Variation in the Structure and Response to Flooding of Root Aerenchyma in some Wetland Plants

The structure and response to flooding of root cortical aerenchyma (air space tissue) in a variety of wetland (flood-tolerant) species was investigated and compared with some flood-intolerant species. In some species aerenchyma consisted of enlarged schizogenous intercellular spaces and in others aerenchyma formation involved lysigeny. Two types of lysigenous aerenchyma were distinguished. In the first the diaphragms between lacunae were arranged radially and consisted of both collapsed and intact cells. In the second type, which was confined to the Cyperaceae, the radial diaphragms contained intact cells, and stretched between them were tangentially-arranged diaphragms of collapsed cells. Flooding in sand culture generally increased root porosity (air space content) although there were exceptions. The flood-intolerant species Senecio jacobaea produced aerenchyma but did not survive long-term flooding. Among the flood-tolerant species, Filipendula ulmaria did not produce extensive aerenchyma even when flooded. Eriophorum angustifolium and E. vaginatum produced extensive aerenchyma under drained conditions which was not increased by flooding. In Nardus stricta root porosity was increased by low nutrient levels as well as by flooding.