SECONDARY GROWTH OF THE LEAF AND BUD TRACES IN HYLOCEREUS UNDATUS (CACTACEAE) DURING THE FORMATION OF BRANCHES OR FLOWERS.

Premise of research. Hylocereus undatus produces two different types of woody structures supporting its lateral shoots. Although a branch and a flower both derive from the same initials, the dormant axillary buds (in cacti aka areoles), the fully developed support structures within the succulent cortex of the parental shoot differ significantly in their macroscopic and microscopic anatomy. The objective of our research was to analyze the secondary growth of these two structures as they develop from their initial state as leaf/bud traces that connect the axillary bud with the stele of the parental shoot. Methodology. Macroscopic anatomy was visualized by a staining and clearing technique for lignified elements of whole-plant organs. The microscopic anatomy was investigated with microtome sections of samples embedded in wax and stained with safranin and fast green. Pivotal results. The vascular tissue of both support structures has a wood dimorphism consisting of an early parenchymatous phase followed later by fibrous wood. In vegetative branches, the woody support structures have the typical ringlike arrangement as found in the stele of the parental shoot, whereas the flower support structures have a reticular arrangement of interconnected woody strands. This fundamentally different anatomy of the support structures can be traced back to very early developmental stages: an interfascicular cambium arises between the leaf/bud traces when a vegetative branch forms but is absent in the case of a flower. Conclusions. (1) Leaf/bud traces of all internodes are initially equivalent; (2) leaf/bud traces undergo either of two kinds of secondary growth, each adapted to the needs of the lateral shoot that develops; and (3) the fate of the axillary bud and its leaf/bud traces is decided early, before the bud is large enough to be visible without dissection. [ABSTRACT FROM AUTHOR]