QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1115 



these measurements were made at 30°C.) However, the break in the 

 p = /(/) curve was now found not exactly at the compensating intensity, 

 but at about twice this intensity (fig. 29.4B(A)). Since i^ught was equal to 

 0.5 Edark, the ratio of the slopes below and above the break is, for these 

 algae, 1.33 rather than 2.0. 



With Chlorella cells grown in glucose solution, the break was below 

 the compensation point (fig. 29.4B(5)) ; the slope below the break was, in 

 this case, exactly one half of that above it, which meant that i^ught was 

 greater than 0.5 /^dark— perhaps reflecting enhanced respiration in the cyto- 

 plasm. A break in the same region {i.e., below the compensation point) 

 was found also in the Haematococcus pluvialis grown in inorganic medium; 

 but in this case, the slope below the break was less than twice that above it. 

 Light curves obtained with Cahomba leaves (floated on carbonate buffer) 

 indicated that the break was present there too, and that -Rught — 0.5 Rdnvk- 

 Kok suggested that these experiments indicate the existence of two hght 

 processes, with the quantum requirement of the "low-light process" (which 

 he called "light respiration," cf. below) exactly one half that of the "high 

 light process" (true photosynthesis). When the slope in the low-light 

 region was less than twice that above it, he interpreted this as indication 

 that the two light processes were occurring simultaneously. When the slope 

 in low hght was exactly H of that in high light, Kok assumed that the high 

 light process did not begin until the low-light process was saturated. 



Kok considered these experiments (which had indicated a probable 

 lowest 1/7-value of 6.75 above the break), as making plausible a quantum 

 requirement of 6 for the high light process (true photosynthesis), and 3 

 for the low light process ("light respiration"). However, according to 

 Franck (1949), Rieke found in Kok's method of light measurement an 

 error which might have reduced the calculated quantum requirements by 

 20%; with this correction, the results become consistent with the assump- 

 tion of quantum requirements of 8 and 4, respectively. 



The sharp breaks in the light curves, found by Kok, are very improbable 

 (cf. the discussion in chap. 26 of the impossibility of a sharp break between 

 the ascending and the horizontal part of the light curve, postulated by 

 Blackman). However, even if the light curves are smoothly curved rather 

 than broken lines the possibility remains that they may decline in the 

 low hght region more steeply than would be expected from their shape in 

 the region of higher light intensities. Once before — in the explanation of 

 the alleged incapacity of cyanide to reduce photosynthesis below the com- 

 pensation point (Vol. 1, page 308)— we have been led to the hypothesis that 

 compensation of respiration in light may not requrie complete photosyn- 

 thesis. We will confront the same situation in the description of the study, 

 by Calvin and co-workers of respiration in light with the help of tracer 



