The occurrence and development of intraxylary phloem in young Aquilaria sinensis shoots

Abstract

ABSTRACT Agarwoods such as Aquilaria spp. and Gyrinops spp. (Thymelaeaceae) produce interxylary phloem in their secondary xylem and intraxylary phloem at the periphery of the pith, facing the primary xylem. We studied young shoots of Aquilaria sinensis and characterized the development of its intraxylary phloem. It was initiated by the division of parenchyma cells localized in the outer parts of the ground meristem immediately following the maturation of first-formed primary xylem. Its nascent sieve plates bore donut-like structures, the individual pores of which were so small (less than 0.1 μm) that they were hardly visible under FE-SEM. Intraxylary phloem developed into mature tissue by means of the division and proliferation of parenchyma cells. During the shoots’ active growth period, the sieve pore sizes were 0.1–0.5 μm, with tubular elements passing through them. In the maturation stage, large clusters of sieve tubes continued to be differentiated in the intraxylary phloem. In the partial senescence stage observed in a three-centimeter-diameter branch, intraxylary phloem cells in the adaxial part became crushed, and sieve plates had pores over 1–2 μm in diameter without any callose deposition. Before and after the differentiation of interxylary phloem in the first and second internodes, callose staining detected more than twice as many sieve tubes in intraxylary phloem than in external phloem. However, after differentiation of interxylary phloem in the eleventh internode, more sieve tubes were found in interxylary phloem than in intraxylary and external phloem. This suggests that prior to the initiation of interxylary phloem intraxylary phloem acts as the principal phloem. After its differentiation, however, interxylary phloem takes over the role of principal phloem. Interxylary phloem thus acts as the predominant phloem in the translocation of photosynthates in Aquilaria sinensis.