Studies on the Mechanism of Injurious Effects of Toxic Gases on Crop Plants : On photosynthesis and dark respiration of the rice plant fumigated with sulfur dioxide for long period

Abstract

The experiment was undertaken to elucidate after-effects of SO₂-treatment for long period on photosynthesis and dark respiration in the rice plant after it was released from the treatment. The measurement of photosynthesis and dark respiration was made 24 hours after the gas had been removed. Treatment with SO₂ was for 90 hours at the tillering stage and 66 hours at the maximum number of tillers stage. The concentrations of SO₂ to which the plant was exposed were 0, 1.2, 4.0 and 10 ppm at the latter stage, and 0, 0.271, 0.719 and 1.415 at the former stage. Under the condition in a single leaf, apparent photosynthesis and dark respiration at 1st, 3 rd and 6 th days after SO₂ had been removed were measured, respectively. Treatment with SO₂ was for 80 hours at the maximum number of tillers stage. The results obtained were as follows: 1. Apparent photosynthesis of the rice plant exposed to SO₂ for long period, as compared with the control, showed a considerable decrease 24 hours after the plant had been released from fumigation with the gas at both the tillering and maximum number of tillers stages, this being true to any concentratrations of the gas. 2. Gross photosynthesis of the rice plant was gradually decreased with an increase in the concentration of the gas and the difference between the control and treated plants in apparent and gross photosynthesis was enlarged with an increase in the light intensity. 3. Light compensation point of the rice plant moved towards a higher light intensity in accordance with the concentraion of the gas. 4. Under the condition in a single leaf, the light-curve of carbon assimilation in the rice plant treated with SO₂ for 80 hours showed always lower values than that of the control (SO₂-Oppm) at 1st, 3 rd and 6 th days after SO₂ was removed, respectively. As increasing in the SO₂ concentrations, apparent photosynthesis decreased proportionally. 5. From the above-mentioned facts it was demonstrated that the decrease of dry matter production in the rice plant exposed to the gas for long period might be resulted not only from a decrease of photosynthesis and an increase of dark respiration during SO₂-treatment, but also from a decrease of photosynthesis after the gas had been removed. In this case, however, the after-effect of the gas treatment on dark respiration was not so large as that on photosyntheis.