5D Library › Ecological Cultivation, Stress-Induced Polysaccharide Synthesis and Fresh-Extract Rheology of Dendrobium officinale · 中文
Chapter 3 Roots, Fungi and Substrate
A partnership that starts at the seed
Orchid seeds are as fine as dust and carry almost no food. In nature they germinate only when a suitable fungus enters them and feeds the embryo with sugars taken from the surroundings, and many orchids keep fungal partners in their roots as adults (Smith and Read 2008).
Evidence
In laboratory culture, D. officinale seeds germinated and grew best with fungi of the genera Tulasnella and Sebacina. Two Tulasnella strains gave the highest germination; one Sebacina strain gave the heaviest seedlings and another the highest polysaccharide content, with smaller seedlings. The fungi used had been isolated from the roots of mature plants (Zhang et al. 2020).
What the fungi do for adult D. officinale growing on bark in a greenhouse is much less studied. It is reasonable to expect that broad-spectrum fungicides, which are designed to kill fungi, can harm beneficial root fungi as well as pathogens, and that heavy fertilising reduces a plant’s dependence on fungal partners. For adult D. officinale in commercial beds, though, neither effect has been measured well enough to say how large it is. The book treats the protection of root fungi as a sensible precaution and a question for research, not as an established cause of yield or quality.
What a substrate holds
Epiphytic orchid roots are covered by a spongy layer of dead cells, the velamen, which soaks up water within seconds of a shower and then dries. In the wild the root is wet briefly and in air most of the time. A cultivation substrate should give the same pattern: water after watering, then plenty of air.
Two curves describe a substrate. The water retention curve gives how much water it holds at each suction, and van Genuchten’s equation describes it with four parameters (Genuchten 1980). The gas diffusivity gives how easily oxygen moves through the air-filled pores to the roots; the Millington–Quirk relation estimates it from the air-filled porosity and the total porosity (Millington and Quirk 1961).
Calculation
Water, air and oxygen diffusivity in three media, at suctions typical of a shallow container.
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 airCoarse bark and bark with gravel drain to a high air content even at small suctions, and oxygen moves through them at a quarter or more of its rate in free air. A fine peat mix that suits many pot plants stays nearly saturated in a shallow bed, and oxygen diffusion falls almost to nothing. This is why orchid growers use coarse media, and why the same watering schedule can suit one medium and drown another. The parameters in the program are illustrative; a grower can measure the real curve of a batch of bark with a simple tension table.
What fertiliser does to the root zone
Fertiliser salts dissolved in the water around the roots lower its water potential, so the plant must work harder to take water up. Growers track this with the electrical conductivity (EC) of the solution draining from the beds.
Calculation
Osmotic and matric potential in coarse bark as it dries, for three feed strengths, using a standard approximation of osmotic potential from EC (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 MPaIn coarse bark the suction from drying is tiny, because the large pores empty at low suction. The salts dominate. As the bark dries, the same salt is concentrated in less water, and a strong feed can lower the root-zone water potential several times more than drying alone. Two consequences follow. “Controlling water” in bark works mainly through how often and how long the roots are wet, not through the suction of the medium. And a heavy feed adds an osmotic stress that the grower may not intend. Drainage EC is therefore one of the four numbers in the weekly check of Chapter 8.
Hypothesis · not established
Not established. In greenhouse D. officinale on pine bark, plants that receive no broad-spectrum fungicide for a full season have higher root colonisation by Tulasnella or Sebacina fungi, and no lower survival, than plants on a routine fungicide schedule. The hypothesis predicts higher colonisation, measured by root sections and DNA sequencing, in the unsprayed beds. If colonisation does not differ, or if survival falls by more than 5 percentage points without fungicide, the hypothesis is refuted.
References
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).