Wetland plants as a subject of interdisciplinary research

Abstract

Habitats that are prone to flooding usually support a distinctive flora. Rushes, sedges and reed are vernacular terms which convey an image of a wetland plant community even to the non-botanist. What features enable these plants to survive months or even years of flooding while the intolerant plants die under such conditions after several days or even hours? A whole range of niches exists even within the wetland environment (Tab. 1, 2). What strategies underlie this niche differentiation, observed and described by field botanists? Waterlogged and flooded soils are usually anoxic, except for a thin surface layer. Due to its low diffusivity in water, oxygen is depleted from most soils in several hours to days after the onset of flooding. Anoxia is associated with the accumulation of potential phytotoxins such as ferrous and manganous compounds, hydrogen sulphide and various organic compounds produced by anaerobic microorganisms in the soil (GAMBRELL & PATRICK 1978, PONNAMPERUMA 1984). Thus, wetland plants face the following major problems: (1) to ensure an alternative oxygen supply for the metabolism of their roots and other underground organs (for example rhizomes); (2) to establish an adequate metabolism under oxygen shortage and re-oxygenation; and (3) to protect themselves against the effects of phytotoxins. Well developed aerenchyma is obviously a solution to the first problem (oxygen supply). Large gas spaces are a characteristic feature of roots and rhizomes of many wetland and aquatic plants (SCULTHORPE 1967, JUSTIN & ARMSTRONG 1987). This system of interconnected gas spaces permits internal aeration, i.e. a supply of oxygen from the atmosphere via aerial shoots down to the rhizomes and roots (ARMSTRONG et al. 1994). In rhizomatous wetland and aquatic plants, a pressure gradient drives the flow of air (internal wind) through influx shoots down to the rhizomes and roots and up again through efflux shoots (GROSSE et al. 1991, ARMSTRONG & ARMSTRONG 1991). Part of the oxygen present in the gas spaces covers respiration of root/rhizome cells. The other part diffuses across the surface tissues to the anaerobic surroundings and creates an oxidised layer around the buried organs. This oxidised layer is regarded as a protection against contact with the anaerobic phytotoxins (KONtALOVA 1990, ARMSTRONG et al. 1994). Even with a well developed system of gas spaces, wetland plants can suffer from oxygen deficiency. During the growing season, hypoxic conditions can occur within plant roots living