根圏ガス環境の制御に関する研究

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It has been emphasized that soil aeration is important to promote plant growth, because the decrease in O_2 concentration in the soil inhibits plant root activity. But O_2 concentration in the soil doesn't usually approach the critical value below which plant growth is inhibited. On the other hand, CO_2 concentration in the soil may be more important for plant growth, becsuse it easily increases up to 2-3%. In this study, the influence of the increase in CO_2 concentration in the rhizosphere on plant growth was investigated. The results made it clear that the increase in CO_2 concentration in the rhizosphere suppressed the photosynthetic rate and plant growth via inhibition of water absorption of the root. In addition, methods to improve the gaseous environment in the rhizosphere in order to promote plant growth are stated in this paper. The results are summarized as follows: (1) CO_2 concentration in the soil increased in proportion to the amount of organic matter contained in the soil. It reached 3% in sand containing 2.5% compost (based on dry weight) after irrigation. Change in CO_2 concentration after irrigation differed among different soils. In sand, the concentration after irrigation was five times as much as that before irrigation, while in vermiculite and Kanuma soil, they were three times and double, respectively. Change in CO_2 concentration in the soil after irrigation was caused by change in gaseous diffusion coefficient in it. In sand,the coefficient was 0 cm^2/sec immediately after irrigation and still retained 0.035 cm^2/sec four days after irrigation. While in vermiculite and Kanuma soil, the coefficients increased rapidly with drying soils and reached 0.05 cm^2/sec one day after irrigation, although they decreased temporarily just after irrigation. In the soil containing small solid particles as sand, CO_2 concentration increased up to high level with a rapid decrease in gaeseous diffusion coefficient by irrigation, and then decreased very slowly because of slow recovery of the coefficient.(2) The effects of CO_2 and O_2 in the rhizosphere on the photosynthetic rate were investigated. The photosynthetic rate of cucumber plant decreased two hours after the beginning of aeration with air containing 3-5% CO_2 into the rhizosphere. After four hours, the photosynthetic rate at the treatment of 10% CO_2 declined up to 85% of that before the treatment, while it retained 95% when treated with 10% O_2. As described above, it was proved that the effect of increasing CO_2 was more significant than that of decreasing O_2 in the rhizosphere. The transpiration rate was also suppressed as CO_2 concentration increased in the rhizosphere. Therefore it was suggested that the plant experienced a water stress by CO_2. Long term effects of 1-2% CO_2 concentration in the rhizosphere which normally exist in the cultivated soil on the photosynthetic rate of sweet potato were investigated. The rate at the treatment of 1% CO_2 declined up to 80% of that before the treatment four days after the beginning of it and that at the treatment of 2% CO_2 declined to the same level after two days. Both declined up to 70% after five weeks. In addition, the effect of CO_2 appeared more significantly at the lower temperature in the rhizosphere. (3) The effect of CO_2 concentration in the rhizosphere on growth of cucumber was investigated. The growth suppression occurred at 0.5-2% CO_2 in the rhizosphere. Total dry weight and leaf area of cucumber plant decreased up to 85% by the increase in CO_2 concentration to 2-5%. Water content in a plant decreased and water vapor diffusion resistance of leaves increased under high CO_2 concentration in the rhizosphere. Therefore the growth suppression may be due to the inhibition of water absorption ability of the root. (4) Fresh air saturated with water vapor was sent into the soil with a compressor in order to decrease CO_2 concentration in rhizospheres of some vegetable crops. Fresh weight of tubers of sweet potato, roots of radish and fruit of cucumber at the treatment of 2.51/min aeration for one meter long ridge increased by 24%, 23% and 17% respectively in comparison with those at non aeration treatment. In addition, the yield of tubers of sweet potato increased by 20% when aerated for only 15 minutes once a day.
1987-03-31

根圏ガス環境の制御に関する研究

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