1952 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



in the earlier studies (where 5 minute Hght-5 minute dark was the shortest 

 cycle used). Some doubts can be entertained concerning the reliability 

 of minute-to-minute measurements because of the physical lag in the re- 

 sponse of the manometer, and of its insufficient precision. (Simple, not 

 differential, manometers were used, so that the pressure changes, which 

 were of the order of 1 mm. per minute, could be read only to ±0.5 mm.) 

 Burk and Warburg believed {cf. p. 1104) that strong shaking eliminated 

 all instrumental lag; and that repetition of measurements assured sufficient 

 reliability of the average. 



These experiments were described in more detail in a second paper 

 (Burk and Warburg 1951). The light beam from a high-pressure mercury 

 or cadmium lamp, made monochromatic by filters, was divided into two 

 beams by four totally reflecting prisms. Each beam had an intensity of 2-3 

 /xeinsteins per minute. They were tested for equality by the Warburg- 

 Schocken actinometer, and for equal absorption in the two vessels, by com- 

 paring the yields of photosynthesis produced in both of them when they 

 were filled with carbonate buffer. Absolute intensity was checked by a 

 bolometer. 



The quantum yield of the actinometer was now found to be significantly smaller 

 than 1 (0.69-0.86), even at low light intensities (cf. p. 839 for the different earlier findings 

 of Warburg and Schocken). Addition of piperidine (6 mg. in 7 ml.), increased this yield 

 to 0.82-1.03 (for beam intensities from 0.05 to 1 jueinstein per minute). With ethyl 

 pheophorbide as sensitizer, and a larger actinometer vessel (120 ml.), the quantum yield 

 was remarkably constant — about 0.70 — between 1 and 5 /^einstein per minute; this 

 system was recommended for future use (cf., however, p. 1949!). 



The quantum requirement of photosynthesis in alternating light (1 

 minute dark-1 minute light) was found to be as high as 7 in carbonate buffer 

 No. 9 (pH 9.4); but in phosphate buffer (pH 4.5, 10% CO2, cf. below), it 

 went down to close to unity. 



In a typical experiment in phosphate buffer, the pressure in the large vessel changed 

 by to 0.5 mm. in 1.5 min. light periods (green light, X 546 m/i), and by —0.5 mm. dur- 

 ing 1.5 min. dark periods; in the smaller vessel, the changes were by +1.5 to —2.0 mm. 

 in the light periods, and —2.0 mm. in the dark periods. Adding together the effects of 

 six light periods, and of six dark periods, and subtracting the second sum from the first, 

 a quantum requirement of 1.27, and a —l/Qp value of 1.15 were calculated. 



With slower alternations, the quantum requirement increased from 1.27 

 (1.5 min. periods) to 1.4 (2.5 min. periods), 4.1 (5 min. periods) and 5.3 

 (7.5 min. periods); the ratio { — 1/Qp) decHned, in the same series, from 

 1.15 to 0.97. The gas exchange in the dark declined with decreasing fre- 

 quency of alternations, in the same proportion as that in light. With the 

 shortest periods used, the oxygen consumption in the dark periods was 

 about 12 times the steady respiration rate. 



