that benefit themselves more than other species. These results, coupled with previous field observations, support hypotheses that plant–soil feedbacks may be a mechanism by which some non‐native species increase their invasive potential and plant–soil feedbacks may influence the vulnerability of a site to invasion.     

Adjusting Nitrogen Application in Accordance with Soil Water Availability Enhances Yield and Water Use by Regulating Physiological Traits of Maize under Drip Fertigation

Knowledge of the interactive effects of water and nitrogen (N) on physio-chemical traits of maize (Zea mays L.) helps to optimize water and N management and improve productivity. A split-plot experiment was conducted with three soil water conditions (severe drought, moderate drought, and fully water supply referring to 45%–55%, 65%–75%, and 85%–95% field capacity, respectively) and four N application rates (N₀, N₁₅₀, N₂₄₀, and N₃₃₀ referring to 0, 150, 240, 330 kg N ha^–1 respectively) under drip fertigation in 2014 and 2015 in the Huang-Huai-Hai Plain of China. The results indicated that drought stress inhibited physiological activity of plants (leaf relative water content, root bleeding sap, and net photosynthetic rate), resulting in low dry matter accumulation after silking, yield, and N uptake, whereas increased WUE and NUE. N application rates over than 150 kg ha^–1 aggravated the inhibition of physiological activity under severe drought condition, while it was offset under moderate drought condition. High N application rates (N₃₃₀) still revealed negative effects under moderate drought condition, as it did not consistently enhance plant physiological activity and significantly reduced N uptake as compared to the N₂₄₀ treatment. With fully water supply, increasing N application rates synergistically enhanced physiological activity, promoted dry matter accumulation after silking, and increased yield, WUE, and N uptake. Although the N₂₄₀ treatment reduced yield by 5.4% in average, it saved 27.3% N under full water supply condition as compared with N₃₃₀ treatment. The results indicated that N regulated growth of maize in aspects of physiological traits, dry matter accumulation, and yield as well as water and N use was depended on soil water status. The appropriate N application rates for maize production was 150 kg ha^–1 under moderate drought or 240 kg ha^–1 under fully water supply under drip fertigation, and high N supply (>150 kg ha^–1) should be avoided under severe drought condition.     

丛枝菌根真菌与植物共生对植物水分关系的影响及机理

自1885年Frank首次提到菌根(mykorhiza)概念以来,大量的试验证实了丛枝菌根真菌(AMF)与植物根系之间形成具有一定结构和功能的共生体,促进植物生长并提高干旱耐受能力,在干旱生态系统中发挥重要的作用。该研究多集中在对宿主植物生理生态的影响及其机制方面,然而菌根共生对宿主植物水分吸收和信号产生、传递的影响研究少而分散,缺少系统总结。综述了最近四十多年丛枝菌根真菌与植物共生体对宿主植物干旱适应性影响研究进展,讨论了菌根共生对植物根冠通讯的影响及机理。干旱胁迫下AMF与植物共生,通过影响宿主植物一系列生理生态过程,提高宿主植物横向根压和纵向蒸腾拉力。经典的Ohm吸水模型是该方向最有代表性的研究成果,该模型揭示了菌根共生的根外菌丝具有不同于根细胞的细胞结构和水分运输性能,这为宿主植物提供一种特殊的快速吸水方式,可提高植物对土壤水分的吸收和运输能力。研究表明,AMF会影响宿主植物根冠通讯过程,如诱发信号级联反应,诱导根系尽早感知水分胁迫并产生非水力根源信号,提高宿主对干旱的耐受性。讨论了AMF在根冠通讯分子机制研究方面存在的问题及可能的解决途径,展望了AMF在干旱农业生产中的应用潜力。     

Simultaneous X-ray imaging of plant root growth and water uptake in thin-slab systems

Direct and simultaneous observation of root growth and plant water uptake is difficult because soils are opaque. X-ray imaging techniques such as projection radiography or Computer Tomography (CT) offer a partial alternative to such limitations. Nevertheless, there is a trade-off between resolution, large field-of-view and 3-dimensionality: With the current state of the technology, it is possible to have any two. In this study, we used X-ray transmission through thin-slab systems to monitor transient saturation fields that develop around roots as plants grow. Although restricted to 2-dimensions, this approach offers a large field-of-view together with high spatial and dynamic resolutions. To illustrate the potential of this technology, we grew peas in 1 cm thick containers filled with soil and imaged them at regular intervals. The dynamics of both the root growth and the water content field that developed around the roots could be conveniently monitored. Compared to other techniques such as X-ray CT, our system is relatively inexpensive and easy to implement. It can potentially be applied to study many agronomic problems, such as issues related to the impact of soil constraints (physical, chemical or biological) on root development.     

Root zone solute dynamics under drip irrigation: A review

Infiltration and subsequent distribution of water and solutes under cropped conditions is strongly dependent on the irrigation method, soil type, crop root distribution, and uptake patterns and rates of water and solutes. This review discusses aspects of soil water and solute dynamics as affected by the irrigation and fertigation methods, in the presence of active plant uptake of water and solutes. Fertigation with poor quality water can lead to accumulation of salts in the root zone to toxic levels, potentially causing deterioration of soil hydraulic and physical properties. The high frequency of application under drip irrigation enables maintenance of salts at tolerable levels within the rooting zone. Plant roots play a major role in soil water and solute dynamics by modifying the water and solute uptake patterns in the rooting zone. Modeling of root uptake of water and solutes is commonly based on incorporating spatial root distribution and root length or density. Other models attempt to construct root architecture. Corn uptake rate and pattern of nitrate nitrogen was determined from field studies of nitrate dynamics under drip irrigation using TDR monitoring. The determined nitrate nitrogen uptake rates are within literature values for corn. This revised version was published online in June 2006 with corrections to the Cover Date.     

The units of currency for plant water status

Abstract. The appropriate measure for plant water status is of fundamental importance in plant water relations. One possible approach is to consider a hypothetical sensor for water status at the cell level. It is argued that such a sensor must respond to an intrinsic variable describing the quantity of water present in the cell. Uncertainty about the variable being sensed has affected our interpretion of plant responses to drought. In general, a more rigorous analysis of the effects of tissue hydraulic parameters and further understanding of the pertinent variable for water status are needed to assess such questions. For example, the significance of changes in the elasticity of cell walls in response to drought depends on the measure for water status. A simple model for water transport suggests that a decrease in the elastic modulus may act to maintain turgor for succulent plant species, but it is not clear how increases in the elastic modulus enhance water uptake for non-succulent species.     

北京山区干旱胁迫下侧柏叶片水分吸收策略

干旱与半干旱地区,水分是限制树木生长的重要影响因子。由于降水稀缺且分配不均,叶片吸收水分是此地区树木吸收和利用小量级降水和凝结水的主要方式。北京山区处于易旱少雨的生态脆弱地带,森林植被经常遭受干旱胁迫,所以对该地区的森林系统而言,叶片直接吸收利用截留的降雨是干旱时期树木获得水分的重要途径。基于野外对比控制试验和室内盆栽模拟试验,选取北京山区的主要造林树种侧柏为研究对象,进行利用天然降雨与模拟降雨试验,研究降雨前后侧柏叶片吸水特征,探究侧柏在干旱环境下如何通过叶片吸水缓解干旱胁迫。结果表明：当侧柏长期处于干旱胁迫状态时,叶片可以利用降雨,从中获益用来缓解树木的干旱胁迫状态;叶片的吸水能力与降雨强度呈正相关关系,与土壤含水率呈负相关关系;重度干旱下侧柏植株在降雨强度为15 mm/h时叶片吸水现象最明显,叶水势变化最大为(1.18±0.17) MPa,叶片含水率变化最大为(8.47±1.00)mg/cm~2;当土壤水率高于20.8%时,基本不发生叶片吸水现象。试验结果说明在干旱地区叶片吸水是树木除根系吸水外的重要水分来源方式,并且对干旱地区有效利用短缺水资源,减轻植物水分亏缺具有重要意义。