Formation of aerenchyma and the processes of plant ventilation in relationto soil flooding and submergence

Enhanced development of gas-spaces beyond that due to the partial cell sepa ration normally found in ground parenchymas and their derivatives creates t issue commonly termed "aerenchyma". Aerenchyma can substantially reduce int ernal impedance to transport of oxygen, nitrogen and various metabolically generated gases such as carbon dioxide and ethylene, especially between roo ts and shoots. Such transport lessens the risk of asphyxiation under soil f looding or more complete plant submergence, and promotes radial oxygen loss from roots leading to oxidative detoxification of the rhizosphere. Aerench yma can also increase methane loss from waterlogged sediments via plants to the atmosphere. This review of the formation and functioning of aerenchyma particularly emphasises research findings since 1992 and highlights prospe cts for the future. Regarding formation, attention is drawn to how little i s known of the regulation and processes that create schizogenous aerenchyma with its complex cell arrangements and differential cell to cell adhesion. More progress has been made in understanding lysigenous aerenchyma develop ment. The review highlights recent work on the processes that sense oxygen deficiency and ethylene signals, subsequent transduction processes which in itiate cell death, and steps in protoplast and wall degeneration that creat e the intercellular voids. Similarities between the programmed cell death a nd its causes in animals and the predictable patterns of cell death that cr eate lysigenous aerenchyma are explored. Recent findings concerning functio n are addressed in terms of the diffusion aeration of roots, rhizosphere ox ygenation and sediment biogeochemistry, photosynthesis and ventilation, pre ssurised gas-flows and greenhouse gas emissions and aspects of ventilation related to secondary thickening.