An attempt has been made to correlate respiration rate, cytochrome oxidase activity and mitochondrial structure in coleoptiles of rice, Oryza sativa L., germinated under aerobic and anaerobic conditions. The rice coleoptiles emerge from the grain and elongate considerably even under complete anaerobiosis, which totally suppresses root and leaf growth. Cell number, dry weight and nitrogen content per coleoptile are all lowered, although some cell division and translocation of reserves into the coleoptile does take place under anaerobiosis. Compared with coleoptiles from air-grown seedlings, anaerobically grown coleoptiles have a much lower capacity for respiratory oxygen uptake and their cytochrome oxidase activity is depressed even more. Mitochondria, however, are still abundant in 4-day-old anaerobic coleoptiles, with a crista density only slightly lower than in cells of aerobically grown coleoptiles. Since, in the embryonic coleoptile of the ungerminated grain, mitochondria show very little internal structure, a considerable amount of elaboration of mitochondrial structure must occur in the rice coleoptile under anaerobiosis, contrasting with the situation in yeast, where mitochondria of normal structure are formed only in aerobic conditions. Since a high crista density develops in rice coleoptile mitochondria with a very much depressed cytochrome oxidase activity, there is no obligate correlation between crista density and cytochrome oxidase activity in this tissue.

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