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SUMMARYNinety-one plant species from wetland (WL), intermediate (INT) and non-wetland (NWL) habitats were grown in flooded and drained soils and responses to flooding were assessed in relation to root anatomy and fractional root porosity (FRP).Flooding response and tolerance were related to FRP. Rooting depth increased with FRP in accordance with diffusion model predictions and emphasized the ventilating efliciency of aerenchyma. Major determinants of FRP were cortical cell conflgurations, closeness of cell packing, the relative proportions of conflguration types, porous: non-porous tissue ratios, aerenchyma development and the type and degree of secondary growth. A classification of cortical types based on cell and aerenchyma patterns is presented.Aerenchyma, both schizogenous and lysigenous, developed preferentially where preaerenchymatous cortical cell configurations (in TS) were radial and particularly, cubic and radial typical of WL and INT species. Aerenchyma rarely formed from hexagonal non-radial arrays which occur chiefly in NWL and in the outer cortical zones of WL and INT plants. The ventilating potential in non-aerenchymatous tissue was shown to be greater for cubic (fractional porosity, FP, max. 0-2146) than for hexagonal arrays (FP max. 0-0931); closer packing greatly accentuated the differences. It is suggested that cubic: hexagonal zonal ratios in roots may reflect a balance between respiratory and mechanical needs.In a majority of WL and INT species, shoot weights were unaffected by or increased with flooding and maximum rooting depths usually much exceeded 50 mm. Cubic packing raised the FRPs, as did aerenchyma which was often much greater under flooding. In the dicotyledonous species, a suppression of secondary growth in some, and a highly porous phelloderm in others, helped maintain high FRP. A minority of species were anatomically and responsively similar to NWL plants; survival under flooding was attributed to shallow rooting.Under flooding, the FRP of almost all NWL species was < 0-055 due to hexagonal packing, a lack of aerenchyma and, in dicotyledonous plants, secondary growth with scanty phelloderm. Shoot weights were reduced in 50 % of cases, rooting depths were < 50 mm, and some species died. Some species were exceptional in having cubic and radial packings; a lack of aerenchyma was associated with continuing meristematic activity in the primary cortex.

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Enhanced development of gas‐spaces beyond that due to the partial cell separation normally found in ground parenchymas and their derivatives creates tissue commonly termed “aerenchyma”. Aerenchyma can substantially reduce internal impedance to transport of oxygen, nitrogen and various metabolically generated gases such as carbon dioxide and ethylene, especially between roots and shoots. Such transport lessens the risk of asphyxiation under soil flooding or more complete plant submergence, and promotes radial oxygen loss from roots leading to oxidative detoxification of the rhizo‐sphere. Aerenchyma 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 prospects for the future. Regarding formation, attention is drawn to how little is 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 development. The review highlights recent work on the processes that sense oxygen deficiency and ethylene signals, subsequent transduction processes which initiate cell death, and steps in protoplast and wall degeneration that create the intercellular voids. Similarities between the programmed cell death and its causes in animals and the predictable patterns of cell death that create lysigenous aerenchyma are explored. Recent findings concerning function are addressed in terms of the diffusion aeration of roots, rhizosphere oxygenation and sediment biogeochemistry, photosynthesis and ventilation, pressurised gas‐flows and greenhouse gas emissions and aspects of ventilation related to secondary thickening.

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