Aerenchyma is a tissue type characterised by prominent intercellular spaces which enhance flooding tolerance in some plant species by facilitating gas diffusion between roots and the aerial environment. Aerenchyma in maize roots forms by collapse and death of some of the cortical cells in a process that can be promoted by imposing oxygen shortage or by ethylene treatment. Maize roots grown hydroponically in 3% oxygen, 1 microl x l(-1) ethylene or 21% oxygen (control) were analysed by a combination of light and electron microscopy. Use of in-situ terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) suggested internucleosomal cleavage of DNA. However, chromatin condensation detectable by electron microscopy was preceded by cytoplasmic changes including plasma membrane invagination and the formation of vesicles, in contrast to mammalian apoptosis in which chromatin condensation is the first detectable event. Later, cellular condensation, condensation of chromatin and the presence of intact organelles surrounded by membrane resembling apoptotic bodies were observed. All these events were complete before cell wall degradation was apparent. Therefore, aerenchyma formation initiated by hypoxia or ethylene appears to be a form of programmed cell death that shows characteristics in part resembling both apoptosis and cytoplasmic cell death in animal cells.

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Abstract. We made an experiment on a 30 cm diameter core of Sphagnum-dominated vegetation and peat to estimate the parameters controlling methane oxidation during movement to the ambient air: 13CH4 was added at the water table, and excess 13CO2 appeared in the gas space above the core. At 20øC in otherwise undisturbed conditions, •22% of CH4 was oxidized to CO2 during passage up through the overlying 10-cm thick unsaturated peat and plants. We simulated the experiment, with seven parameters: transfer coefficients in water, in the gas phase, and through the container wall; the rate of CH4 and of CO2 generation; and the two parameters of a hyperbolic relation between CH4 concentration and the rate of CH4 oxidation. We optimized these parameters to fit the experimental results, and then were able to generalize to any temperature (0ø-25øC) and any depth (0-55 cm) of water table. Changing temperature has important effects on the proportion of CH4 oxidized. IntroductionPeatlands cover •3% of the Earth's land surface [Clymo, 1984].About 3.5 million km 2, enough to form a square of side 1800 km, are in the Boreal zone [Gorham, 1991 ], especially in the former USSR, Fennoscandinavia, Canada, and the northern parts of the United States. The vegetation of much of this Boreal peatland is dominated by Sphagnum that, because it decays unusually slowly, comes to form a disproportionately large part of the peat [Clymo, 1984]. The surface layers of a peatland including the live plants and down to the depth to which the water table drops in a dry summer are collectively called the acrotelm [Ingram, 1978]. Below this is the peat proper: the permanently waterlogged and permanently anoxic catotelm, throughout which the hydraulic conductivity is relatively lower, by several orders of magnitude, than it is in the acrotelm. In the acrotelm the water table moves up and down as the balance among precipitation, evaporation, runoff, and downward percolation dictate, and the attendant transition from predominantly oxic conditions above the water table to anoxic ones below also moves up and down [Clymo and Pearce, 1995]. In this article the unsaturated zone is considered to be the acrotelm, and the saturated zone is considered to be the catotelm. [1993] suggested that 30% of the CH4 produced in high-latitude wetlands was oxidized before it reached the atmosphere. It is clear from these works that the potential for CH4 oxidation is widespread, but most of the observations have attendant difficulties too: they measure only potential or net potential, or they use inhibitors whose specificity may be uncertain [Ormeland and Capone, 1988], or the experiments were made in highly disturbed or far from natural conditions. One cannot extrapolate the results to different depths of acrotelm because they reveal little about the rates of those processes that contribute to the overall result: the rate of CH4 production, the rate of transport, and the rate of oxidation. As far as we know nobody has yet tried to make direct measurements of the rate of CH4 ...

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