Influence of Cover-Culture on the Photosynthesis of Tobacco Plants

Abstract

Leaf tobacco cultivation in eary growing season has been based on the cover-culture technique. Young tobacco plants, after transplanted to field, are generally covered with polyethylene-film or melamin coated paper, and tightly closed in the cover-culture. Under such enviromental closed system, the ploblems which should be clarified are that what kind of methods and materials for cover-culture are required in order to protect young plants from frost or low temperature and to make photosynthetic condition favorable for promorting the growth of plants. As the first approach to this problems, we carried out the work with the purpose of how to manage the photosynthesis of young tobacco plants under the eviromental closed system on the field. (1) The photosynthesis, plant respiration and some enviromental factors under the cover-culture which has been brought into practical use in early season cultivation of leaf tobacco were measured on the field. The cover-culture has been found generally in Kyushu district and many other production area of leaf tobacco, and the ordinary cover-materials were polyethylene-film and melamin coated paper. (2) The diagram for measuring was based on the circulation system of the air (closed system); the air in the cover-culture was circurated through two chambers both of photosynthesis and soil respiration, and on the process of air circuration photosynthesis of tobacco plants and soil respiration were measured separately and simultaniously. Besides, by passing the outside air through the chamber (open system), the photosynthesis, soil respiration and plant respiration were measured to compare them with the results of the closed system. (3) In the closed system of the cover-culture, CO2 concentration decreased as plants grew and the day went by. In addition, the heigher the air temperature rises in. day time, the more the amount of plant respiration increased. Consequently, the photosynthesis were strongly surpressed; the amount of photosynthesis per plant never increased or rather decreased in spite of growing of plants during the latter half of covering period, and the amount of photosynthesis per unit leaf area remarkably decreased as the leaf area per plant increased. (4) In the open system of the cover-culture, the photosynthesis per plant remakably increased and the rate of plant respiration decreased relatively. The amount of apparent photosynthesis per plant was, 2.6 times that of closed system at the end of covering period. (5) The greater amount of soil respiration was obtained when the air of low CO2 concentration was sent to the chamber of soil respiration in the closed system as compared to the open system. The amount of soil respiration which goes in such closed system should be grasped not only as the biochemical activity of soil micro-organism, but also the diffusion of CO2 from soil to the ground. (6) CO2 concentration under the cover-culture could be an index of degree of causal relation between photosynthesis and soil respiration. (7) Covering method which would positively produce enviromental conditions favourable for photosynthesis under the cover-culture in early season cultivation of leaf tobacco should be re-discussed on the following viewpoints; giving the air of heigher CO2 concentration to tobacco plants; promoting soil respiration; and not giving rise to excessive air temperature.