Mycorrhizal Experiments: Plant-Fungal Symbiosis | 菌根实验：植物与真菌的共生关系

📚 Mycorrhizal Experiments: Plant-Fungal Symbiosis | 菌根实验：植物与真菌的共生关系

Mycorrhizae are mutualistic associations between plant roots and fungi, representing one of the most widespread and ancient symbioses on Earth. Over 90% of terrestrial plant species form mycorrhizal relationships, making this a cornerstone concept in both ecology and practical experimental biology.

菌根是植物根系与真菌之间形成的互利共生体，代表着地球上分布最广、历史最古老的共生关系之一。超过90%的陆生植物物种能形成菌根关系，这使得菌根成为生态学和实验生物学中的核心概念。

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1. What Are Mycorrhizae? | 什么是菌根？

Mycorrhizae (singular: mycorrhiza) literally mean “fungus-root” (from Greek: mykos = fungus, rhiza = root). In this symbiotic relationship, the fungus colonizes the plant’s root system, extending the plant’s access to soil nutrients, while the plant supplies the fungus with carbohydrates produced during photosynthesis.

菌根（单数：mycorrhiza）字面意思是”真菌-根”（源自希腊语：mykos = 真菌，rhiza = 根）。在这种共生关系中，真菌定殖于植物根系，扩展植物获取土壤养分的范围，而植物则为真菌提供光合作用产生的碳水化合物。

Key benefits to the plant include enhanced water and mineral absorption — especially phosphorus, nitrogen, and micronutrients. The fungal hyphae are much finer than root hairs, penetrating soil pores inaccessible to roots and increasing the absorptive surface area by up to 1000-fold.

植物获得的主要益处包括水分和矿物质吸收能力的增强——尤其是磷、氮和微量元素。真菌菌丝比根毛细得多，能穿透根系无法到达的土壤孔隙，使吸收表面积增加多达1000倍。

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2. Types of Mycorrhizae | 菌根的类型

There are several types of mycorrhizae, but the two most commonly studied in educational experiments are arbuscular mycorrhizae (AM) and ectomycorrhizae (EcM). They differ fundamentally in structure and host preference.

菌根有若干类型，但在教学实验中最为常见的是丛枝菌根（AM）和外生菌根（EcM）。两者在结构和宿主偏好上有着本质区别。

Feature | 特征	Arbuscular Mycorrhizae (AM) | 丛枝菌根	Ectomycorrhizae (EcM) | 外生菌根
Host plants | 宿主植物	~80% of land plants (grasses, crops, herbs) | 约80%的陆生植物（禾草、作物、草本）	~2% (mostly trees: pines, oaks, birches) | 约2%（主要是树木：松、栎、桦）
Fungal penetration | 真菌穿透	Penetrates root cortical cells | 穿透根皮层细胞内部	Does NOT penetrate cells; forms mantle around root | 不进入细胞内部；在根外形成菌套
Key structure | 关键结构	Arbuscules (tree-like structures) and vesicles | 丛枝（树状结构）和泡囊	Hartig net (intercellular hyphae) | 哈蒂氏网（细胞间菌丝）
Fungal group | 真菌类群	Glomeromycota (obligate biotrophs) | 球囊菌门（专性活体营养）	Basidiomycota & Ascomycota | 担子菌门和子囊菌门

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3. Designing the Classic Mycorrhizal Experiment | 设计经典菌根实验

The standard educational experiment aims to compare plant growth with and without mycorrhizal inoculation. A common model system uses leek (Allium porrum) or clover (Trifolium) with AM fungi such as Rhizophagus irregularis (formerly Glomus intraradices).

标准教学实验旨在比较有无菌根接种条件下植物的生长差异。常用模式系统是用韭菜（Allium porrum）或三叶草（Trifolium）与丛枝菌根真菌如不规则根孢囊霉（Rhizophagus irregularis，原名Glomus intraradices）组合。

The experiment requires at least two treatment groups: (1) control — sterilized soil without fungal inoculum, and (2) treatment — sterilized soil amended with mycorrhizal inoculum. A third group — non-sterilized soil — is often added to assess the effect of other soil microorganisms. Each group should contain at least 5-10 replicate plants to account for biological variation.

实验至少需要两个处理组：(1)对照组——灭菌土壤，不含真菌接种物；(2)处理组——灭菌土壤中添加菌根接种物。通常还增设第三组——未灭菌土壤——以评估其他土壤微生物的影响。每组至少应有5-10个重复植株，以考虑生物学变异。

Critical design considerations include: using the same soil type for all treatments to ensure comparability, sterilizing soil by autoclaving at 121 °C for 15-20 minutes to eliminate existing microbes, and maintaining identical light, temperature, and watering regimes across all treatments.

关键设计考虑包括：所有处理使用相同类型土壤以确保可比性；用高压灭菌锅在121 °C下灭菌15-20分钟以消除原有微生物；所有处理保持相同的光照、温度和浇水方案。

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4. Inoculation and Cultivation Procedure | 接种与培养程序

Fungal inoculum is typically prepared from colonized root fragments mixed with soil, or from commercial spore suspensions. For a classroom setting, inoculum can be obtained from an established mycorrhizal pot culture. The inoculum should be mixed thoroughly into the sterilized substrate at a ratio of approximately 10% (v/v) before planting.

真菌接种物通常由已定殖的根段与土壤混合制备，或使用商业孢子悬浮液。对于课堂实验，可从已建立的菌根盆钵培养物中获取接种物。种植前应将接种物按约10%（体积比）的比例与灭菌基质充分混合。

Seeds should be surface-sterilized with 70% ethanol for 30 seconds, followed by 2-3% sodium hypochlorite for 5 minutes, then rinsed thoroughly with sterile water. This prevents contamination from seed-borne microbes and ensures that any observed difference is attributable to the inoculated fungus.

种子需用70%乙醇表面消毒30秒，再用2-3%次氯酸钠处理5分钟，最后用无菌水彻底冲洗。这可以防止种子携带微生物的污染，确保观察到的差异归因于接种的真菌。

Plants are typically grown for 6-12 weeks before harvest. During this period, weekly measurements of plant height, leaf number, and leaf color should be recorded. At harvest, both above-ground (shoot) and below-ground (root) biomass are measured — with shoot dry weight serving as the primary indicator of plant performance.

植株通常生长6-12周后收获。在此期间，每周记录株高、叶片数和叶色。收获时，分别测量地上部（茎叶）和地下部（根）生物量——地上部干重作为植株生长表现的主要指标。

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5. Quantifying Mycorrhizal Colonization | 定量菌根定殖率

After harvest, roots must be cleared and stained to visualize the fungal structures. The standard protocol involves: (1) clearing roots in 10% KOH at 90 °C for 15-30 minutes, (2) acidifying in 2% HCl for 5 minutes, (3) staining with 0.05% trypan blue or ink-vinegar solution, and (4) destaining in lactoglycerol.

收获后，根必须经过透明和染色处理以显现真菌结构。标准流程包括：(1)在90 °C的10% KOH中透明处理15-30分钟；(2)用2% HCl酸化5分钟；(3)用0.05%台盼蓝或墨水-醋溶液染色；(4)在乳酸甘油中脱色。

Colonization is quantified using the gridline intersect method. Stained roots are cut into 1 cm segments, spread evenly on a 1 cm × 1 cm grid plate, and examined under a dissecting microscope. For each gridline intersect, record whether AM structures (arbuscules, vesicles, or hyphae) are present. The percentage colonization is calculated as:

定殖率用网格线交叉法进行定量。将染色后的根切成1厘米长片段，均匀铺在1 cm × 1 cm网格板上，用体视显微镜检查。对每条网格线交叉点，记录是否存在AM结构（丛枝、泡囊或菌丝）。定殖百分率计算如下：

% Colonization = (Number of intersects with fungus / Total intersects examined) × 100%

定殖率（%）=（含真菌的交叉点数 ÷ 检查的总交叉点数）× 100%

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6. Measuring Nutrient Uptake Effects | 测定养分吸收效应

To demonstrate that mycorrhizae improve nutrient uptake, specifically phosphorus (P), dried shoot tissue should be analyzed for P concentration. The ammonium-molybdate colorimetric method is commonly employed. Finely ground shoot samples are digested with acid, and the digest reacts with ammonium molybdate to produce a blue color whose intensity is proportional to phosphorus content.

为证明菌根促进养分吸收，特别是磷（P），应对干燥的地上部组织进行磷浓度分析。常用钼酸铵比色法：将研磨细的茎叶样品用酸消解，消解液与钼酸铵反应生成蓝色络合物，其颜色深浅与磷含量成正比。

The results typically show that mycorrhizal plants have significantly higher shoot phosphorus concentrations than non-mycorrhizal controls, even when shoot biomass appears similar. This demonstrates that the benefit is not merely increased size but enhanced nutritional status per unit biomass.

结果通常显示菌根植物地上部的磷浓度显著高于非菌根对照，即使两者地上部生物量看似相近。这证明菌根的效益不仅是植株变大，更是单位生物量中营养状况的改善。

Nitrogen uptake, particularly ammonium (NH₄⁺) and nitrate (NO₃⁻), can be measured similarly. The Kjeldahl method or a CHN elemental analyzer provides total nitrogen content in dried tissues. Mycorrhizal fungi also produce extracellular enzymes such as phosphatases that release P from organic compounds, making more P available to the host plant.

氮吸收，特别是铵离子（NH₄⁺）和硝酸根离子（NO₃⁻），可类似地加以测定。凯氏定氮法或CHN元素分析仪可提供干组织中的总氮含量。菌根真菌还会产生磷酸酶等胞外酶，从有机化合物中释放磷，使更多磷可供宿主植物利用。

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7. Data Analysis and Interpretation | 数据分析与解读

Before statistical analysis, data should be checked for normality using tests such as Shapiro-Wilk. The primary comparisons — shoot dry weight, root dry weight, P concentration, and % colonization — between mycorrhizal and non-mycorrhizal treatments are typically analyzed using Student’s t-test (for two groups) or one-way ANOVA (for three or more groups) followed by post-hoc tests like Tukey’s HSD.

在统计分析前，应使用Shapiro-Wilk检验等检查数据的正态性。菌根与非菌根处理之间的主要比较指标——地上部干重、根部干重、磷浓度和定殖率——通常使用Student’s t检验（两组时）或单因素方差分析（三组及以上时）并进行Tukey’s HSD等事后检验。

A key interpretation step is to examine the dose-response relationship between % colonization and plant growth. If a strong positive correlation (r > 0.7) exists between colonization rate and shoot biomass, this supports a causal role of mycorrhizae in promoting growth. However, correlation does not equal causation — always consider whether environmental factors might explain the pattern.

关键的解读步骤是考察定殖率与植物生长之间的剂量-反应关系。如果定殖率与地上部生物量之间存在强正相关（r > 0.7），则支持菌根促进生长的因果作用。但相关性不等于因果性——始终要考虑环境因素是否可能解释这一模式。

Graphical presentation should include: (1) bar charts with means ± standard deviation or standard error for growth parameters, (2) a scatter plot of % colonization versus shoot dry weight, and (3) a line graph showing temporal changes in plant height. Include significance asterisks (*p < 0.05, **p < 0.01, ***p < 0.001) above the bars.

图形展示应包括：(1)生长参数的均值±标准差或标准误的柱状图；(2)定殖率对地上部干重的散点图；(3)株高随时间变化的折线图。在柱子上方标注显著性星号（*p < 0.05，**p < 0.01，***p < 0.001）。

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8. Common Errors and Controls | 常见误差与对照

One frequent error is cross-contamination between treatments. Mycorrhizal plants and control plants must be watered separately, and pots should be arranged in a randomized block design with sufficient spacing. Porous pots should be placed on individual saucers, and care should be taken to prevent splash transfer of soil during watering.

一个常见误差是处理间的交叉污染。菌根植物和对照植物必须分开浇水，花盆应以随机区组设计排列并保持足够间距。多孔花盆应放在各自的托盘上，浇水时注意防止土壤飞溅转移。

The “sterilized soil + inoculum” treatment can confound results because the inoculum adds organic matter and nutrients beyond just fungi. To control for this, a third treatment — sterilized soil plus autoclaved (dead) inoculum — should be included. This control contains the same organic matter amendment but lacks living fungi, isolating the biological effect of the symbiosis.

“灭菌土壤 + 接种物”处理可能混淆结果，因为接种物不仅引入真菌，还添加了有机物和养分。为控制这一因素，应增设第三组——灭菌土壤加高温灭菌（灭活）的接种物。该对照含有相同的有机物添加但缺乏活真菌，从而分离出共生的生物学效应。

Measurement consistency matters. Dry weights must be measured after oven-drying at 70 °C to constant mass, not air-drying, as moisture content varies. Root staining should use a consistent batch of reagents, and microscopic scoring should be performed by the same observer or with inter-observer validation to minimize subjectivity.

测量一致性至关重要。干重必须在70 °C烘箱中烘至恒重后测定，不能用风干，因为含水量有差异。根染色应使用同一批试剂，显微评分应由同一观察者完成或进行观察者间校验以尽量减少主观性。

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9. Extensions: Soil Ectomycorrhizal Observation | 拓展：土壤外生菌根观察

For ectomycorrhizae, a simpler observational experiment can be conducted. Collect fine root tips from soil beneath oak or pine trees, gently rinse off soil, and examine under a stereomicroscope. Ectomycorrhizal root tips are typically swollen, branched, and covered with a dense fungal mantle, often yellow, white, or brown in color.

对外生菌根，可进行更简单的观察实验。从橡树或松树下收集细根尖，轻轻洗去土壤，用体视显微镜观察。外生菌根的根尖通常肿胀、分枝，并被致密的真菌菌套覆盖，颜色常为黄色、白色或棕色。

Cross-sectioning stained root tips reveals the diagnostic Hartig net: fungal hyphae growing between cortical cells but not entering them. This anatomical arrangement is the defining feature distinguishing EcM from AM. Students can prepare freehand sections with a razor blade and view them under a compound microscope at 100-400× magnification.

对染色根尖进行切片可显示诊断性特征——哈蒂氏网：菌丝在皮层细胞之间生长但不进入细胞内。这种解剖学排列是区分外生菌根与丛枝菌根的决定性特征。学生可用剃须刀片制作徒手切片，在100-400倍复合显微镜下观察。

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10. Ecological Significance and Applications | 生态意义与应用

Mycorrhizae play a critical role in ecosystem functioning. They facilitate plant succession, improve soil structure through glomalin production (a glycoprotein that binds soil particles), and create underground networks — often called the “wood wide web” — that connect multiple plants, allowing resource transfer between individuals.

菌根在生态系统功能中发挥关键作用。它们促进植物演替，通过产生球囊霉素（一种结合土壤颗粒的糖蛋白）改善土壤结构，并形成地下网络——常被称为”树木互联网”——连接多株植物，实现个体间的资源转移。

In agriculture, arbuscular mycorrhizal inoculants are commercially produced as biofertilizers that reduce the need for chemical phosphate fertilizers. In reforestation programs, seedlings are routinely inoculated with mycorrhizal fungi before outplanting — dramatically improving survival rates, especially on degraded soils. This practical significance makes mycorrhizal experiments highly relevant to real-world environmental challenges.

在农业中，丛枝菌根接种剂被商业化生产为生物肥料，可减少化学磷肥的使用。在再造林项目中，苗木在野外定植前常规接种菌根真菌——可大幅提高存活率，尤其在退化土壤上。这种实际意义使菌根实验与实际环境挑战高度相关。

Mycorrhizae also confer enhanced resistance to root pathogens and improved tolerance to drought and heavy-metal stress. These additional benefits arise through multiple mechanisms: competition for root colonization sites, induced systemic resistance in the host, and improved water relations via the fungal hyphal network spanning larger soil volumes.

菌根还赋予植物增强的抗根部病原体能力和对干旱及重金属胁迫的耐受性。这些额外益处源于多种机制：竞争根部定殖位点、诱导宿主系统性抗性，以及通过覆盖更大土壤体积的真菌菌丝网络改善水分关系。

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11. Exam Readiness: Common Questions and Approaches | 应试要点：常见问题与答题思路

In examinations, students are frequently asked to: (1) describe the mutualistic nature of the mycorrhizal relationship, stating specific benefits to each partner; (2) explain how the fungal hyphae increase water and mineral uptake; (3) suggest why mycorrhizal plants show greater drought tolerance; and (4) design a controlled experiment to test the effect of mycorrhizae on plant growth.

考试中，学生常被要求：(1)描述菌根关系的互利性质，说明双方各自获得的具体益处；(2)解释真菌菌丝如何增加水分和矿物质吸收；(3)提出菌根植物表现出更强抗旱性的原因；(4)设计一个受控实验来测试菌根对植物生长的影响。

Model answers should use precise terminology: “the plant provides carbohydrates and amino acids to the fungus; the fungus provides phosphate, nitrate, ammonium, and water to the plant.” Mention specific structural features — arbuscules maximize surface area for exchange; extraradical hyphae extend beyond the phosphate-depletion zone. Always cite a quantification method, such as the gridline intersect method, when discussing measurement.

参考答案应使用精确术语：”植物向真菌提供碳水化合物和氨基酸；真菌向植物提供磷酸盐、硝酸盐、铵离子和水。”提及具体结构特征——丛枝最大化交换表面积；根外菌丝延伸超出磷耗竭区。讨论测量时，始终引用如网格线交叉法等定量方法。

For practical-based exam questions, remember the golden rules: identify independent variable (presence/absence of mycorrhizal fungi), dependent variable (plant biomass, % colonization, P concentration), controlled variables (soil type, water, light, temperature, seed source), and include sufficient replicates with appropriate statistical testing. State that at least 3 groups are needed: sterile soil, sterile soil + live inoculum, sterile soil + dead inoculum.

对于基于实验的考题，牢记黄金法则：明确自变量（有无菌根真菌）、因变量（植物生物量、定殖率、磷浓度）、控制变量（土壤类型、水分、光照、温度、种子来源），并设置足够的重复和适当的统计检验。说明至少需要三组：灭菌土壤、灭菌土壤+活接种物、灭菌土壤+灭活接种物。

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12. Conclusion and Summary | 结论与总结

The mycorrhizal experiment is a powerful demonstration of mutualistic symbiosis, integrating concepts of plant physiology, fungal biology, soil ecology, and experimental methodology. It teaches students how to design controlled experiments, quantify microscopic structures, and connect biological mechanisms to ecosystem-level processes.

菌根实验是互利共生的有力示范，整合了植物生理学、真菌生物学、土壤生态学和实验方法学的概念。它教学生如何设计受控实验、定量微观结构，并将生物学机制与生态系统层面的过程联系起来。

Beyond the laboratory, understanding mycorrhizae is essential for sustainable agriculture, forest restoration, and climate change mitigation through improved carbon sequestration in soils. Mastery of this topic demonstrates not only factual knowledge but also systems-level thinking — a key skill assessed in A-Level and IB examinations.

在实验室之外，理解菌根对于可持续农业、森林恢复以及通过改善土壤碳固存来减缓气候变化至关重要。掌握这一主题不仅体现事实性知识，更展现系统级思维——这是A-Level和IB考试中评估的关键能力。

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