ANATOMY OF BRANCH ABSCISSION IN LAGERSTROEMIA MICROCARPA WIGHT

Abstract

SUMMARYSome anatomical details of branch abscission in Lagerstroemia microcarpa Wight (Lythraceae) are discussed. Repeated abscission of numerous annual twigs and subsequent healing of their scars produce irregular growth of the subjacent branch portions to give gall-like structures. In the abscission zone, secondary xylem fibres are thin walled and poorly lignified, with dense protoplasmic contents and closely spaced septa. Disintegration of pith parenchyma cells and shrinkage of bark and wood tissues contribute towards weakening of abscission zone. The protective zone situated proximal to the abscission zone is strongly lignified and rich m extractives. Detachment occurs immediately above the protective zone leaving the encircling dormant buds intact. Abscission scars are healed centripetally by the usual method of callus formation from the cambial tissue. It appears that branch abscission in L. microcarpa is a mechanism to withstand drought by reducing the transpiring surface.

In vitroabscission of kauri (Agathis australis) branches

“…The 3-5 layers of densely-cytoplasmic cortical parenchyma, above which distal regions of explant internodes detach, are anatomically similar to the abscission zones in other species. Rows of apparently meristematic cells are present in the swollen bases of kauri seedling branches (Licitis-Lindbergs 1956), and separation-zone parenchyma are associated with the abscission of twigs and branches in Lagerstroemia microcarpa (Bhat et al 1986) and Taxodium distichum (Barnard 1926). 4.…”

“…The separation face is not linear; both in situ and in vitro the distal portion tapers to a horn. Perhaps the most significant departures from the normal course of branch abscission were that (i) delignification, suberisation, and the accumulation of polyphenols, resins, and tannins at the separation face (Barnard 1926;Licitis-Lindbergs 1956;Bhat et al. 1986) were not evident in kauri explant tissue; and (ii) only those explants derived from the latest growth flush were capable of forming abscission zones (Table 1), whereas abscission in situ only occurs at the base of the older, heavily-lignified branches (Wilson et al 1998).…”

“…Relative to intact controls, those branches were swollen at proximal regions, had a much dilated cortex and pith, and had an attenuated vascular system. Workers on branch abscission in other species have reported the reduced lignification of cells in a discrete abscission zone (e.g., Osborne 1984), well-defined protective layers of periderm adjacent to this zone (e.g., Bhat et al 1986), and lignification of the exposed scar, either before or after branch fall (e.g., Barnard 1926). However, the full sequence of developmental events is not apparent from these isolated comparisons, and more detailed anatomical investigation is required if we are to understand the significance of cellular events that ultimately lead to the shedding of a branch.…”