Diurnal and seasonal transitions of water and ice content in apple stems: Field tracking the radial location of the freezing- and thawing-fronts using a noninvasive smart sensor

Documentation in the field of the longitudinal progression of freezing in woody stems during winter has recently become feasible due to development of advanced instrumentation. Similar techniques to detect the radial progression of freezing fronts in the stem have remained challenging and, so far, unresolved. Here we address this problem with a smart sensor, based on an inner fringing dielectric technique, but combined with a new analytical model of radial freezing-thawing-fronts. These fronts, occurring in night and daytime of winter, respectively, are considered as vectors, both moving radially from the outside of the stem inward. We present laboratory calibration data and a field experiment in a 14-year old apple orchard to demonstrate the novel capability of this noninvasive sensor for in situ determination of ice content. We track the diurnal and seasonal variation of ice content, identify the radial location of the freezing/thawing-front, link this to anatomical features of the stem, and characterize hysteresis in freezing and subsequent thawing. We document homeostasis of total tissue water content during winter which allowed quantification of ice and liquid water content over both diurnal and seasonal cycles. Moreover, the progressive, day-to-day, decrease in liquid water during cold acclimation in autumn and increase during de-acclimation in spring are shown, along with the associated transpirational fluxes. These data contribute to fundamental knowledge of the water relations of trees, demonstrate a practical technique to monitor multiple trees in the field, and thus provide insight into the ecophysiological processes involved as trees overwinter.