Each mesophyll chloroplast fixes CO₂ using light-driven ATP/NADPH. Net CO₂ uptake follows a light-saturation curve modulated by CO₂ availability and dark respiration, while the paired guard cells set how much CO₂ can diffuse in and how much water vapor escapes.
A_net = [α·PAR·Amax /(α·PAR+Amax)] · [CO2/(CO2+Km)] − Rd
gs = gs_max · aperture(PAR, CO2, mode) + g0
E = gs · VPD(T) / Patm × 1000 [mmol H2O m⁻² s⁻¹]
VPD(T)= 0.6108·exp(17.27T/(T+237.3)) · (1 − RH)
- Light (PAR) — photosynthetic photon flux; raises A_net along a rectangular-hyperbola light-response curve and biases stomata toward opening.
- CO₂ concentration — substrate for carbon fixation; also feeds back to partially close stomata as it rises (in Auto mode).
- Leaf temperature — shifts the photosynthetic optimum (Gaussian response) and raises respiration (Q10) and the saturation vapor pressure deficit driving transpiration.
- Stomata mode — Auto lets guard-cell turgor respond to light/CO₂; Open/Closed force the pore fully wide or nearly shut, isolating the effect of stomatal control on gas exchange.
This coupling — photosynthesis pulling CO₂ in while transpiration pushes water vapor out through the same pore — is exactly the water-for-carbon trade-off real plants balance every day.