Interaction between secondary phloem and xylem in gravitropic reaction of lateral branches of Tilia cordata Mill. trees

Abstract

Abstract The tension wood (TW) of Tilia cordata (lime tree) does not contain gelatinous fibers. Based on anatomical studies of secondary phloem (secPhl) and xylem by means of microscopy, digital imaging, and biomechanical tests, it was hypothesized that there is an interaction between the phloem and xylem as a response of gravitropic forces on lateral branches. The goal of the present study was to check this hypothesis. The results demonstrated that dilated phloem rays are longer and wider on the upper side (US) of a branch compared to the lower side (LS) and that the ratio of fiber/ray parenchyma in the phloem is lower on the US of the branches. Bark strips consisting of secPhl with periderm have higher elastic modulus (MOE) on the US of branches. The results support the hypothesis that the compression stress of ray parenchyma may cause phloem fibers to stretch, which may result in the development of axial tensile stresses that are higher on the US of branches. However, the wider rings of xylem formed on the US of branches and the results of biomechanical tests can be interpreted that a higher MOE of wood in the US of lateral branch are the main factors responsible for gravitropic reaction of Tilia branches. TW can be considered as a kind of biotensegrity.