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J Integr Plant Biol, 1959, 8 (3): -, Research Article
Studies on Plant Respiration. IV. Some Morphological, Physiological and Biochemical Changes During Germination of Rice at Different Partial Pressures of Oxygen
C. Chu and P. S. Tang
Morphological, physiological and biochemical changes during germination of rice (var. Yinfang) were studied as function of oxygen tension. Seeds left to germinate on moist quartz-sand fin the dark at 22 ℃ under 21%, 5% and 0–0.2% oxygen (continuous flow) were taken out for examination on the lst, 5th and the 10th days of germination. The results of the experiments are analyzed and interpreted as follows. 1. The process of germination in rice seeds (var. Yinfang) includes the following three sequential steps none of which may be considered separately a. The elongation of tissues or organs which already possess the full complement of cells prior to soaking. These organs include coleoptile and coleorhyza. This step is characterized by its indifference to oxygen supply. In nitrogen, the coleoptile elongates even faster and for a longer duration than when it is in air. The primary root also elongates under anaerobic conditions but only to the extent limited by cell elongation. The rapid and extensive elongation of the coleoptile in absence of oxygen is here considered as a physiological adaptation which ensures the rice embryo to obtain oxygen needed for completing the other oxygen-requiring steps involved in the entire process of germination. The stomata on the coleoptile, together with the aerenchyma tissues developed later, facilitate oxygen supply from the air above to the organs submerged under the water layer of the flooded seedling bed. The elongation of the coleoptile is here considered only as one of the steps in the entire process of germination, and is not, as considered by some earlier workers, the complete process, nor an "abnormal germination". b. The growth of tissues and organs involving cell division. This includes the growth of leaf and root primordia, of root tips, and formation of branch roots and root hairs. This is strictly an oxygen-dependent step requiring the presence of oxygen, albeit a small amount (less than 2%) for cell division. No cell division occurs in absence of oxygen. In presence of the latter, both cell division and cell elofigation occur. c. Energy and food supply for the germination process. This step involves (1) the digestion of the compound starch grains stored in the endosperm and its mobilization across the scutellum tissue. This forms the main source of food and energy, supply; and (2) the utilization of the small amounts of simple starch grains and protein reserves already present in the scutellum tissue. Step (1) is aerobic, requiring oxygen for enzymic digestion of the starch, while step (2) may proceed under nitrogen. It is possible however, that both steps (1) and (2) are strictly oxygen-dependent, and the disappearance of the small amount of materials in the scutellum tissue itself may be due to oxygen residual in the seeds from the start of the experiment. 2. It may be concluded from the above analysis of the results that germination of rice seeds is an oxygen-requiring process, similar to that of up-land crops such as wheat. When the supply of oxygen is cut off, or seriously reduced, as when seeds are submerged under a layer of water, cell division as well as digestion and mobilization of food reserves are prevented. In such case, although the coleoptile may break the seed coat and elongate the entire process of germination cannot proceed to completion due to failure of the other two oxygen-requiring steps to take place. This conclusion is at variance with the views of some earlier workers who maintain that rice seeds can germinate at the sole expense of energy supplied from fermentation, and when the carbohydrate reserves are exhausted through this inefficient process germination ceases. Our observation that the bulk of the food and energy supply in the endosperm cannot be digested and mobilized during anaerobiosis and the observation that cell division does not occur in absence of oxygen appear to support the views presented in this account. 3. Based on our view that rice germination is an aerobic process it is logical to conclude, as: already pointed out in the first report of this series, that excessive flooding of seedling beds during germination and early growth is not only unnecessary, but undesirable due to depletion of oxygen supply to the embryo. In the light of this conclusion irrigation of rice seedling beds should be conducted with the aim of supplying maximum aeration, while flooding of the beds should be made only as required for temperature regulation and the maintenance of moisture supply necessary for proper water relations of the seedlings. 4. The results and conclusions presented in this account may throw some light on the causes of seedling damages observed in flooded beds. Many factors contribute to the three most commonly observed pathogenic conditions, but from the view point of oxygen supply, the following causes based on depletion of oxygen supply may be offered as a contributing factor to the damage. Excessive flooding of seedling beds seriously reduces oxygen supply to the seedlings. This in turn causes excessive growth of the coleoptile over the coleorhyza. The balance of the seedlings are thus tipped so that roots fail to ground. Under such conditions, the aerotropic tendency of the roots (upward) is stronger than their geotropic tendency so that further growth, if any, of the roots is toward the surface of the water layer. This causes the phenomenon of "topling over". The stoppage of food supply to the partially germinated seedlings due to lack of oxygen causes degeneration of the cells in the elongated organs. This weakens their resistance and prepares the way for infection of the tissues by pathogenic microorganisms such as Achlya, resulting in the "rotting" of the seedlings. Growth and formation of new organs by cell division is an oxygen-requiring process. The inhibition of aerobic respiration by respiratory inhibitors such as azide and fluoride blocks the function of the terminal, oxidases of the cells. This has the same effect as cutting off of oxygen supply. The toxic effect produced on the roots of the seedlings by hydrogen sulfide which is a well known respiratory inhibitor may be explained as due to the inhibition of cell division especially in the root tips, by blocking the activities of terminal oxidases. 5. Finally, based on the results presented in this and the previous accounts of this series of studies on plant respiration, the concept of "multilineal path" and respiration as an adaptive physiological function may be recapitulated in the following words: The adaptation of the plant to its environment is mediated through its changing metabolic patterns which in turn are brought about through the activities of the enzymes within the organism, including the formation and action of adaptive enzymes. The mode, direction and intensities of these enzyme activities vary according to the stages of development in the plant as well as to variations in the environment. The multiplicity of the activities of the enzymes results in varied metabolic patterns which ensure the pliability of adaptation. Variations in the metabolic patterns reflect themselves ultimately in the differences observed in the morphological behavior and physiological performance of the plant as a whole.
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