Aerenchyma tissue development and gas-pathway structure in root of Avicennia marina (Forsk.) Vierh.

The aerenchyma differentiation in cable roots, pneumatophores, anchor roots, and feeding roots of the mangrove plant, Avicennia marina (Verbenaceae) was analyzed using a light microscope and scanning electron microscope. In all types, cortex cells were arranged in longitudinal columns extending from the endodermis to the epidermis. No cells in the cortex had intercellular spaces at the root tip (0–150 m), and aerenchyma started developing at 200 m from the root apex. The aerenchyma formation was due to cell separation (schizogeny) rather than cell lysis. The cell separation occurred between the longitudinal cell columns, forming long intercellular spaces along the root axis. During aerenchyma formation, the cortex cells enlarged longitudinally by 1.8–3.9 times and widened horizontally by 2.2–2.9 times. As a result, the aerenchyma had a pronounced tubular structure that was radially long, elliptical or oval in cross section and that ran parallel to the root axis. The tube had tapering ends, as did vessel elements, although there were no perforated plates. The interconnection between neighboring tubes was made by abundant small pores or canals that were schizogenous intercellular spaces between the wall cells. All aerenchyma tubes in the root were interconnected by these small pores serving as a gas pathway.     

Root system architecture and gravity perception of a mangrove plant,Sonneratia alba J. Smith

We describe the features of the root system and the gravitropism of roots produced bySonneratia alba. The root system consists of four root types with different growth directions: (a) Pneumatophores, which are negatively orthogravitropic and their statocytes are very large (922 μm²) and the statolith is located near the proximal wall, (b) Cable roots and (c) Feeding roots which are both diagravitropic and their statoliths are settled along the longitudinal wall, and (d) Anchor roots which are positively orthogravitropic. The statocyte cells are the smallest (420 μm²) and statoliths settled at the distal wall. We found that all roots with marked gravitropism have statoliths that settle along different walls of the statocyte. This implies that the statoliths sensing of gravity is done by gravity on mass, and that they are denser than surrounding cytoplasm and this position is related to root growth direction. This finding matches the statoliths sediment under the effect of gravity. Irrespective of statolith, position and direction of growth may be stable.