Exposure to oxygen deficits is more widespread in biological systems than is commonly believed. Until recently, the general perception of anaerobic metabolism was often limited to the induction of alcoholic or lactic acid fermentation as the sole biochemical response to hypoxia/anoxia. Developments in the physiology, biochemistry, and molecular biology of anaerobic responses in invertebrates, lower plants, and higher plants have demonstrated that, depending upon the species, anaerobic metabolism may encompass much more than simple glycolytic metabolism. Here, recent progress in elucidating the mechanism(s) determining tolerance versus intolerance to anaerobic environments in higher plants is discussed, drawing most heavily on experimental systems using seeds or seedlings.
In apple (Mais domestica Borkh.) sorbitol is the primary product of photosynthesis, the major translocated form of carbon, and a common fruit constituent and storage compound. Previous work on sorbitol metabolism has revealed a NADPH-dependent aldose 6-phosphate reductase (A6PR) in green tissues, and a NAD-dependent sorbitol dehydrogenase in nongreen tissues. Results here show a decrease in sorbitol dehydrogenase activity and an increase in A6PR activity as leaves developing in the spring undergo the transition from sink to source. Sorbitol dehydrogenase activity reached a minimum as A6PR peaked. These changes were related to increases in leaf carbohydrate levels, especially sorbitol, and to increases in rates of net photosynthesis. Studies conducted in the autumn on senescing leaves also showed changes in enzyme activites, leaf carbohydrate levels, and photosynthesis. At this time, however, sorbitol dehydrogenase increased in specific activity, whereas A6PR activity, leaf carbohydrates, and photosynthetic rates all decreased substantially. Other experiments showed differences in the ability of young and mature leaves to metabolize sorbitol and in the distribution of sorbitol enzymes in leaves at transitional developmental stages. The results suggest that sorbitol metabolism in apple is tightly controlled and may be related to mechanisms regulating partitioning or source and sink activity.Changes in the activities of enzymes responsible for synthesis and degradation of translocatable carbohydrates have been studied in several plants. Justification for such studies is due in part to the role these enzymes may play in the capacity of a photosynthesizing leaf to export carbohydrates. Equally important is the role these enzymes must play in mechanisms determining sink activity. Studies of the factors controlling activities of the enzymes involved could lead to a better understanding of control processes in photosynthesis, translocation, and storage (9).For both theoretical and practical reasons, developing leaves represent interesting models for studying metabolism of translocatable carbohydrates. Theoretically, information on transformation of a leaf from a heterotrophic, importing organ to an autotrophic, exporting organ should also provide insight into those mechanisms associated with onset of export. From a practical viewpoint a leaf can contribute to plant yield only when it is autotrophic and has begun to export.
Early in its life cycle, the marine mollusc Elysia chlorotica Gould forms an intracellular endosymbiotic association with chloroplasts of the chromophytic alga Vaucheria litorea C. Agardh. As a result, the dark green sea slug can be sustained in culture solely by photoautotrophic CO 2 fixation for at least 9 months if provided with only light and a source of CO 2 . Here we demonstrate that the sea slug symbiont chloroplasts maintain photosynthetic oxygen evolution and electron transport activity through photosystems I and II for several months in the absence of any external algal food supply. This activity is correlated to the maintenance of functional levels of chloroplast-encoded photosystem proteins, due in part at least to de novo protein synthesis of chloroplast proteins in the sea slug. Levels of at least one putative algal nuclear encoded protein, a light-harvesting complex protein homolog, were also maintained throughout the 9-month culture period. The chloroplast genome of V. litorea was found to be 119.1 kb, similar to that of other chromophytic algae. Southern analysis and polymerase chain reaction did not detect an algal nuclear genome in the slug, in agreement with earlier microscopic observations. Therefore, the maintenance of photosynthetic activity in the captured chloroplasts is regulated solely by the algal chloroplast and animal nuclear genomes.
Crassulacean acid metabolism (CAM) (16,17,32). CAM has been shown to be affected by salt treatment (29, 34), water stress (1,11,15,17,26,34) photoperiod (19), .temperature fluctuations between hot days and cool nights (11,14), stage of maturity (6,17,20), and flowering (3).The purpose of the present study was to examine the possible occurrence of CAM or facultative CAM in a succulent C4 plant species. Portulaca oleracea or purslane was chosen for this study for several reasons. First, many aspects of its C4 physiology are well established, such as four-carbon acid metabolism (9, 10), enzyme activities (8, 9), compensation point (9, 31), anatomy and cytology (7), photorespiration (8), photosynthetic rate (9), and response to salt and water stress (9). Also, previous work has ' This study was supported by Sigma Xi Research Society of North America, and by National Science Foundation Grants PCM-77-25 100 and PCM-79-05937. shown that the water use efficiency of P. oleracea is particularly high, even for a C4 plant (24) and is close to that of a CAM plant. Finally, the Portulacaceae is a likely group in which to find a species with both C4 and CAM activity since this family contains C3, C4, and CAM plants (2,5,29).MATERIALS AND METHODS P. oleracea L. plants were grown in the greenhouse until 3 to 4 weeks old. Young, uniform plants with three to four leaf pairs were then individually potted and transferred to a growth chamber for a minimum of 7 weeks before experimental use (3). Incandescent and fluorescent bulbs provided an intensity of 380 t,E m 2S-I at plant level. Temperatures of 30 C day/15 C night were maintained with RH of 45%. Six different combinations of daylength and watering regimes were tested.
Although rice has long been recognized to be uniquely adapted for growth in low oxygen environments of flooded rice fields, rice weeds of the Echinoehloa crus-galli complex appear to be at least as well specialized for germination and growth under such unusual biological conditions. Seeds of two varieties of E. erus-galli germinate and grow for prolonged periods in a totally oxygen-free environment. E. erus-galli germinates as well as rice {Oryza sativa) under a total nitrogen atmosphere and produces as large a seedling in spite of its much smaller seed size. Like rice, the seedlings of E. erus-galli are unpigmented, the primary leaves do not emerge from the coleoptile and no root growth occurs without oxygen. Of particular interest is the ultrastructure of mitochondria from anaerobically-grown seedlings. Mitochondrial profiles from the primary leaf of seedlings grown continuously in nitrogen are very similar to those grown aerobically. The size and shape of the mitochondria are similar and the cristae are numerous and normal in appearance. This is in sharp contrast to previous studies of other species which have reported that mitochondria were vesiculate and tended to lose their normal fine-structure after similar periods without oxygen.Finally, based on ultrastructure and ' "^C labeling studies, anaerobically-grown seedlings are highly active metabolically, which may explain, at least for E. erus-galli var. oryzieola, its ability to germinate and emerge from flooded rice fields.
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