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

Abstract

Volatile (VOC) flux from leaves may be expressed as GSΔP, where GS is stomatal conductance to specific compound and ΔP partial pressure gradient between the atmosphere and substomatal cavities. It has been suggested that decreases in GS are balanced by increases in ΔP 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 m3 mol-1). Our analysis demonstrates that highly volatile compounds such as isoprene and monoterpenes with H values on the order of 103 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 10-2-101 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 ΔP in response to a decrease in GS 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.