In tobacco mutants which contain ⅛ to 1/30 of the normal chlorophyll content per leaf area the content of yellow pigments (carotenoids) is also diminished but less in proportion to the chlorophyll content. The pale yellow-green mutant grows and matures provided that light intensity and temperature make up for the chlorophyll deficiency. In most green plants and algae light saturation of photosynthesis is reached between 5000 and 12,000 ergs/sec·cm2. The mutants continue to give higher photosynthetic rates until the incident intensity reaches 50,000 ergs/sec·cm2. While often unable to compensate their respiration at intensities at which the normal green plant approaches saturation, the pale yellow-green leaves are able to provide the mutant plant with two to three times the absolute amount of carbon dioxide assimilation per hour and leaf area at 50,000 ergs/sec·cm2 and 20° to 25°C. These observations are valid for red light λ > 600 mµ. In blue light λ < 575 mµ (below saturation levels) the mutants separate into two classes, one in which absorption by some carotenoid enhances the photosynthetic rate and the other in which the absorbing pigments are inactive and therefore depress the rate strongly. The unusual kinetics of photosynthesis in these chlorophyll-deficient tobacco mutants is reflected in the structure of their chloroplasts which we found to be of a kind thus far not described for healthy, normally growing, higher plants.

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