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J Integr Plant Biol, 1959, 8 (3): -, Research Article
Studies on Plant, Respiration. III. lnfluence of Oxygen Tension on Organ Formation, Material and Energy Transformation During Germination of Rice Seeds
P. S. Tang, F. C. Wang and F. C. C. Chih
Rice seeds (var. Yinfang) were germinated on moist quartz sand in the dark, at 25–28℃ for 5 to 10 days. Changes in dry weight, available carbohydrate and heat of combustion in the seeds and organs were investigated under 0–0.2%, 2%, 5%, 8.5% and 20.8% (air) oxygen with continuous streaming gas mixtures. Amounts and efficiencies of material transformation in relation to organ formation in the germinating seeds as well as energy relations were compared under these conditions. With ample oxygen supply (2–20.8%), for each milligram of organ formed approximately another milligram of stored material is left from the embryo in the process so that the efficiency of transformation is about 50%. This efficiency is significantly lowered at 0–0.2% O2 tension. The amounts of materials left from the endosperm and the weight of the organs formed decreased with decreasing oxygen tension. Lowered oxygen tension hinders root formation. At 20.8% O2, the root/top ratio is 1:1, at 2% O2 the ratio becomes ca 1:3. Coleoptile growth is favored under lowered oxygen tention. With insufficient oxygen supply (0–0.2%) loss in dry weight in the endosperm is significantly retarded and very little or no increase in dry weight occurred in the organs. Under such conditions growth is limited to the elongation of the coleoptile. No root formation occurred under such conditions. Since under atmospheric conditions the solubility of oxygen is from 0.7% (at 15 ℃) to 0.5% (at 30℃) and since these values are lowered by dissolved substances and by respiration, seeds submerged under flooded seedling beds are surrounded by partial pressures of oxygen of less than 1%, and still more probably it is in the neighborhood of 0.2%. This condition is that which is found in these experiments to be unfavorable for rice seed germination. Of the 25 milligrams of air-dry weight of a rice grain, the hull occupies 4 mg (15%), the embryo weighs only 0.26 mg (1%) and the bulk is made up of the endosperm which comprises 84% (22 mg) of the total weight. Of the 22 mg of endosperm tissue, only 18.5 mg is "available carbohydrate". The extent and efficiency with which this material is used for organ formation is affected by partial pressure of oxygen in the surrounding atmosphere. In air, on the 5th day of germination, 13.75 mg is left of the 18.80 mg of "available carbohydrates'' originally present in the seed; under 5% O2, 14.96 mg is left, and under 0–0.2% O2, 15.58 mg is left. On the 10th day of germination the figures are respectively, 7.48, 8.07 and 11.95 mg. More storage materials are left over at lowered oxygen supply, not less, nor exhausted, as assumed by some earlier workers. The embryo obtains 8 times its own weight of material from the endosperm in 5 days at 20.8% oxygen. On the 10th day, this ratio becomes over 24. This rate of translocation and transformation is lowered with oxygen lack so that under 0--0.2% oxygen, it is only one-eighth of that in air. In presence of oxygen 1.2–2.3 calories of heat is lost for each mg of organ formed. On the basis of energy change measured as heat of combustion, the efficiencies of transformation are: on the 5th day of germination, 67.1% at 20.8% O2; 78% at 5% O2; and 68% at 0–0.2% O2. The values obtained on the 10th day of germination are: 68.5%, 70% and 19% respectively. The heat of combustion per gram of endosperms and seedlings increases during the course of germination, indicating conversion of a part of the material into that with a higher heat content. The changes in heat energy of the seeds as actually measured by calorimetry differs from that calculated from carbohydrate loss. This "residual" energy is considered as due to simultaneous presence, to varying degrees, of aerobic and anaerobic respiration at all oxygen tensions. The possibility of transformation of this residual energy into "energy of organization" is also considered. The results of the experiments presented in this account support our contention that the germination of rice seeds (Yinfang, a lowland variety) is an oxygen requiring process. Although rice seeds, due to possession of a strong alcoholic fermentation system can withstand anaerobic conditions for a few days, for good germination and healthy growth of the seedlings oxygen supply is essential. In presence of oxygen (5–20.8%), material and energy transformation is more efficiently carried out, and organ formation in the embryo and seedlings are such as to meet the demand for healthy seedling culture. These results are also in accord with the concept of "multilineal path" and respiration as an adaptive physiological function of the living plant as postulated in the previous accounts of this series of studies on plant respiration.
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