Ms. Reddy Prasanna

Interfaces in xylem occur due to aspiration of air bubble from the xylem sap or due to weakening of water column under high tension. Refilling of embolized may rely on sap’s surface activity at the liquid-air interface. However, until recently, the surface tension (ST) of xylem sap is assumed to be equal to pure water (72mN m-1). Sap, on the other hand, is a complex solution made up of various molecules that are amphiphilic in nature which may adsorb at the liquid-air interface and reduce the ST as a function of their aqueous concentration and temperature. However, reduction in ST is not instantaneous, but rather dynamic, as amphiphilic molecules tend to diffuse from bulk and adsorb at the freshly formed interface. However, there is a lack of quantitative knowledge on the rate and extent of dynamic and equilibrium ST (EST) of sap. The main objective of our study is to quantify DST & EST as a function of their aqueous concentration and temperature. Results showed an exponential decay in ST of xylem sap indicating significant surface activity. The magnitude of decay of ST depended on concentration and temperature, while reaching EST as low as 43- 45 mN m-1. Furthermore, the slope of EST values deviated from pure water at different temperatures implying multi-molecular dispersion at the interface. Our results revealed pronounced surface activity of xylem sap and its dependency on concentration and temperature. These dependencies may be relevant in xylem vulnerability analyses, where high ST solutions overestimate hydraulic safety.