INFLUENCE OF OXYGEN TENSION ON RESPIRATION, FERMENTATION, AND GROWTH IN WHEAT AND RICE

THE CEREAL grains constitute the most important group of food plants for man. Wheat (Triticum spp.) is the leading cereal food of North America and Europe, and rice (Oryza sativa) is the chief food of the Orient, where it sustains half the population of the earth. Even in the United States rice has become a crop of considerable importance. Rice is normally cultivated in shallow ponds, filled with water to a depth of six to eight inches during the greater part of the growing season (Jones, 1937). Wheat, however, is always grown under ordinary dry-land conditions, and it is unable to survive more than a brief period of flooding. Submersion in water markedly decreases aeration, and it seems likely that this factor is responsible for the difference in the reactions of wheat and rice to aquatic conditions. Some fundamental difference between the respiratory systems of these closely related grasses might well be expected to account for their responses to decreased oxygen in the environment. Rice must either be more efficient than wheat in using low concentrations of oxygen, or it must possess a growth mechanism that is only partially dependent upon the limited supply of oxygen. It has long been known that wheat, rice, and other cereal grains can carry on some anaerobic activity. In an attempt to ascertain the physiological basis for this difference between wheat and rice, a study was made of the influence of oxygen tension, from 0 to 20.8 per cent, on respiration, fermentation, and growth in seedlings of each of these cereal grains. MATERIALS AND METHODS.-Respiration studies. -The studies reported in this paper were carried out during a period of one year, beginning in May, 1941, with seeds of the 1940 crop of Leap's Prolific winter wheat (obtained from A. H. Hoffman, Inc., Landisville, Pa.) and Early Prolific lowland rice (supplied by the Texas Agricultural Experiment Station, Beaumont, Texas). Both are among the leading commercial varieties of these cereal grains. Tests near the close of the work showed that storage of the seeds had produced no significant alteration of the respiratory activity of seedlings grown from them. Seeds used in the experiments were selected for uniformity in appearance and size. After being soaked for five minutes in a freshly filtered 0.55 molar calcium hypochlorite solution and then rinsed with distilled water, the seeds were placed in large glass culture dishes, on three thicknesses of sterile filter paper moistened with a slight excess of distilled water. These germinators were kept at a temperature of 300C. in a dark incubator. Approximately three times as many seeds as were required for an experiment were selected and germinated. 1 Received for publication June 6, 1942. Prepared in the Department of Botany, Columbia University, under the direction of Professor Sam F. Trelease. Good germination and three-day development of both wheat and rice occurred on filter paper moistened with distilled water. Preliminary tests indicated no important advantage of adding salts or sugar to the germination substrate. Media with ionic balance gave indications of superiority, and the pH value of the substrate had a more marked influence. Wheat germination and development were most rapid on slightly acid substrates (pH 5.0-6.0), while rice development was best in nearly neutral substrates (pH 6.0-7.0). A period of 25 hours was allowed for wheat germination and a period of 33 hours for rice germination, in order to obtain seedlings of each with roots 5 to 7 mm. long. Secondary roots were evident on the wheat seedlings, but not on those of rice; coleoptiles were developed to a length of 4 or 5 mm. Groups of wheat and rice seedlings, 25 of wheat or 30 of rice, of the character described were selected and pretreated by soaking them in a sucrose buffer for one hour before they were transferred to each of the respiration vessels charged with the same buffer. The buffer had a pH value of 5.8 and a K/Ca balance of 7.3/1. The buffer solution had the following composition: 0.009 M KH2PO4; 0.001 M K2 HPO4; 0.0015 M Ca(H2PO4)2; 0.117 M sucrose. It was employed in all manometric tests in order to provide a stable medium conducive to maximum seedling activity. The composition of the buffer was based upon recommendations in the literature supplemented by the tests on seed germination previously mentioned. Numerous studies (Jones and Shive, 1922; Trelease and Trelease, 1935) have shown that good development of wheat occurs at the pH of this buffer. Similar phosphate buffers have been employed by other workers in manometric respiration studies on tissues of higher plants (Mlarsh and Goddard, 1939; Goodwin and Goddard, 1940; James and James, 1940). Best germination of rice seeds and growth of young seedlings is reported by Alam (1936) to occur at pH 6.4-6.5. Dastur and Winifred (1938) found best growth of rice in the pH range of 6.4 to 7.0. The importance of the physiological balance of salts is well known and has been amply demonstrated for wheat seedlings by Trelease and Trelease (1926). Ion balance has been taken into consideration in the preparation of substrates in a number of recent respiration studies (Goodwin and Goddard, 1940; Turner, 1938a; James and James, 1940). Several investigators have demonstrated increase in respiratory activity, especially anaerobic activity, in substrates enriched with carbohydrates. The maximum effect in barley seedlings resulted when the sucrose concentration of the buffer was 0.117 molar (Brown and