Stomatal Conductance Dynamics & Farquhar-von Caemmerer-Berry (FvCB) Photosynthesis Coupling

1. Limiting Factors of Carbon Assimilation: The CO2 Saturation Plateau

In high-intensity commercial crop production, light and carbon dioxide represent the primary substrates for the Calvin-Benson cycle. Ambient atmospheric CO₂ (~420 ppm) is biochemically rate-limiting for C3 greenhouse crops (tomatoes, cucumbers, peppers, strawberries). Supplying supplemental carbon dioxide up to 800–1,200 ppm elevates net CO₂ assimilation (\(A_{net}\)) by 25–40%, provided canopy stomata remain open and photosynthetically active.

2. The A-Ci Curve, Rubisco Carboxylation Kinetics, and Ball-Berry Stomatal Resistance

The biochemical model of Farquhar, von Caemmerer, and Berry (FvCB) establishes that net carbon assimilation is limited by either Rubisco enzyme kinetics (\(W_c\)) or RuBP regeneration via electron transport (\(W_j\)):

A_net = min(W_c, W_j) - R_d [μmol/m²·s]
W_c = V_cmax × [ (C_i - Γ*) ÷ (C_i + K_c·(1 + O_i / K_o)) ]
g_s = g_0 + a_1 × [ (A_net × RH) ÷ C_s ] [mol/m²·s]

The Ball-Berry model couples stomatal conductance (\(g_s\)) directly with carbon assimilation (\(A_{net}\)), relative humidity (\(RH\)), and surface CO₂ concentration (\(C_s\)). Under high CO₂ concentrations, guard cells naturally decrease aperture to conserve water, yet internal intercellular carbon (\(C_i\)) remains high, drastically elevating instantaneous Water Use Efficiency (WUE).

3. Engineering Daytime CO2 Dosing & Boundary Layer Exchange Optimization

• Ventilation Compromise: When greenhouse roof vents open above 15–20% to manage temperature, injected pure liquid CO₂ rapidly escapes into the atmosphere, making dosing economically unfeasible. Dosing is most cost-effective during morning closed-ventilation periods.
• Boundary Layer Scrubbing: Without active internal air movement, stagnant boundary layers (exceeding 3–5 mm) form over leaf surfaces, creating severe localized CO₂ depletion. Sustaining horizontal air circulation at \(0.3 ext{--}0.5 ext{ m/s}\) thins the leaf boundary layer, slashing diffusive resistance.