Studies on the Effect of Wind Speed upon the Photosynthesis

Abstract

In the previous paper, the authors described the relationship between the thickness of boundary layer near the leaf surface and wind speed.When the transport of carbon dioxide from atmosphere toward chloroplast depends only on the thickness of boundary layer, the photosynthesis varies in accordance with the wind speed.Generally, when the amount of photosynthesis of single leaf is measured, a narrow assimilation box have been used in order to increase the wind speed in the box. However, it is very difficult to measure the natural photosynthesis, because the thickness of boundary layer of leaf surface is not developed in the box and air stream near the leaf surface differs entirely from that under the natural condition.The authors have constructed a Gottingen type wind tunnel to measure the photosynthesis under the natural condition of air flow. The wind tunnel is control able of wind speed from 15 to 300cm/sec, light intensity up to 0.6cal/cm2 min or up to 85klux, air temperature from 17 to 40°C, relative humidity from 30 to 95%, carbon dioxide concentration up to 15% and the dissolve oxygen concentration in culture solution up to maximum saturation.Cucumber (Cucumis sativas L.) was grown in Hoagland solution in green house of over 20°C minimum temperature. The leaves were cut off except the first and second ones after the second leaf grew over 100 cm2. The gross photosynthesis of the first and second leaves were measured from respiration by dry weight loss in the dark (16 hours) and net photosynthesis by increment of dry weight in the light (8 hours).Under the conditions of 25°C of air temperature, 0.6cal/cm2 min of light intensity and 65% of relative humidity, the gross photosynthesis was measured. The amount of gross photosynthesis, was nicreased with the wind speed and showed the maximum value at 50-60cm/sec of wind speed, and over that wind speed, the amount decreased gradually. Namely, when the wind speed exceeds a certain value, the photosynthesis-wind curve deviates from the photosynthesis-wind curve calculated from the relation between thickness of boundary layer and wind speed. This means that the transpiration is increased and that the content of leaf water is decreased when the boundary layer becomes thinner. In order to know the effect of transpiration, the net photosynthesis was measured under the various relative humidities. When the relative humidity is raised, the maximum value of the net photosynthesis is increased and the wind speed is also elevated showing the maximum value. And the net photosynthesis gradually closes to the photosynthesis-wind curve calculated from the boundary layer. These results mean that the photosynthesis of real leaf is not only dependent on the light intensity but also both the wind speed and relative humidity.