The Effect of Carbon Dioxide Concentration of Atomospheric on Growth of Leaf Beet with Various Plant Densities

Abstract

Dry matter production and plant density relationships of leaf beet plants were examined under a range of carbon dioxide concentrations from 0.03% to 0.6% CO2. Leaf beet plants (Beta vulgaris L, var. flavascens D. C.) were raised in gravel culture beds in polythin growth chambers from June 11 to July 10, 1965. The carbon dioxide concentrations in four chambers employed in this experiment were 0.03, 0.07, 0.15 and 0.6%, respectively. The each bed in the chambers was partitioned into four sections and the leaf beet plant with the height of 6 cm were transplanted in these sections at the densities of 50, 125, 292 and 500plants/m2, respectively.As can be seen in Fig. 1, the dry weight per unit area increased throughout the range of plant density values applied, although the initial increase is more rapid. The increase rate of the dry weight is larger for the chambers with high CO2 concentrations than for the chamber with usual CO2 concentration, indicating that the dry weight for the chamber with usual CO2 concentration is approximately constant in the range of plant densities over 200plants/m2 and LAI 6. On the other hand, the dry weight for the chamber with CO2 concentration of 0.6% did not show any saturating values throughout the range of plant density values applied.The value of absorptivity of incoming solar radiation of the plant canopy was measured for respective plots. The absorptivity decreases throughout the range of plant density values applied, although the initial decrease is more rapid (see Fig. 2.). In the range of plant density over 200plants/m2, no difference in the absorptivities was found among the chambers.The above results suggest that a principal cause for the plateau phenomenon in photosynthesis is not the limitation of absorption of incoming solar radiation by plant canopy, but the limitation of the supply of carbon dioxide from the atmosphere and soil to plant communities. This proposition is applicable to plant communities with foliage over LAI 6 under summer conditions in Japan.