Past research into flooding tolerance and oxygen shortages in plants has been motivated largely by cultivation problems of arable crops. Unfortunately, such species are unsuitable for investigating the physiological and biochemical basis of anoxia-tolerance as selection has reduced any tolerance of anaerobiosis and anaerobic soil conditions that their wild ancestors might have possessed. Restoration of anoxia-tolerance to species that have lost this property is served better by physiological and molecular studies of the mechanisms that are employed in wild species that still possess long-term anoxia-tolerance. Case studies developing these arguments are presented in relation to a selection of crop and wild species. The flooding sensitivity and metabolism of maize is compared in relation to rice in its capacity for anaerobic germination. The sensitivity of potato to flooding is related to its disturbed energy metabolism and inability to maintain functioning membranes under anoxia and postinoxia. By contrast, long-term anoxia-tolerance in the American cranberry [Vaccinium macrocarpon) and the arctic grass species Deschampsia beringensis can be related to the provision and utilization of carbohydrate reserves. Among temperate species, the sweet flag {Acorus calamus) shows a remarkable tolerance of anoxia in both shoots and roots and is also able to mobilize carbohydrate and maintain ATP levels during anoxia as well as preserving membrane lipids against anoxic and post-anoxic injury. Phragmites australis and Spartina altemiflora, although anoxia-tolerant, are both sulphide-sensitive species which can pre-dispose them to the phenomenon of die-back in stagnant, nutrient-rich water. Glyceria maxima adapts to flooding through phenological adaptations with a seasonal metabolic tolerance of anoxia confined to winter and spring which, combined with a facility for root aeration and early spring growth, allows rapid colonization of sites with only shallow flooding. The diversity of responses to flooding in wild plants suggests that, depending on the life strategy and habitat of the species, many different mechanisms may be involved in adapting plants to survive periods of inundation and no one mechanism on its own is adequate for ensuring survival.

The behavior of purified potato mitochondria toward the main effectors of the animal mitochondrial permeability transition has been studied by light scattering, fluorescence, SDS-polyacrylamide gel electrophoresis, and immunoblotting techniques. The addition of Ca 2؉ induces a phosphate-dependent swelling that is fully inhibited by cyclosporin A if dithioerythritol is present. Mg 2؉ cannot be substituted for Ca 2؉ but competes with it. Disruption of the outer membrane and release of several proteins, including cytochrome c, occur upon completion of swelling. Ca 2؉ -induced swelling is delayed and its rate is decreased when pH is shifted from 7.4 to 6.6. It is accelerated by diamide, phenylarsine oxide, and linolenic acid. In the absence of Ca 2؉ , however, linolenic acid (<20 M) rapidly dissipates the succinate-driven membrane potential while having no effect on mitochondrial volume. Anoxic conditions favor in vitro swelling and the concomitant release of cytochrome c and of other proteins in a pH-dependent way. These data indicate that the classical mitochondrial permeability transition occurs also in plants. This may have important implications for our understanding of cell stress and death processes.Since the late 1970s, it has been known that animal mitochondria can experience a sudden increase in the permeability of their inner membrane to low and medium molecular weight compounds via the opening of a pore (1-3). This mitochondrial permeability transition pore (PTP) 1 is viewed as a multiprotein complex composed at least of the voltage-dependent anion channel, the adenine nucleotide translocator (AdNT), and cyclophilin-D, at the contact sites between outer and inner membranes (4). When the pore opens, solutes up to about 1.5 kDa can pass through the inner membrane, a process known as the mitochondrial permeability transition (MPT). Subsequently, the membrane potential (⌬⌿) decays, oxidative phosphorylation is uncoupled from electron flow, intramitochondrial ions and metabolites are released, and a large amplitude swelling can occur, disrupting the outer membrane and releasing intermembrane compounds.Although pore opening primarily requires the accumulation of Ca 2ϩ in the mitochondrial matrix, it is also modulated by numerous factors. For instance, P i , low ⌬⌿, thiol-oxidizing reagents, low ATP level, fatty acids, anoxia, and reaeration stress all favor pore opening, whereas thiol-reducing agents, low pH, high ⌬⌿, and divalent cations other than Ca 2ϩ counteract it (5). Inhibition of MPT is readily achieved with submicromolar concentrations of cyclosporin A (CsA) (6, 7). This highly specific effect has decisively contributed to the acceptance of the pore theory (6, 7) and is used today as the primary diagnostic trait of the classical MPT (5). The implication of mitochondria and PTP in mammalian cell death gave a new impetus to the research. For instance, cytochrome c has been shown to be released from the mitochondrial intermembrane space into the cytosol (8, 9), where it can trigger apoptosis (10). How ...

In this paper we report on our study of the changes in biomass, lipid composition, and fermentation end products, as well as in the ATP level and synthesis rate in cultivated potato (Solanum tuberosum) cells submitted to anoxia stress. During the first phase of about 12 h, cells coped with the reduced energy supply brought about by fermentation and their membrane lipids remained intact. The second phase (12-24 h), during which the energy supply dropped down to 1% to 2% of its maximal theoretical normoxic value, was characterized by an extensive hydrolysis of membrane lipids to free fatty acids. This autolytic process was ascribed to the activation of a lipolytic acyl hydrolase. Cells were also treated under normoxia with inhibitors known to interfere with energy metabolism. Carbonyl-cyanide-4-trifluoromethoxyphenylhydrazone did not induce lipid hydrolysis, which was also the case when sodium azide or salicylhydroxamic acid were fed separately. However, the simultaneous use of sodium azide plus salicylhydroxamic acid or 2-deoxy-D-glucose plus iodoacetate with normoxic cells promoted a lipid hydrolysis pattern similar to that seen in anoxic cells. Therefore, a threshold exists in the rate of ATP synthesis (approximately 10 mol g ؊1 fresh weight h ؊1 ), below which the integrity of the membranes in anoxic potato cells cannot be preserved.O 2 deprivation becomes a frequent stress for plants submitted to unpredictable heavy rainfalls and flooding. The diffusion of O 2 to their submerged underground organs is severely limited, so that plants must cope with hypoxic or even anoxic conditions. Whereas ATP is produced with a high efficiency by respiration in nongreen cells, its synthesis is much lower under anoxia, with fermentation as the sole energy provider. For most higher plants, the latter condition eventually becomes lethal. The multifarious effects of O 2 deprivation stress on plants sensitive and resistant to anoxia are fairly well understood and have been extensively reviewed in this decade (Armstrong et al., 1994; Sachs, 1994; Ratcliffe, 1995; Crawford and Braendle, 1996; Drew, 1997; Vartapetian and Jackson, 1997).Aside from the obvious interest given to the responses of energy metabolism, the role of macromolecules, and in particular, of gene expression and protein synthesis, has received considerable attention (Sachs, 1994; Drew, 1997). In contrast, the behavior of membrane lipids under anoxia has scarcely been investigated. This is surprising knowing how important it is for a living cell to maintain its membrane integrity. In potato (Solanum tuberosum) tubers, for instance, changes in membrane lipids have mostly been studied during aging, and have been related to overall lipid unsaturation, lipid degradation, and peroxidation processes (Knowles and Knowles, 1989; Spychalla and Desborough, 1990a, 1990b; Kumar and Knowles, 1993; Dipierro and De Leonardis, 1997). Although these effects are not directly relevant to anoxia (Kumar and Knowles, 1996), they are probably related to the effects likely to occur unde...

Mont, L. S., Braendle, R. and Crawford, R. M. M. 1987. Catalase activity and post-anoxic injury in monocotyledonous species.-J. exp. Bot. 38: 233-246. Three anoxia-intolerant species, Glyceria maxima, Juncus effusus and Iris germanica (var. Quechei), and three anoxia-tolerant species Schoenoplectus lacustris, Acorus calamus and Iris pseuaacorus were chosen for investigation. Rhizomes of anoxia-intolerant species show increased catalase activities when returned to air after periods of prolonged anoxia. Levels of catalase remained fairly constant in anoxia-tolerant species under the same conditions. In the anoxia intolerant G. maxima, the postanoxic increase in catalase activity was reduced by circulating the anaerobic atmosphere. This treatment also reduced the ethanol content of the tissue under incubation, and increased the survival of the rhizomes as seen in their ability to resume growth in the post-anoxic phase. Exposure of anaerobic G. maxima rhizomes to ethanol vapour increased post-anoxic levels of catalase activity and when this produced a 5-fold increase always resulted in death of the rhizomes. Acetaldehyde vapour applied in the same way gave rise to increases in catalase activity followed by rapid death of the rhizomes.It is suggested that post-anoxic oxidation of anaerobically accumulated ethanol may result in a surge of acetaldehyde production, which could exert a toxic effect on the recovering tissues. The possible role of catalase in an ethanol-oxidation reaction, which is well documented in animals, is discussed in the light of the association between the natural accumulation of large concentrations of ethanol and subsequent post-anoxic death in some plant tissues.

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