Concurrent measurements of bark and xylem water contents in Malus pumila Mill. stems using improved flexible sensors

The pattern of radial water transport in tree stems, specifically the interaction between bark and xylem, remains elusive because few measurement techniques are not capable of independently sensing bark water content (BWC) and xylem water content (XWC). To investigate the water variations in stem, two improved flexible sensors operating at 100 MHz and equipped with small interdigital-electrode (IE) probes were developed to measure BWC and XWC independently. The performances of the flexible sensors were tested under laboratory conditions. Software simulations and laboratory measurements were performed to evaluate the volume of sensitivity (VOS) and to assess the impact of variations in stem diameter (SD) on the sensors. Concurrent measurements of BWC and XWC were performed on three trees in Malus pumila Mill. (Red Fuji). The measurements revealed that the difference in predawn water content between the bark and xylem gradually increased as the water deficit intensified. The maximum daily variation of BWC from predawn to afternoon (MDV_BWC) was greater than that of the maximum daily variation of XWC from predawn to afternoon (MDV_XWC). In addition, the dehydration-rehydration loops exhibited a time lag between BWC and XWC under water deficit conditions, with earlier dehydration and rehydration of the bark than of the xylem. Concurrent measurements of BWC and XWC provide a new perspective for examining the pattern of stem radial water transport. However, the time lag between BWC and XWC might need to be validated in other tree species.