Abstract
The hypothesis that net photosynthesis is diminished in many plant species because of a high rate of CO2 evolution in the light has been tested further. High rates of CO2 output in CO2-free air in comparison with dark respiration were found in Chlamydomonas reinhardi, wheat leaves, tomato leaves, and to a lesser extent in Chlorella pyrenoidosa by means of the 14C-photorespiration assay. In tobacco leaves high photorespiration was characteristic of a standard variety, Havana Seed, and a possibly still higher rate was found in a yellow heterozygous mutant, JWB Mutant. However, the dark homozygous sibling of the latter, JWB Wild, had a low photorespiration for the tobacco species. The relative rates of photorespiration were in the same sequence when measured by the 14CO2 released in normal air from leaf disks supplied with glycolate-1-14C in the light.
As would be predicted by the hypothesis, the maximal net rate of photosynthesis at 300 ppm of CO2 in the air in JWB Wild leaves was greater (24%) than in Havana Seed, while JWB Mutant had less CO2 uptake than the standard variety (21%). At 550 ppm of CO2 the differences in net photosynthesis were not as great between the 2 siblings as at 200 ppm. The relative leaf expansion rates of seedlings of the 3 tobacco varieties in a greenhouse had the same relationship as their rates of CO2 assimilation.
Thus within the tobacco species, as in a comparison between tobacco and maize, low photorespiratory CO2 evolution was correlated with higher photosynthetic efficiency. Therefore it seems that increased CO2 uptake should be achieved by genetic interference with the process of photorespiration.
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Selected References
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