Photosynthesis and transpiration of euphorbia geniculata compared with corn and euphorbia hirta.

Abstract

Photosynthesis and transpiration activities of E. geniculata were examined under several conditions in comparison with E. hirta and corn plants. The results obtained are as follows:1) E. geniculata, a C3 plant, had a lower saturation light intensity for photosynthesis, ca. 50klux, while those of E. hirta and corn, both C4 plants, were over 70klux and 75klux, respectively. The apparent photosynthetic rate under a strong light was limited by mesophyll resistance, rm, in E. geniculata, but by stomatal resistance, rs, in E. hirta and corn. The highest transpiration rate of E. geniculata was about two times that of E. hirta and corn. Stomatal closing reaction due to darkening was slower in E. geniculata than in the other two species.2) A young leaf of E. geniculata had a higher saturation light intensity, maximum transpiration rate and maximum apparent photosynthetic rate than an old leaf did.3) When grown under shading treatment, saturation light intensity for apparent photosynthesis was lower in E. geniculata than in corn. The degree of decrease in apparent photosynthesis under a strong light condition was larger in E. geniculata than in corn. The decreased apparent photosynthesis was attributable to increased rm in E. geniculata, but to both increased rm and rs in corn. Shading treatment decreased transpiration rate in corn but not in E. geniculata.4) The short-term water requirement of E. geniculata was more than twice that of corn under normal light, and further increased to approximately three times that of corn by shading treatment.5) Maximum apparent photosynthesis in E. geniculata became smaller when day-length was increased. The highest maximum transpiration rate was obtained from 12-hour day-length, giving the lowest water use efficiency. Stomatal closing reaction due to darkening was far quicker in 8-hour than in longer hour plots.