An ultrastructural study using anhydrous fixation of Eragrostis nindensis, a resurrection grass with both desiccation-tolerant and -sensitive tissues

Abstract

The ability of tissues to survive desiccation is common in seeds but rare in vegetative tissues. In this study the ultrastructure of hydrated and dehydrated tissues were examined at different stages of the life cycle of the resurrection grass, Eragrostis nindensis Ficalho & Hiern. Conventional fixation techniques are unsuitable for dry tissues as rehydration occurs during fixation in aqueous fixatives. Thus a cryofixation and freeze-substitution method was developed. As a result of the improved fixation methods, it was possible to identify the stage and nature of the damage in the desiccation-sensitive tissues. E. nindensis has desiccation-tolerant orthodox seeds, but the young seedlings are not tolerant to extreme water loss. However, like the seeds, most of the leaves of the adult plant are tolerant to desiccation (only the oldest outermost leaf on a tiller are not). Desiccation-induced damage in these outer leaves was observed in the later stage of dehydration, dominated by the appearance of abundant cell wall fractures (1 wall fracture per 50 μm2). Unlike the outer leaves, the leaves of seedlings appeared similar to those of the hydrated ones upon desiccation. Irreparable damage occurred on rehydration of these tissues possibly as a result of the absence of protection mechanisms observed during desiccation of the inner desiccation-tolerant leaves of the mature plants. The mesophyll tissues of these leaves become compact with extensive cell wall folding on drying. The bundle sheath cells maintained their shape with desiccation but became packed with small vacuoles.