Drought Stress & Stomatal Closure (2D)

Environment

Light intensity (PAR) 80%

Atmospheric dryness (VPD) 1.5 kPa

Root-zone water buffer 60

Soil→leaf hydraulic conductance 6.0

Stomatal drought sensitivity isohydric

Plant State

ClosedFully open

Soil moisture θ

40%

Soil Ψ_soil

0.00 MPa

Leaf Ψ_leaf

0.00 MPa

Stomatal g_s

0 mmol

Photosynthesis A

0 µmol

Transpiration E

0 mmol

Water conserved (stomatal closure)

How it works

This is the same coupled soil–plant–atmosphere physiology as the 3D guard-cell model, drawn as a real-time 2D dashboard instead of 3D cells. The soil dries as the plant transpires water out of its own limited root-zone buffer — a genuine feedback loop, not a scripted animation.

Ψ_soil  =  Ψ_e · (θ/θ_s)^(−b)                      [soil water characteristic]
C_leaf · dΨ_leaf/dt  =  K_plant·(Ψ_soil − Ψ_leaf) − E   [soil→leaf hydraulic transport]
g_s  =  g_smax / (1 + exp(−k·(Ψ_leaf − Ψ50)))       [sigmoidal stomatal closure]
E  =  g_s · VPD                                     [transpiration]
A  =  A_sat · light/(light+k_L) · (g_s / g_smax)     [CO2 uptake ∝ g_s]
dθ/dt  =  −E / buffer                               [plant water use dries the soil]

Ψ_soil — soil water potential, computed from soil moisture via a Brooks–Corey retention curve: it falls fast as the last of the available water is used up.
Ψ_leaf — leaf water potential, integrated over time from the balance between water arriving through the xylem and water lost to transpiration — it lags Ψ_soil, it doesn't just copy it.
g_s — stomatal conductance, a sigmoid (logistic) function of Ψ_leaf: near-fully open until a threshold, then closes sharply — the same ABA/turgor closure curve as the 3D guard-cell model, here driving a real CO₂-vs-water tradeoff.
Stomatal drought sensitivity — shifts Ψ50, the leaf water potential at 50% closure: isohydric species (low value) close early and conservatively; anisohydric species (high value) keep stomata open longer at the cost of more negative Ψ_leaf.

Photosynthesis A Transpiration E Stomatal conductance g_s