Controls on the emission of plant volatiles through stomata: Differential sensitivity of emission rates to stomatal closure explained

Volatile (VOC) flux from leaves may be expressed as G S Δ P , where G S is stomatal conductance to specific compound and Δ P partial pressure gradient between the atmosphere and substomatal cavities. It has been suggested that decreases in G S are balanced by increases in Δ P such that stomata cannot control VOC emission. Yet, responses of emission rates of various volatiles to experimental manipulations of stomatal aperture are contrasting. To explain these controversies, a dynamic emission model was developed considering VOC distribution between gas and liquid phases using Henry's law constant ( H , Pa m 3 mol −1 ). Our analysis demonstrates that highly volatile compounds such as isoprene and monoterpenes with H values on the order of 10 3 have gas and liquid pool half‐times of a few seconds, and thus cannot be controlled by stomata. More soluble compounds such as alcohols and carboxylic acids with H values of 10 −2 –10 1 are controlled by stomata with the degree of stomatal sensitivity varying with H . Inability of compounds with high solubility to support a high partial pressure, and thus to balance Δ P in response to a decrease in G S is the primary explanation for different stomatal sensitivities. For compounds with low H , the analysis predicts bursts of emission after stomatal opening that accord with experimental observations, but that cannot be currently explained. Large within‐leaf VOC pool sizes in compounds with low H also increase the system inertia to environmental fluctuations. In conclusion, dynamic models are necessary to simulate diurnal variability of the emissions of compounds that preferably partition to aqueous phase.