854 METHODS OF KINETIC MEASUREMENTS CHAP. 25 



determination of total yield of photosynthesis over the whole growth per- 

 iod, and is widely used in field experiments ; but it is obviously unsuitable 

 for exact kinetic detemiinations, in which the organic material present at 

 the beginning of the experiment, cannot be neglected. 



However, a different method of measurement of photosynthesis, also 

 based on the determination of energy conversion, appears feasible, although 

 it probably cannot compete in simplicity and exactness with the methods 

 based on the determination of carbon dioxide or oxygen : 



Of the total light energy ( A//) absorbed by the plants, one part ( A//c) is 

 transformed into chemical energy (latent heat of combustion of the syn- 

 thesized organic material) and the rest ( A//,) is converted into heat. We 

 stated above that the exact determination of Ai/^ is difficult because of the 

 initial weight of organic material ; but the determination of A//^^ is possible. 

 It can be carried out, without affecting the plants, by placing them in a 

 sensitive calorimeter ; Ai/^ can then be calculated by subtracting Ai7, from 

 A/jT. Magee, DeWitt, Smith and Daniels (1939) measured the heat pro- 

 duced in a small quartz vessel in a thermostated container in consequence 

 of the absorption of a certain amount of light by a suspension of Chlorella 

 cells, and compared it with the amount of heat produced in the same ap- 

 paratus when the light is absorbed by India ink or another inert absorber. 

 In these experiments, the rate of energy absorption was of the order of 

 1000 erg/sec, and AHi was found to be smaller than AH by about 20%. 

 Similar methods were developed by Arnold and by Tonnelat (c/. chapter 29, 

 page 1122). 



6. Application of Isotopic Indicators 



The use of isotopic indicators — stable isotopes such as H-, C^'' or 0^*, 

 and radioactive isotopes such as H^, C'^ and C^^ — may make possible the 

 development of several new sensitive methods for measuring the yield of 

 photosynthesis. All that seems to be needed is to supply the plants with 

 carbon dioxide or water enriched in one of these isotopes and measure 

 either the accumulation of this isotope in the plant cells, or its disappear- 

 ance from the supplied material. However, the phenomena of isotopic 

 exchange and isotopic discrimination make the task less simple than it may 

 appear at first. 



Apphcations of the isotopes H^, C^^ C^* and O^^ to the solution of some 

 qualitative problems in the chemistry of photosynthesis (such as the forma- 

 tion of a carbon cUoxide acceptor complex, and the origin of the oxygen 

 evolved in photosynthesis) were described in Volume I (see particularly 

 pages 54, 202 and 241). New investigations with C^^ as tracer, aimed at 

 the identification of the organic intermediates of the reduction of carbon 

 dioxide, will be described in chapter 36. 



