Drought Stress & Stomatal Closure

Environment

Light intensity (PAR) 70%

Soil water potential Ψ_soil -0.20 MPa

Guard-cell ABA concentration 2 µM

Guard-Cell State

ClosedFully open

Stomatal aperture

0.0 µm

Guard-cell turgor P_g

0.00 MPa

Relative transpiration rate

How it works

A stoma is a pore flanked by two kidney-shaped guard cells. Their turgor pressure (internal water pressure against the cell wall) sets how far they bow apart, which sets the pore width. Light drives H⁺-ATPase pumps that pull K⁺ and malate into the guard cells, lowering their osmotic potential, pulling in water and raising turgor — the cells bow outward and the pore opens.

Drought reverses this. Falling soil water potential triggers abscisic acid (ABA) synthesis in roots and leaves; ABA binds guard-cell receptors and activates SLAC1 anion channels, depolarising the membrane and opening outward K⁺ channels. K⁺ and anions leave the cell, water follows osmotically, turgor collapses, and the pore closes — cutting water loss at the cost of CO₂ uptake.

P_g  =  max(0,  P_light  −  k_ABA·[ABA]  −  k_Ψ·max(0, Ψ_thr − Ψ_soil))
P_light = P_rest + k_L·(light/100)
Aperture = A_max · clamp(P_g / P_full, 0, 1)
Transpiration ∝ Aperture  (stomatal conductance g_s)

Light — raises the turgor ceiling the guard cells can reach (ion-pump driven opening).
Ψ_soil — below a threshold, hydraulic and ABA-mediated signalling starts pulling turgor down directly.
ABA — the direct closure signal: higher concentration collapses turgor regardless of light.
Small spheres show K⁺/anion efflux (outward, closing) or influx (inward, opening) across the guard-cell membrane.

Guard cells K⁺ / anion flux

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