Physiological Studies on the Tolerance of Rice Plants to Salinity. : Part 2. Effects of salinity on the absorption of water and of chloride ion
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A general suppression of growth is probably the most common plant response to salinity. On the mode of action of salinity, particularly the excess soluble salt in root medium, two working hypotheses have been proposed, one is to interpret it as all osmotic phenomenon and the other as a physiological effect of the excess salt accumulated in plant tissues. The primary cause of growth suppression in the presence of excess salt is not evident from any information available at the present time. In this paper is reported an attempt, using Oryza sativa L. cultiva Eiko as material, to clear up the points left in doubt by earlier workers. Rice plants cultured in the basic nutrient solution were treated with nutrient solution containing 1.0% NaCl for a week at various growth periods. Amounts of water and of chloride ion absorbed were measured quantitatively. The experimental results obtained may be summarized as follows; 1. The salt present in excess in the culture solution caused a remarkable suppression of growth. In detail increases of top length, tiller number, fresh and dry weights and new leaf formation were wholly depressed; such harmful effects due to the salt treatment were more significant when the rice plants were treated at their earlier growing stages than at later one. 2. The amount of water absorbed decreased remarkably as a result of the treatment with saline solution, but no definite trend could be observed for such an inhibiting effect. Accordingly the differences between their tolerance to salinity at various growing stages are not satisfactorily explainable from this evidence. On the other hand, the water contents in rice plants showed little change even after the treatment with salt solution. So it seems very reasonable to assume that water deficiency may not be the primary cause of salt injury. 3. The absorption of chloride ion from the nutrient solution containing NaCl by rice plant (mg/gm dry weight/week) was significant at earlier growing stages, followed by a gradual depression toward later stages, particularly a steep decline was observed at the time of boot-leaf formation. It is interesting to note that these evidences are in good agreement with the reduction of increases in dry and fresh weights and with the increase in leaf withering due to the treatment with saline solution. 4. Judging from the facts mentioned above, it seems probable that the excess accumulation of salt in rice plants may be a primary cause of salt injury, and in this case the accumulation of chloride ion is mainly localized in the shoot of rice plants, while the accumulation is very poor in grains.
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