1448 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



condensor discharges through gas-filled tubes (Emerson and Arnold 1932), 

 and appropriately shaped rotating sectors (in combination wth a constant 

 light source of high intensity, e. g., high-pressure mercury arcs) (Pratt and 

 Trelease 1938, Weller and Franck 1941, Tamiya and Chiba 1949). In the 

 discharge tube technique, the flashes are shorter (of the order of 10"^ sec- 

 ond) and more intense, but the rotating disc technique offers easier access 

 to a wide range of integrated flash energies. 



In both tyi)es of experiments, the yield — determined by the usual tech- 

 nique, such as manometric measurement — is the average yield for a large 

 rmmber of identical flashes. Franck, Pringsheim and Lad (1945) were 

 able to determine, by means of the phosphorescence-quenching technique, 

 the oxygen production of a single flash. They used for this purpose photo- 

 flash bulbs, which produce much stronger flashes than are obtainable by the 

 other methods. The flashes last for about 0.04 second, and reach a peak 

 intensity of 1.4 million lumen. 



It is easy to show that in flashing light (as in alternating light) the factor 

 in must always be < 1 if the rate limitation is caused by catalyst deficiency. 

 (The maximum yield in flashing light is reached when the rate-limiting 

 catalyst is practically fully occupied for the whole duration of the dark 

 intervals; it is then equal to the maximum jdeld in continuous light.) 

 The factor irs (which, in alternating light, had a limiting value of 2) can 

 reach much higher values in flashing light (c/. equation 34.1). For example, 

 if in — 1, and t*/ta = 10-=^ {e. g., t* = 10"^ second and.^^ = IQ-^ second), 

 iiE is approximately 1000. 



However, these high values of ijs are without real significance, for when 

 the light periods are shorter than the "Emerson- Arnold period" the yield 

 per flash depends only on the total energy of the flash (the time integral of its 

 intensity) and not on these two factors separately. In other words, plants 

 do not distinguish between a flash of a certain intensity that lasts for 10""* 

 second, and a flash of tenfold intensity that lasts for only 10~^ second. 

 Under these conditions, the two intermittency factors for equal intensity, 

 in and ijE, lose their importance, and the only factor with which we need 

 to be concerned is ist- 



These considerations appear not to apply to intense flashes lasting for 

 several milliseconds or longer; in this case, the yield becomes a function 

 of the duration of the flash, and is affected by dark periods of the order of 

 0.1 or 1 second. We mentioned this complicating effect in discussing the 

 "alternating light" phenomena in part A, and will return to it below in dis- 

 cussing the experiments and theoretical considerations of Weller and 

 Franck, Tamiya, and Gilmour et at. We will also see that Burk and co- 

 workers denied the validity of the photochemical "reciprocity law" in 

 photosynthesis even for the very brief condensor discharge flashes. 



