Carbohydrate metabolism and anoxia tolerance in cereal grains

Few plant species can survive when the availability of oxygen drops below the needs of aerobic respiration. Under these conditions, arising frequently from soil flooding, a limited number of plant species can avoid a rapid death, thanks to morphological and metabolic adaptations allowing them to avoid or tolerate the anaerobic stress. In the recent years, a number of reviews about plant responses to anaerobiosis have been published (Drew 1990; Kennedy et al. 1992; Perata & Alpi 1993; Armstrong et al. 1994; Crawford & Braendle 1996; Drew 1997). This review will therefore not include reference to many aspects on this subject but will rather focus on the response of a group of cultivated plants (cereals), with particular emphasis on their carbohydrate metabolism during grain germination. Indeed, in recent years, increasing attention has been paid to the possible role of carbohydrates availability and utilization in conferring anoxia tolerance (Perata & Alpi 1993; Armstrong et al. 1994; Ricard et al. 1994; Hanhijarvi & Fagerstedt 1995). One of the critical aspects when reviewing data about stress physiology arises from the non-homogeneous conditions used by researchers to impose the stress condition. Anoxic/hypoxic conditions can be obtained by incubating the plant material in anaerobic incubators, in flasks flushed with pure nitrogen gas, or by flooding under air-free water or buffer. All these methods are reliable ones, but the data obtained using different methods are not easy to compare (e.g. leakage of metabolites is different depending on the volume of liquid media in which the plant material is incubated, and obviously the volume is greater when using flooding as a method to get the anaerobic environment). An additional source of variability arises from the heterogeneous variety of plant material used; dry seeds/grains incubated under anoxia since the imbibition time, aerobically germinated seedlings transferred to anoxia, dissected tissues (coleoptiles, roots, root tips, etc.). This is fully justified by the different hypotheses to be tested, but comparison of data obtained from different plant species/cultivar/tissues is hardly possible. Most of the data discussed in this review were obtained in the author’s laboratory using comparable anoxia-treatment conditions (pure nitrogen gas atmosphere) and thus allowing an unambiguous comparison of data.