Description Usage Arguments References Examples

A sigmoidal-exponential function, which describes the relative conductivity as a function of the plant water potential. The relative conductivity is scaled to be 1 when water potential is zero. This function was used by Pammenter and vander Willigen (1998), but note that this implementation gives the relative conductivity, not the PLC (but relK = 1 - PLC). The slope of relK versus P at the inflection point can be calculated from the shape parameter (a) as slope = -a/4.

1 | ```
fsigmoidal(P, PX, a, X = 50)
``` |

`P` |
Water potential (positive-valued MPa) |

`PX` |
Water potential at X loss of conductivity (positive valued). |

`a` |
Shape parameter, related to the slope at the inflection point (see Description). |

`X` |
If 50, PX is the P50. |

Pammenter, N.W., Willigen, C.V. der, 1998. A mathematical and statistical analysis of the curves illustrating vulnerability of xylem to cavitation. Tree Physiol 18, 589-593. doi:10.1093/treephys/18.8-9.589

1 2 3 4 5 6 | ```
curve(fsigmoidal(x, PX=-2, a=5), from=0, to=-5)
curve(fsigmoidal(x, PX=-2, a=2), add=TRUE)
# Comparison to Weibull
curve(fweibull(x, PX=3, SX=40), from=0, to=6)
curve(fsigmoidal(x, PX=3, a=4*(40/100)), add=TRUE, col="red")
``` |

```
Loading required package: nlme
Loading required package: car
```

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