Studies have been carried out intending to clarify the physiological mode of the effect of hydrogen sulphide (H
2S) on the yield of rice plants through root rot. H
2S was added for two weeks to the culture solution in which rice plants were cultured, at different period from young-ear-formation stage to maturing. 1) The plants grown under higher level of nitrogen-fertilization were more severely affected by the H
2S-treatment in their yield (weight of fully ripened grains per hill) than those grown under lower level of nitrogen. In case of the plant treated at young-ear-formation-stage, at flowering stage or at ripening stage, the low yield was attributed mainly to the lower percentage of fully ripened grains. In case of those treated at booting stage, the low yield was brought about through the decreased number of spikelets per ear instead of decreased percentage of fully ripened grains. 2) Although H
2S-treated plants showed lower physiological activity in their root during the H
2S-treatment, such as respiration rate, cytochrome oxidase activity, α-naphtylamine-oxidizing and tetrazoriumchloride-reducing activity, they recovered their activity some time after the treatment and even became more active than the control plant. 3) In case of the leaves, on the other hand, H
2S-treatment brought about a considerable decrease in the ratio of photosynthesis to respiration as a result of diminution in the former and increase in the latter. Furthermore, it would be an evidence to show the occurrence of something unusual in energy metabolism that the cytochrome oxidase activity was decreased whereas the respiration fate and peroxidase activity were increased. 4) It may be concluded from these results that an aspect of the influence of H
2S on the yield of rice plants reveals itself in the diminution of photosynthesis and increase of respiration resulting in the lowered photosynthesis/respiration ratio of leaves which means a net diminution in carbohydrate production. It is quite possible that this diminution in carbohydrate production during and even after treatment may ultimately bring about the decrease in percentage of fully ripened grains.
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