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aerenchyma
Aerenchyma is a spongy tissue that forms spaces or air channels in the leaves, stems and roots of some plants, which allows exchange of gases between the shoot and the root.〔Sculthorpe, C. D. 1967. The Biology of Aquatic Vascular Plants. Reprinted 1985 Edward Arnold, by London.〕 The channels of air-filled cavities (see image to right) provide a low-resistance internal pathway for the exchange of gases such as oxygen and ethylene between the plant above the water and the submerged tissues. Aerenchyma is also widespread in aquatic and wetland plants which must grow in hypoxic soils.〔Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p〕〔Kozlowski, T. T. (ed.) 1984. Flooding and Plant Growth. Orlando, FL: Academic Press.〕
== Aerenchyma formation and hypoxia ==
When soil is flooded, hypoxia develops, as soil microorganisms consume oxygen faster than diffusion occurs. The presence of hypoxic soils is one of the defining characteristics of wetlands. Many wetland plants possess aerenchyma, and in some, such as water-lilies, there is mass flow of atmospheric air through leaves and rhizomes.〔Dacey, J. W. H. 1980. Internal winds in water lilies: an adaptation for life in anaerobic sediments. Science 210: 1017–19.〕 There are many other chemical consequences of hypoxia. For example, nitrification is inhibited as low oxygen occurs and toxic compounds are formed, as anaerobic bacteria use nitrate, manganese, and sulfate as alternative electron acceptors.〔Patrick, W. H., Jr. and Reddy, C. N. 1978. Chemical changes in rice soils. In Soils and Rice, pp. 361–79. Los Ban˜ os, Philippines: International Rice Research Institute.〕 The reduction-oxidation potential of the rhizhosphere decreases and metal ions such as iron and manganese precipitate.
In general, low oxygen stimulates trees and plants to produce ethylene.〔Kozlowski, T. T. (ed.) 1984. Flooding and Plant Growth. Orlando, FL: Academic Press.〕 Ethylene slows down primary and adventitious root elongation and formation.
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