Tissue Responses and Solution Movement After Stem Wounding in Six Cycas Species

Abstract

We made drill holes in the stems of six Cycas species and used a standard microtechnique and microscopy methods to determine the structural responses of the soft-wooded, parenchymatous (manoxylic) stems after 2 and 12 months of recovery. We also injected an aqueous stain to one plant per species to determine the transverse hydraulic pathways among the discrete tissue categories. Expanding secondary tissue decay and insect larva infestations were evident in some wounds after 2 months. Wounds that lacked the secondary complications had recovered as a result of wound periderm after 2 months. Large areas of secondary rotting after 2 months were bounded by wound periderm after 12 months. The wound periderm formed a thick, leathery phellem (cork) that was continuous across cortex, vascular, and pith tissues. The six species represent a range in ease of horticultural management, yet the form of recovery from the wounds was similar for all species. Mucilage exudation from cut surfaces was copious, and species differences in volume of mucilage were also not related to extent or form of recovery from the wounds. Stain injected into the hole moved longitudinally and laterally within a vascular cylinder to adjacent vascular cylinders and to the cortex by way of persistent leaf traces. Results indicate a transverse hydraulic connection among the concentric vascular cylinders and leaf traces that extend into the cortex. Moreover, the succulent nature of this manoxylic stem is highly susceptible to secondary infection whenever the protective bark is removed or the internal tissues are injured. Therefore, use of prophylactic treatments to minimize the risk of secondary complications is warranted whenever pruning or amputation of adventitious shoots for propagation remove the protective bark from the succulent cortex tissue.