Regulation of Aerenchyma Formation in Roots and Shoots by Oxygen and Ethylene

Abstract

Excess water, especially around roots and the lover parts of the shoot, is a frequent cause of abnormally slow rates of gaseous exchange between plants and their environment (Armstrong, 1979) in a wide range of habitats that embrace tidal estuaries, river and lake margins, swamp and marshland, poorly drained agricultural land, rice paddies and irrigated land in arid areas. The challenge this presents to survival is compounded by activities of the soil microflora that initially competes with roots for oxygen and later, when the soil is rendered anaerobic, chemically reduces iron and sulphur, in sequence, to soluble phytotoxic forms (Ponnamperuma, 1972). Thus, it is not surprising that most species of land plants are severely injured by temporary soil flooding and are excluded completely from areas that experience prolonged waterlogging. However, some dryland species such as wheat, maize, tomato and sunflower can survive soil flooding by adaptation (Jackson and Drew, 1984). Their survival does not usually reflect a metabolic tolerance to anoxia per se. Instead, new roots and existing shoot bases avoid anoxia by forming highly porous (aerenchymatous) interiors that permit gas exchange along internal pathways of small resistance linking root cells with the air or with photosynthetically generated oxygen in the leaves. These pathways constitute a linearized form of cell separation.