Stomatal regulation in greenhouse crops

We study the impact of the stomatal opening variations on the transpiration of the entire canopy in protected crops cultivation. Three effects contribute to a reduction of this impact: a damping effect due to the boundary layer adhering to the leaf, a thermal negative feedback effect linked to the energy balance and a hydric negative feedback effect linked to the water balance. Summing up these effects gives a coupling factor between the relative stomatal conductance changes and the relative transpiration rate modifications.

Detailed expression of the coupling factor is established in the frame of a dynamic greenhouse climatic model (GGDM) and its value is calculated for typical days of a temperate climate. The results show that large stomatal conductances are poorly effective in the control of the canopy transpiration (coupling factor =.1 ). Significant values of the coupling factor are only obtained for very small stomatal conductances (gST < 10-3 ) . These values are independant of the climatic conditions but are strongly affected by the development stage of the vegetation.