Positive pressure in xylem and its role in hydraulic function

Although transpiration-driven transport of xylem sap is well known to operate under absolute negative pressure, many terrestrial, vascular plants show positive pressure above atmospheric on a seasonal or daily basis, during early developmental stages. The actual location and mechanisms behind remain largely unknown, both in that before leaf flushing, those diurnal periodicity bleeding guttation. Available evidence shows can be driven based purely physical forces, osmotic exudation into conduits, hydraulic parenchyma cells associated with conduits. latter two may not mutually exclusive understood similar modelling scenario. Given the renewed interest this review aims provide constructive way forward by discussing similarities differences mechanistic models, evaluating available for functions, such as rehydration tissues, refilling water stores, embolism repair providing recommendations future research, including methods avoid minimise cutting artefacts.