A second pathway for gas out of the pressure chamber—what is being squeezed?

We report a qualitative description of the flows of gas that occur through a leaf when its balance pressure is measured in the pressure chamber. There are two distinct pathways: (a) a bulk flow of gas through the intercellular air spaces, and (b) a diffusion-driven pathway where gas is dissolved into solution under high pressure and comes out of solution at the liquid/atmosphere surface of the cut end where the pressure is atmospheric. The intercellular space flow is well known. It is argued that this flow shows to a reasonable approximation, that the externally supplied gas is squeezing the non-gaseous part of the leaf, and the outer boundary of the non-gaseous material is the boundary of the system that is being manipulated. The second pathway, the diffusion-driven flow, has not (we believe) been described before, and is analogous to a diver getting the bends. The diffusion-based flow demonstrates that gas spaces can and do form inside the outer boundary of the non-gaseous part of the leaf when a balance pressure is measured. These interior gas spaces alter the value recorded for the balance pressure, and complicate any interpretation of what this measurement tells us about the water status of the plant. A hypothesis is proposed that the diffusion-based flow from the xylem comes from vessels that are embolized, and that percentage embolisms might be measured by the proportion of vessels showing the diffusion-driven flow.