# Acoustic Emissions to Measure Drought-Induced Cavitation in Plants
## 2 The Importance of Water  
On a daily basis, plants extract water from the soil via the roots from where it is transported in the stem towards the leaves, where it eventually transpires into the atmosphere [18]. This seemingly wasteful process is vital for plant survival. Water is the transport medium that carries nutrients from the soil towards the plant organs and distributes generated carbohydrates throughout the plant. The evaporation of water in the stomata of leaves provides a cooling function, which is necessary to prevent overheating of leaves during sunny days. The water in living cells also provides a crucial role in the firmness and elasticity of soft tissues. The positive pressure that is exerted on the cell walls, called turgor pressure, is essential for growth (through cell growth and cell division) and fulfills the role of backbone in non-woody tissues such as leaves and petioles. Only a minor fraction of the transported water (<1%) is used to make new plant material through photosynthesis [19; 20]. Given this multitude of functions, it is hence no wonder that water shortage is one of the main causes of plant mortality [21; 22]. Water is transported in the xylem tissue of the plant following a gradient in water potential. Xylem is a porous structure of dead cells containing a network of parallel conduits, interconnected by pits [23]. The xylem conduits operate under negative pressure, or tension. According to the cohesion-tension theory [18], the origin of this tension is the evaporation of water in the stomatal region. When the stomata in the leaves are open, water is transpired due to the difference in water vapour pressure between the atmosphere and the substomatal cavity. As water evaporates into the air spaces in the leaf, water menisci in the small capillaries (nanometer scale) in adjacent cell walls are retracted and capillary forces (due to strong adhesion) pull the menisci back towards the surface. The network of many small capillaries in the cell wall acts thus as a wick for water rise [24]. Thanks to the strong cohesion between water molecules, the tension is transmitted downwards and water can be drawn towards the leaves. The tension in the xylem conduits may increase enormously when faced with a dry soil or with a great transpirational demand that exceeds the rate of water supply from the roots or from internal water reserves. This involves a risk of gas bubbles entering the conduits, which may expand and quickly fill the whole conduit. The water released from the conduits during this process may contribute to the transpiration stream, but on the other hand, the water conducting system will be locally interrupted. Because adjacent conduits are interconnected via pits in the conduit walls, the sap may circumvent the embolized conduit. However, when too many conduits are embolized, this will impair plant functioning. The formation of air emboli that block sap flow in xylem conduits is currently of high interest because it is one of the key processes leading to plant mortality during drought [22]. The phenomenon is called cavitation, which is the mechanical breakage of the continuous xylem water column and occurs when the tensile strength of the column is exceeded [25]. According to the current knowledge, the main cause of cavitation is the failure of a pit in the conduit wall to prevent gas from entering the conduit at strong tension [26], known as the air-seeding hypothesis [27]. Recently, [PERSON] _et al._[28] postulated that nanobubbles are snapped off during air-seeding. These nanobubbles are stabilized by surfactants and may exist in plant sap under tension. They may eventually result in an embolism when the size of the nanobubble exceeds a critical threshold due to increasing tension or when many nanobubbles coalesce.