The Influence of Transpiration upon the Photosynthesis in Several Crop Plants

Many investigations have been made on transpiration and photosynthesis of plants. However, the relationship between transpiration and photosynthesis has not been completely estabilished. The study reported here was planned to elucidate the effect of transpiration on the apparent photosynthesis and the relation of temperature and photosynthesis in several crops:sweet potato, rice, soybean and peanut. 1) Experiments were conducted outdoors under full sunlight using an apparatus which has a humidity-controlling system (Fig.1). In early growth stage, photosynthesis was not affected by restraint of the trans-piration caused by increasing humidity in the chamber, but in late growth stage a marked increase of photosynthetic rates occurred by restraining the transpiration (Figs.2-5). These results suggest that water supply from root to the photosynthetic organ turns insufficient in the late growth stage. 2) Fig.6 and Fig.7 indicate the relation of leaf temperature and photosynthesis in sweet potato and soybean plant at various growth stages. In early growth stage, apparent photosynthesis was insensible of high temperature from 30 to 40°C. In late growth stage, however, apparent photosynthetic rates showed a rapid decrease at high-temperature range. These phenomena were also recognized in other crops, e.g., rice and peanut. From results of the investigation for the stomatal opening and fluctuation of water level in sweet potato leaves under various temperature condition, it may be concluded that depression of photo-synthetic rate at high-temperature range was due to a deficit of water content and high stomatal resistance in leaf. 3) In the series of experiments as shown in Figs.2-5, water vapour transfer coefficient (D) was calculated for several crops from the equation presented in the note of Fig.9. There was a close relationship between the transpiration and the ‘D' value. The‘D' values, selected under approximately constant conditions with leaf temperature of 35°C and air volume passing through a chamber of 8-11 m3/hr, showed high correlations to photosynthetic rates in three crops except soybean plant (Fig.1O). The‘D' value is a reciprocal of diffusion resistance(r) which can be divided into two components: (i) resistance in external air during diffusion away from leaf(ra), and (ii) resistance through the stomata(rs). It is considered that the ‘D' mainly depends on 1/rs in the case of Fig.10. Accordingly, when photosynthetic rate is high at high leaf temperature, stomata are widely open. 4) Two factors, total sugar content in root and the ratio of root dry weight (R) to leaf area (F), must be concerned with the water supply from root to photosynthetic organ. Therefore, both factors, whose changes with time are shown in Fig.14, would be correlated to the‘D' values of each growth stage