114 The Maximum Efficiency of Photosynthesis 



not materially reach the lowest layers of the cell suspensions, so that the measured 

 red light from below was absorbed in a cell layer of noncompensated respiration, 

 and hence the efficiency calculated for the light increment AJ was possibly still the 

 efficiency of an inhibition of respiration. Although this question, owing to the rapid 

 motion of the cells, had to be answered in the negative, it has been tested experi- 

 mentally as a result of the discussion. Respiration was also compensated mainly 

 from below, instead of from above, by white light filtered through a red filter that 

 transmitted wavelengths longer than 560 mu. This time the increment of measured 

 red light from below was definitely absorbed by the very cells whose respiration 

 was overcompensated. The same efficiencies for the increment AJ were obtained 

 when the compensation was effected by light from below as from above. 



In the same discussion objections were raised against the procedure of complete 

 absorption, and experiments with optically thin cell suspensions were suggested. 

 But the methods used to measure light absorption in turbid media seem to be still 

 unsatisfactorily developed in connection with manometry.* It maybe remembered 

 that 10 years ago Noddack and coworkers 15 " 19 determined quantum requirements 

 of thin suspensions of Chlorella that absorbed only about 10% of the incident light. 

 But Noddack observed quantum requirements of 4 in carbonate buffer, which 

 suggests, according to our knowledge of today, that his light absorption measure- 

 ments were possibly in error by more than 100" ( „ unless his cultures, which were 

 maintained in an unusual manner, had actually become adapted to give high 

 efficiencies in carbonate buffer. 



The only serious objection raised against complete absorption was too high a 

 respiration. But since efficiencies are determined with compensated respiration, 

 this objection is no longer valid. 



9. The Measurement of the Quantum Intensities 



Light measurements were carried out manometrically as described in 1949 13 . Two hundred mg. 

 of thiourea and 3 mg. of ethyl chlorophyllide,** dissolved in 7 ml. pyridine, were placed in a rectan- 

 gular vessel of about 8 cm. 2 bottom area, with 2 in the gas phase. A beam of red light of 630 — 660 

 m// and of a cross section of 3 cm. 2 entered the Solution in a vertical direction through the bottom 

 of the vessel and was completely absorbed in the Solution. With an adequate rate of shaking, the 2 

 consumption of the Solution was proportional to the light intensity and did not increase with 

 increasing shaking rates. 



Let the decrease in 0± pressure in the actinometer be /; mm. in t minutes of illumination and 

 let &o 2 (mm 2 be the vessel constant for oxygen in pyridine a 2°° = 0.092), then the total inten- 

 sity . \J of the light beam is obtained by the equation: 



AJ = h &o 2 /22.4 t (//mole quanta/min.) [12] 



where 22.4 is the volume of one //mole of gas in cu. mm. The quantum intensity was usually 

 determined at the order of magnitude of about 1 //mole/10 min. (i.e., 22.4 cu. mm./lO min.), as 

 arranged by the use of suitable neutral screens. 



Because our application does not require the conversion of cu. mm. of O2 to //moles of quanta, 

 we may express the light intensities most simply by the cu. mm. of O2 absorbed/min. in the actino- 

 meter . \J = h kojt (mm 3 <>/min.) [13] 



* Zusatz 1961. Vergleiche Arbeit Nr. 25 dieses Buchs, wo die Anwendung der 

 Ulbrichtschen Kugel für Absorptionsmessungen in Chlorella beschrieben ist. 



** The crystallized ethylchlorophyllide was prepared by Walter Christian of the Kaiser 

 Wilhelm Institute at Berlin-Dahlem. Chlorophyll itself was not used owing to the foaming of the 

 preparation at hand in pyridine. 



