Differences in the Apparent Permeability of Spore Walls and Prothallial Cell Walls in Onoclea sensibilis

Abstract

The spores of Onoclea sensibilis L. undergo marked changes in their apparent permeability during germination. For example, spores do not stain with an acetocarmine-chloral hydrate mixture until germination has proceeded for about 20 hr (Towill & Ikuma, 1975; Fisher & Miller, 1978), whereas after that time stain is absorbed readily. During the early stages of germination, the spores of Onoclea, Matteuccia struthiopteris (L.) Tod., and other species are difficult to prepare for electron microscopy because embedding resins fail to penetrate adequately (Marengo, 1973; Gantt & Amott, 1976; Raghavan, 1976). Spores in later stages can be processed with no difficulty. This impermeability seems to be associated with the inner spore coat (intine). Fisher and Miller (1978) noted that when the intine of Onoclea spores was artificially ruptured during the early stages of germination and the protoplast was directly exposed to acetocarmine-chloral hydrate, the protoplast stained rapidly, whereas no intact spores could be stained. The time at which Matteuccia and Onoclea spores normally become penetrable by embedding resins coincides with the time the intine ruptures naturally during germination (Gantt & Amott, 1965; Marengo, 1973). If the intine of Onoclea is caused to open by treatment with sodium hypochlorite, even dormant spores and those in early stages of germination may be infiltrated readily with embedding resin (Bassel, Kuehnert & Miller, 1981). Dormant spores of both Onoclea and Matteuccia have a loose outer spore coat and a thick intine, along which there is a longitudinal seam, the raphe (laesura), on the flattened, proximal face of the spore; the spore protoplast is naked within the intine (Gantt & Arnott, 1965; Bassel, Kuehnert & Miller, 1981). Germinating spores synthesize a new wall around the protoplast inside the intine between 8 and 16 hr. At the time the intine ruptures and is cast off, this new wall becomes the bounding wall of the young protonema. Clearly there is a difference in which materials will penetrate the spore intine and which will cross the normal prothallial cell wall. Carpita et al. (1979) published a method for obtaining quantitative information about the apparent capillary pore size of plant cell walls, which limits the passage of solutes. The cells are placed in a solution of a non-ionic solute having a water potential lower than that of the cells, which causes water to leave the cells. If the solute can pass through the wall, and thus the solution can be in contact with the plasma membrane, one observes plasmolysis (retraction of the protoplast from the cell wall). If, however, the solute particles are too large to penetrate the wall, exit of water from the cell causes cytorrhysis (collapse of the cell wall around the protoplast as it shrinks). I applied this technique to spores and young gametophytes of Onoclea and observed differences in the apparent capillary pore sizes of the spore intine, prothallial cell walls, and rhizoid walls.