Aerenchyma Development in the Freshwater Marsh Species Sagittaria Lancifolia L.

Abstract

Aerenchyma tissue consists of gas channels that provide a pathway for O$\sb2$ transport to waterlogged roots. Aerenchyma gas spaces are formed by cell separation or cell lysis during development. Light microscopy was used to determine how aerenchyma develops in Sagittaria lancifolia, a freshwater marsh species, when plants were grown in hydroponic solution. Petiole aerenchyma was formed by cell separation caused by differential cell expansion and division. In the root cortex, certain radial files of cells separated slightly from adjacent files and then lysed to form gas spaces. Development of root aerenchyma was characterized in S. lancifolia, Zea mays, and Oryza sativa grown at normal and low O$\sb2$ concentrations in the root zone. Root aerenchyma was formed at both O$\sb2$ concentrations in S. lancifolia and O. sativa, but only at low O$\sb2$ in Z. mays. The relative volume of root aerenchyma increased in S. lancifolia at low O$\sb2,$ but not in O. sativa at low O$\sb2.$ In S. lancifolia, the initial changes in root cortex cells undergoing lysis were fragmentation and condensation of the nucleus as observed by transmission electron microscopy. Breakdown of cytoplasmic organelles and disruption of the plasma membrane followed, but the cell wall remained intact. Cortex cells of Z. mays and O. sativa roots appeared similar to those in S. lancifolia at late stages of cell lysis, although the tonoplast in Z. mays cells disintegrated earlier. These observations of cell lysis were consistent with published descriptions of programmed cell death. Cortical microtubule (CMT) arrays in root cortex cells of S. lancifolia were oriented as expected, except in radial layers of cells termed diaphragms, where CMT arrays were parallel to the root axis. This orientation would lead to the radial expansion of these cells and could produce the separation of the files of cortex cells that precedes cell lysis. It is concluded that there are multiple mechanisms of aerenchyma formation. Three mechanisms of root lysis found in these studies are: constitutive cell lysis (O. sativa), induction of lysis by low O$\sb2$ (Z. mays), and both constitutive cell lysis and induction of cell lysis by low O$\sb2$ (S. lancifolia).