ExoHeat: A continuous heating solution for bi-directional sap flow in small-diameter plant organs demonstrated in tomato peduncles

• ExoHeat, a continuous-heating sap flow sensor for small-diameter plant organs. • Accurate bidirectional flow measurements up to 2 g h −1 in tomato peduncles. • Zero-flow correction to account for ambient temperature changes and peduncle diameter. • Whole-plant validation showed a close agreement with gravimetric transpiration. • High-resolution measurements revealed distinct sap flow patterns in the peduncle. Accurate sap flow measurements in small-diameter plant organs are essential for understanding water transport and source-sink dynamics, yet existing methods are limited by their temporal resolution, reduced sensitivity to low or reverse flow, and incompatibility with small organ dimensions. In this study, the ExoHeat sensor, a continuous-heating solution was developed for bidirectional sap flow measurements in small-diameter plant organs. Its performance was validated on tomato truss peduncles ( Solanum lycopersicum L.). Zero-flow corrections accounting for ambient temperature and peduncle diameter ensured robust baseline adjustment, while gravimetric calibrations revealed a strong linear relationship between the sensor-measured temperature difference and sap flow rate up to 2 g h −1 , corresponding to a sap flux density of 5.8 10 -3 cm 3 cm −2 s −1 . Whole-plant validation further demonstrated close agreement between ExoHeat-derived sap flow and gravimetric transpiration data. Anatomical imaging showed an asymmetrical distribution of xylem vessels in the tomato truss peduncle, underscoring the importance of correct sensor orientation. High-resolution measurements on ripening trusses successfully captured dynamic bidirectional flow patterns. The ExoHeat sensor thus provides a novel, high-temporal-resolution tool for accurate monitoring of sap flow in small-diameter organs, with promising applications in plant physiology, irrigation optimisation and stress detection.