¹ From the Biology Division, Oak Ridge National Laboratory, Oak Ridge

1. The kinetics of the inactivation of photosynthesis by 2537 Å in Chlorella pyrenoidosa and Scenedesmus D₁ indicate that, while the destruction process is largely a first order effect, higher order effects also occur, which become evident at low exposures. In agreement with previous observations, endogenous respiration is insensitive to exposures which inactivate photosynthesis.

2. In Scenedesmus D₁ a solid dose of ultraviolet has no more effect on the photosynthetic apparatus than a dose of equal total duration interrupted by periods of photosynthesis. Nor is any difference noted if the cells are in a different buffer, e.g. 0.05 M KH₂PO₄, or carbonate-bicarbonate buffer 9.

3. In C. pyrenoidosa, a solid dose and an interrupted dose cause equal effects on photosynthesis when neutral phosphate buffer is used. If the ultraviolet exposure schedules are identical, equal effects are also noted in cells suspended in buffer 9, and in 0.05 M phosphate (pH 6.2). Solid exposures are, however, much more effective than interrupted exposures, when buffer 9 is used.

4. Oxygen evolution (Hill reaction), photosynthesis, and photoreduction in Scenedesmus D₁ are equally sensitive to a given dose of ultraviolet. The mechanism responsible for adaptation to hydrogen metabolism is not more sensitive to ultraviolet than is the photosynthetic mechanism. The O₂/H₂/CO₂ reaction in darkness is less sensitive to ultraviolet than any of the above reactions.

5. Glucose oxidation by C. pyrenoidosa, and colony formation in Scenedesmus D₁ are far more sensitive to a given dose of ultraviolet than photosynthesis in these organisms.

6. The photosynthetic apparatus of C. pyrenoidosa is more sensitive to ultraviolet than that of Scenedesmus D₁.

7. The Hill reaction in chloroplast fragments is also inactivated by 2537 Å by a first order process. Exposures which inactivate this reaction completely have no effect on polyphenol oxidase, cytochrome oxidase, or catalase in the same chloroplast preparation.

8. After irradiation, the survival of photosynthesis in Scenedesmus D₁ and of the Hill reaction in chloroplast fragments are independent of the light intensity used to measure these processes.

9. No significant changes occur in the ultraviolet absorption of chloroplasts after an exposure to 2537 Å, which completely inactivates the Hill reaction.

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