Studies on the Photosynthesis and Respiration of Some Horticultural Crop Plants Grown in the Sand Field

Variations in the concentration of CO2 in the atomosphere of melon plants which were grown under tunnel shaped coverings of plastic sheet (abbreviated as tunnel) were investigated. The tunnels were laid in an ordinary plastic greenhouse (18m× 50m=900m2 in size) set up on the sand dune field.1. The CO2 concentration at nighttime showed the highest value in the non-hole poly tunnel with bare ground condition. At 17:00, when the tunnels were closed, the CO2 concentration increased by a ratio of 65ppm/hour and reached 1700ppm at 7:00 the next morning. Then with the increase of solar radiation intensity, the CO2 concentration decreased, in turn, by a ratio of 230ppm/hour.2. Under the non-hole-poly tunnel with mulched ground condition, the pattern was the same as for the non-hole-poly tunnel with bare ground condition, but the CO2 concentration was about 200ppm lower than the concentration for the bare ground condition. The decreasing rate of CO2 concentration after sunrise was larger and constant, than the concentration for the bare ground condition, showing 280ppm/hour.3. During the daytime when solar radiation was high the non-hole-poly tunnel with mulched ground conditions showed a lower CO2 concentration (after 11:00 ca. 50ppm) than that of the surrounding air of the plastic greenhouse. On the other hand, in the tunnel with bare ground conditions, it showed a higher value of ca. 300ppm. In the rainy afternoon it showed a higher value of ca. 900ppm.4. At nighttime the CO2 concentration in the hole-poly tunnel usually remained very low, as compared to that in the non-hole-poly tunnel. At one hour after closing the hole-poly tunnel in the evening, the CO2 concentration increased to 560 ppm under the bare ground condition and to 500ppm under the mulched ground condition, respectively. At sunrise the CO2 concentration began to decrease again with the increase of the solar radiation intensity, reaching a level similar to that of the surrounding atomosphere of the greenhouse.5. The CO2 concentration of the atomosphere in the non-hole-poly tunnel with bare ground and non vegetational conditions showed a steady increase to over 550 ppm 3 hours after shutting the tunnel. Thereafter, no change was seen. This suggests that the sand bed provided a considerable supply of CO2 without vegetation.