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第三章 根、真菌与基质

5D Library › 铁皮石斛的生态栽培、逆境多糖合成与鲜汁流变学 · English

第三章 根、真菌与基质

从种子开始的共生

兰科植物的种子细如尘埃，几乎不带养分。在自然界，只有当合适的真菌侵入种子、把从周围环境获取的糖分输送给胚时，种子才能萌发；许多兰科植物成年后仍在根中保留真菌伙伴 (Smith and Read 2008)。

证据

在实验室培养中，铁皮石斛种子与胶膜菌属（Tulasnella）和蜡壳耳属（Sebacina）真菌共培养时萌发和生长最好。两个胶膜菌菌株的萌发率最高；一个蜡壳耳菌株使幼苗最重，另一个使多糖含量最高、但幼苗较小。所用真菌分离自成年植株的根 (Zhang et al. 2020)。

对于在温室树皮上生长的成年铁皮石斛，真菌起什么作用，研究要少得多。可以合理预期，以杀灭真菌为目的的广谱杀菌剂既会伤害病原，也会伤害有益的根际真菌；大量施肥则会降低植株对真菌伙伴的依赖。但对于商业种植床中的成年铁皮石斛，这两种效应都还没有测量到足以说明其大小的程度。本书把保护根际真菌视为合理的预防措施和一个研究问题，而不是产量或品质的既定原因。

基质能保持什么

附生兰的根外包着一层由死细胞组成的海绵状组织，即根被（velamen）。阵雨后几秒钟内它就吸满水，然后逐渐变干。在野外，根只是短暂湿润，大部分时间暴露在空气中。栽培基质应提供同样的模式：浇水后有水，然后有充足的空气。

描述基质要用两条曲线。持水曲线给出基质在各吸力下保持多少水，范·赫努赫滕（van Genuchten）方程用四个参数描述它 (Genuchten 1980)。气体扩散率表示氧气通过充气孔隙到达根部的难易，米林顿–奎尔克（Millington–Quirk）关系式由充气孔隙度和总孔隙度估算它 (Millington and Quirk 1961)。

推导

三种基质在浅层种植床典型吸力下的含水量、含气量和氧气扩散率。

def theta(h, ts, tr, a, n):
    """Water content (m³/m³) at suction h (cm of water), van Genuchten (1980)."""
    m = 1 - 1 / n
    return tr + (ts - tr) / (1 + (a * h) ** n) ** m

def rel_diffusivity(air, porosity):
    """Gas diffusivity in the medium relative to free air, Millington & Quirk (1961)."""
    return air ** (10 / 3) / porosity ** 2

# Illustrative parameters (θs = total porosity, θr, α in cm⁻¹, n); real media should be measured.
media = {"coarse pine bark (1–2 cm)": (0.78, 0.08, 0.40, 2.2),
         "bark + gravel 1:1":         (0.55, 0.05, 0.50, 2.4),
         "fine peat mix":              (0.88, 0.20, 0.04, 1.6)}
for name, p in media.items():
    print(name)
    for h in (5, 10, 30):                     # 10 cm ≈ the suction at the base of a shallow pot
        w = theta(h, *p)
        air = p[0] - w
        print(f"   suction {h:2d} cm: water {w:.2f}, air {air:.2f} m³/m³, "
              f"O2 diffusivity {100 * rel_diffusivity(air, p[0]):5.2f} % of free air")
# coarse pine bark (1–2 cm)
#    suction  5 cm: water 0.35, air 0.43 m³/m³, O2 diffusivity  9.59 % of free air
#    suction 10 cm: water 0.21, air 0.57 m³/m³, O2 diffusivity 25.34 % of free air
#    suction 30 cm: water 0.12, air 0.66 m³/m³, O2 diffusivity 42.10 % of free air
# bark + gravel 1:1
#    suction  5 cm: water 0.18, air 0.37 m³/m³, O2 diffusivity 11.98 % of free air
#    suction 10 cm: water 0.10, air 0.45 m³/m³, O2 diffusivity 22.76 % of free air
#    suction 30 cm: water 0.06, air 0.49 m³/m³, O2 diffusivity 30.40 % of free air
# fine peat mix
#    suction  5 cm: water 0.86, air 0.02 m³/m³, O2 diffusivity  0.00 % of free air
#    suction 10 cm: water 0.83, air 0.05 m³/m³, O2 diffusivity  0.01 % of free air
#    suction 30 cm: water 0.69, air 0.19 m³/m³, O2 diffusivity  0.47 % of free air

粗树皮和树皮加砾石即使在很小的吸力下也排水到很高的含气量，氧气在其中的扩散速率达到自由空气中的四分之一以上。适合许多盆栽植物的细泥炭混合基质，在浅床中几乎保持饱和，氧气扩散几乎降到零。这就是兰花种植者使用粗基质的原因，也说明为什么同一个浇水方案在一种基质中合适、在另一种基质中会淹死根系。程序中的参数是示意值；种植者可以用简单的张力台测出一批树皮的真实曲线。

肥料对根区的作用

溶解在根周围水中的肥料盐分会降低溶液的水势，植株吸水就要更费力。种植者通过测量种植床排出液的电导率（EC）来跟踪这一点。

推导

粗树皮逐渐变干时，三种营养液浓度下的渗透势与基质势；渗透势由 EC 按一个标准近似式估算 (U.S. Salinity Laboratory Staff 1954)。

# Fertiliser salts lower the water potential of the root-zone solution. A common rule of thumb
# (US Salinity Laboratory 1954): osmotic potential ≈ −0.036 MPa per dS/m of electrical conductivity.
# As the medium dries, the same salt is dissolved in less water, so the EC rises.
def osmotic(ec):
    return -0.036 * ec

def matric(h_cm):
    return -h_cm * 9.81e-5          # 1 cm of water suction ≈ 0.0000981 MPa

water_full = 0.35                    # m³/m³ just after watering (coarse bark, see the last program)
for ec_feed in (0.3, 0.8, 2.0):
    print(f"feed solution {ec_feed} dS/m")
    for water, h in ((0.35, 5), (0.21, 10), (0.12, 30)):
        ec = ec_feed * water_full / water
        total = osmotic(ec) + matric(h)
        print(f"   water {water:.2f}: EC {ec:4.2f} dS/m, osmotic {osmotic(ec):6.3f} MPa, "
              f"matric {matric(h):6.4f} MPa, total {total:6.3f} MPa")
# feed solution 0.3 dS/m
#    water 0.35: EC 0.30 dS/m, osmotic -0.011 MPa, matric -0.0005 MPa, total -0.011 MPa
#    water 0.21: EC 0.50 dS/m, osmotic -0.018 MPa, matric -0.0010 MPa, total -0.019 MPa
#    water 0.12: EC 0.88 dS/m, osmotic -0.032 MPa, matric -0.0029 MPa, total -0.034 MPa
# feed solution 0.8 dS/m
#    water 0.35: EC 0.80 dS/m, osmotic -0.029 MPa, matric -0.0005 MPa, total -0.029 MPa
#    water 0.21: EC 1.33 dS/m, osmotic -0.048 MPa, matric -0.0010 MPa, total -0.049 MPa
#    water 0.12: EC 2.33 dS/m, osmotic -0.084 MPa, matric -0.0029 MPa, total -0.087 MPa
# feed solution 2.0 dS/m
#    water 0.35: EC 2.00 dS/m, osmotic -0.072 MPa, matric -0.0005 MPa, total -0.072 MPa
#    water 0.21: EC 3.33 dS/m, osmotic -0.120 MPa, matric -0.0010 MPa, total -0.121 MPa
#    water 0.12: EC 5.83 dS/m, osmotic -0.210 MPa, matric -0.0029 MPa, total -0.213 MPa

在粗树皮中，变干带来的吸力很小，因为大孔隙在低吸力下就已排空。起主导作用的是盐分。树皮变干时，同样的盐分溶在更少的水中；浓的营养液使根区水势的下降幅度可以是单纯变干的好几倍。由此有两个结论。在树皮基质中「控水」主要是通过控制根湿润的频率和时长，而不是通过基质的吸力。重肥会额外施加一种种植者可能并不想要的渗透胁迫。因此，排出液 EC 是第八章每周核查的四个数字之一。

假说 · 尚未验证

尚未确立。 在松树皮上温室栽培的铁皮石斛，整个生长季不施用广谱杀菌剂的植株，其根部被胶膜菌或蜡壳耳真菌定殖的程度高于按常规施药的植株，存活率也不更低。该假说预测：通过根切片和 DNA 测序测得的定殖程度在未施药床中更高。如果定殖程度没有差别，或者不施杀菌剂使存活率下降超过 5 个百分点，假说就被推翻。

参考文献

Genuchten, M. Th. van. 1980. “A Closed-Form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils.” Soil Science Society of America Journal 44 (5): 892–98.
Millington, R. J., and J. P. Quirk. 1961. “Permeability of Porous Solids.” Transactions of the Faraday Society 57: 1200–1207.
Smith, Sally E., and David Read. 2008. Mycorrhizal Symbiosis. 3rd ed. Academic Press.
U.S. Salinity Laboratory Staff. 1954. Diagnosis and Improvement of Saline and Alkali Soils. Edited by L. A. Richards. Agriculture Handbook 60. United States Department of Agriculture.
Zhang, Yue, Yuan-Yuan Li, Xiao-Mei Chen, et al. 2020. “Effect of Different Mycobionts on Symbiotic Germination and Seedling Growth of Dendrobium Officinale, an Important Medicinal Orchid.” Botanical Studies 61: 2.

Fan, Y. (2026). Ecological Cultivation, Stress-Induced Polysaccharide Synthesis and Fresh-Extract Rheology of Dendrobium officinale: CAM Photosynthesis, VPD Water Control, Growth-Differentiation Balance and Nocturnal Core-Body-Temperature Regulation [铁皮石斛的生态栽培、逆境多糖合成与鲜汁流变学] (Version 1.0.0). 5D Library. https://doi.org/10.5281/zenodo.23729740 · Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0).