1922 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



certain localized group of "reduction centers" — such as ACO2 complexes — 

 so that "knocking out" a center leaves the attached Eb molecule "un- 

 employable"; cf. Rabinowitch 1951.) 



Holt, Brooks and Arnold (1951) found that in Scenedesmus Di, a given 

 dose of 235.7 m;u reduces, in the same proportion, the rates of photosyn- 

 thesis, photoreduction (with H2 as reductant) and Hill reaction (with qui- 

 none as oxidant). The dark oxyhydrogen reaction (p. 138) is much less 

 sensitive. Measurements with different cell densities (6.5 to 65 mm.^ 

 Scenedesmus cells in the vessel) showed no difference in the extent of inacti- 

 vation; since the rate of absorption (of visible light) increased with in- 

 creasing density, it was concluded that ultraviolet light affects photosyn- 

 thesis equally at all light intensities (cf. below for direct confirmation of 

 this conclusion in the case of chloroplasts). When photosynthesis was 

 reduced to one third of its full rate by cyanide, or to one-half of its full 

 rate by hydroxylamine, the effect of ultraviolet irradiation was propor- 

 tionally the same as in normal cells. 



Chlorella pyrenoidosa was found to be much more sensitive to 253.7 m^i 

 photons than Scenedesmus Di. At a certain intensity of irradiation, the 

 rate of photosynthesis in Chlorella was reduced to 54% after 50 sec, 5% 

 after 150 sec, and to zero after 240 sec. of exposure, while in Scenedesmus, 

 the rate was 82% of the initial one after 150 sec, and 40% after 240 sec of 

 the same irradiation. 



Glucose respiration also was much less affected by ultraviolet irradiation 

 in Scenedesmus than in Chlorella pyrenoidosa. The exogenous respiration 

 of the latter was even more sensitive to ultraviolet than its photosynthesis. 

 (The endogenous respiration is relatively insensitive to X 253.7 mju in Chlo- 

 rella as well as in Scenedesmus, cf. fig. 37D.11.) The curve of log R as 

 function of time consisted, in the case of exogenous respiration of Chlorella, 

 of two straight sectors, as if two different first-order inactivation processes 

 were involved. The capacity of Scenedesmus Di for colony formation was 

 also reduced by exposure to 253.7 m/x much more strongly than its photo- 

 synthesis. 



One peculiar effect, noted in these experiments, was the enhanced 

 influence of continuous (as compared to interrupted) ultraviolet irradiation 

 on the rate of photosynthesis of Chlorella in carbonate buffer No. 9. (For 

 example, two 90 sec irradiation periods, interrupted by a 20 min. photo- 

 synthesis period, resulted in an inactivation by 46%, while a solid 3 min. 

 exposure resulted in 78% deactivation.) No such difference was noted in 

 acid phosphate medium or with Scenedesmus, in either acid or alkaline me- 

 dium. In the latter species, the inactivation was the same at low and at 

 high pH values, ^vith interrupted as well as with continuous irradiation. 



The log (P/Pq) = f(t) curves showed a definite deviation from straight 



