Plant Water-Carbon Balance: Photosynthesis vs Transpiration

Stomata

Stomatal aperture 70%

Environment

Soil moisture 60%

Light (PAR) 70%

Vapor pressure deficit 1.4 kPa

Live Readouts

Transpiration E

0.0 mmol·m⁻²s⁻¹

Photosynthesis A

0.0 µmol·m⁻²s⁻¹

Leaf water potential Ψ_leaf

0.00 MPa

Sugar reserve

How it works

Water and carbon move through a plant along the soil→root→xylem→leaf→air continuum, gated by the stomata — microscopic pores in the leaf surface, each bounded by two guard cells whose turgor pressure controls the pore's width.

Transpiration:   E  = g_s · VPD
Photosynthesis:  A_j = α·PAR / (1 + PAR/PAR_sat)      (light-limited)
                 A_c = g_s · (C_a − C_i) · k            (CO2-limited)
                 A  = min(A_j, A_c) − R_d
Leaf potential:  dΨ_leaf/dt = (Ψ_soil − E·R_h − Ψ_leaf) / τ
Guard cells:     g_s = g_s,max · aperture · turgor(Ψ_leaf)

The same pore that lets CO2 diffuse in for photosynthesis also lets water vapor diffuse out — every stoma is a trade-off. Opening it wider raises A but also raises E, pulling leaf water potential Ψ_leaf down. If uptake from dry soil can't keep up, Ψ_leaf falls past the wilting threshold and the guard cells lose turgor, forcing the pore to close on its own — visible here as the leaves drooping and the stomatal-conductance term shrinking even while the aperture slider stays put.

Stomatal aperture — direct guard-cell control; wider = more CO2 in, more water out.
Auto control — approximates a plant's own water-use-efficiency strategy: aperture tracks toward whatever value maximizes A while keeping Ψ_leaf above the wilting point.
Soil moisture / VPD / light — the three environmental drivers real stomata respond to.
Sugar reserve — net carbon gain (A minus respiration) integrated over time; it drives visible growth pulses once enough has accumulated.

Real-world relevance: this coupled control is exactly why crops wilt in a heatwave even with wet soil (VPD-driven closure), and why breeders select for stomata that hold water-use efficiency high under drought.

Xylem water flow Transpired vapor Leaf canopy

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