1924 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



posing the same problem as the above-mentioned equahty of inactivation 

 of photosynthesis in continuous and intermittent hght. 



Holt, Brooks and Arnold discussed whether fractional inactivation by 

 ultraviolet irradiation means equal partial inactivation of all cells, or com- 

 plete inactivation of a fraction of cells. A priori, it seems unlikely that a 

 single hit can destroy the photosynthetic apparatus in the whole cell. 

 The observation that, in Scenedesmiis suspensions partially inhibited by 

 253.7 m/i and then adapted to hydrogen, reversion to normal photosynthesis 

 in strong light required a longer time than in nonirradiated cells, supported 

 the concept that all cells had been partially incapacitated (rather than that 

 certain cells had been knocked out altogether) . 



In addition to immediate effects of irradiation, further damage to the 

 photosynthetic apparatus results from slow processes, initiated by irradia- 

 tion but not becoming manifest for several hours. These affect endogenous 

 respiration as well as photosynthesis. 



Redford and Myers (1951) also made a study of the effect of 253.7 m^ 

 irradiation on growth, respiration and photosynthesis of Chlorella pyrenoi- 

 dosa. They, too, found endogenous respiration to be insensitive (except 

 for slight transitory stimulation) and exogenous respiration (supported by 

 acetate or glucose), highly sensitive to ultraviolet irradiation (table 

 37D.VI). The inactivation of exogenous respiration was an exponential 

 function of total dosage, intensity X time, and independent of the specific 

 values of the two factors. 



Photosynthesis after irradiation was found to remain constant for SO- 

 SO min. and then to fall off (delayed effect). The rate in the initial, steady- 

 rate period after irradiation was an exponential function of irradiation time, 

 up to a certain limit, but showed strong deviations at higher dosages. 

 As shown in fig. 37D.12, a certain dosage had a stronger effect if given 

 within a shorter time (i. e., at higher intensity). 



Table 37D.VI 



Comparative Sensitivity to 253.7 mn of Different Functions of Chlorella 



pyrenoidosa (after Redford and Myers 1951) 



Relative sensitivity calculated from 

 Slope of Time needed for 



Function survival curve half-inactivation 



Glucose respiration 1 1 



Growth 0.27 0.30 



Initial photosynthesis 0.04 . 045 



Frenkel (1949) compared the effect of ultraviolet irradiation on photosynthesis 

 with its effect on catalase activity of Chlorella pyrenoidosa in bicarbonate solution. 

 While 6 min. exposure to a certain flux reduced the rate of photosynthesis of a cell sus- 



