Photosynthetic and respiratory exchanges of carbon dioxide by leaves of the grain amaranth.
The maximum rate of apparent photosynthesis in recently expanded leaves of unshaded Amaranthus edulis plants at 310 ppm. CO2 and 7500 f.c. was 55 ± 5 mg CO2/dm2 per h. The rate of photosynthesis increased with increasing illumination up to 7500 f.c. at normal and higher concentrations of CO2; at this level of illumination the effect of CO2 concentration on photosynthesis was linear over the range 0-500 ppm. No apparent respiration was detected in CO2-free air, but when DCMU (3-(3, 4-dichlorophenyl)-1, 1-dime-thylurea) was used to inhibit photosynthesis completely, respiratory CO2 was released in the light at a rate equivalent to dark respiration in CO2-free air. Rate of photosynthesis decreased markedly at temperatures above 40°C. Old leaves assimilated CO2 more slowly than recently expanded ones; at high CO2 their light saturation was at 5000 f.c. and in CO2-free air they released respiratory CO2. Recently expanded leaves from shaded plants resembled old leaves in their photosynthetic characteristics. The photosynthetic behaviour of A. edulis resembled that of tropical monocotyledons such as maize and sugar-cane.-R.B.