NONMANOMETKIC MEASUREMENTS OF QUANTUM YIELD 1123 



3. Although the medium (nutrient solution, pH 5.5) satisfied Warburg's require- 

 ments of "physiological" conditions, presence of mercury drops introduced a danger of 

 poisoning. In fact, such poisoning has been observed, but deemed too slow to affect the 

 measurements. 



(c) Calorimetric Method 



The basis of the calorimetric determination of the yield of photosynthe- 

 sis — which is a direct measurement of the energy conversion yield, e, rather 

 than of the quantum yield, 7 — was described in chapter 25 (page 854). 

 The first to carry out such measurements was Arnold in 1936-1937; how- 

 ever, so strong was the beUef at that time in the correctness of Warburg's 

 value, y = 14, that Arnold took his inability to obtain this yield as indica- 

 tion of a failure of the method, and did not pubhsh his results until 1949 

 (reference to them was made by Franck and Gaffron 1941). Arnold used 

 a modified Callender's radiobalance, originally designed to measure heat 

 production by radioactive materials. Its period was so small that com- 

 plete measurements could be made in from 1 to 10 minutes. Between 

 0.05 and 4 mm.^ of cells, in Knop's solution or carbonate buffer, were used. 

 One run was made with healthy ChloreUa cells, and one run with the same 

 cells inhibited by ultraviolet irradiation. Respiration was assumed to be 

 unaffected by ultraviolet light (c/. Vol. I, page 344). The results are shown 

 in Table 29.Vl. 



Table 29.VI 

 Calorimetric Determination of Quantum Yield (after Arnold 1949) 



Extra heat 

 evolved in light" 



With With 100 Aff^ 



inhibited healthy 1 ' 



cells, cells, a 



Cells /„ J„ - AH^ AH^ % 1/7 



HEAT PRODUCTION IN MICROAVATTS 



C. pyrenoidosa 5^08 3^70 Os 27^ 9.5 



4.60 3.80 0.80 17.4 14.8 



4.46 3.90 0.56 12.5 20.6 



2.24 1.90 0.34 15.2 16.9 



Avocado leaf 0.786 0.656 0.130 16.5 15.6 



C. vulgaris 0.912 0.700 0.212 23.2 11.1 



C. pyrenoidosa 1.86 1.34 0.52 27.9 9.2 



16.2 12.9 3.3 20.4 12.6 



" The illuminating light was from a neon arc (ten to fifteen red lines isolated by a 

 red filter), and had a very low intensity — from 61 to 126 erg/sec. cm.^ 



