Structure of axial and radial sieve tubes and its functional significance for the phloem network in Aquilaria sinensis (Thymelaeaceae)

Summary The structure of axial and radial or ray sieve tubes was examined by fluorescence microscopy and SEM. Two types of sieve areas were found on the lateral walls of axial and radial sieve tubes: one consists of a cluster of larger, more highly differentiated sieve pores, corresponding to sieve plates, and the other consists of scattered or clustered minute sieve pores, arranged in a linear array in the latter along the cell axis, covered by slime bodies, making a new structure complex associated with nutrient pathways. Migration of cytoplasmic slime bodies in axial and radial sieve tubes was observed to take place by changing shape into filamentous or tubular slimes and passing through the sieve pores, depending on the size of the pores at the xylem differentiation stage. Axial sieve tubes in the interxylary phloem were observed to be arranged axially in dense bundles rather than sparsely scattered. Rarely, it was observed that axial sieve tubes themselves branched off and merged with other axial sieve tubes without being bridged by radial sieve tubes in the ray tissue. Parenchyma cells and ray tissue in the interxylary phloem were unlignified in all the secondary xylem from the 12-year-old tree of Aquilaria sinensis . The division of ray parenchyma cells was often observed in the mature secondary xylem. Furthermore, two types of radial sieve tubes at different stages of differentiation were observed in the same ray tissue, suggesting that at least younger radial sieve tubes differentiate from ray parenchyma cells in the secondary xylem. Based on the positioning of radial sieve tubes and axial sieve tubes in the secondary xylem, three types of connections were identified; (1) radial sieve tube strand in interxylary phloem connects to axial sieve tube strand; (2) radial sieve tube strand bridges two interxylary phloems; (3) radial sieve tube strand anastomoses with interxylary phloem and connects to axial sieve tube strand. The highest frequency of (1) among the three types suggests that it is crucial for the functioning of the phloem network and that radial sieve tubes and axial sieve tubes located inside the interxylary phloem are connected to establish longitudinal transport channels.