1480 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



preparations, only about 10% of the normal saturation rate of photosyn- 

 thesis (for equal amount of chlorophyll). In this sense, the observed 

 yields in very strong flashes were, similarly to those of Tamiya, much 

 higher than expected from the Franck-Emerson-Arnold relation between 

 the yields in constant and flashing hght. Reminiscent of Tamiya's obser- 

 vations was also the fact that in (relatively) weak flashes (about 100 klux) 

 the flash yield was in the "proper" relation to the rate in constant light 

 (one O2 per 20,000 chlorophyll molecules per flash, or 10% of the Emerson- 

 Arnold flash yield); complete saturation in respect to dark time was 

 reached, in this case, after 0.04 sec. at 15.6° C, also in good agreement 

 with the Emerson-Arnold observations on photosynthesis. With flash 

 energies » 100 lux sec, on the other hand, the flash yields kept growing 

 with flash duration far past the Emerson-Arnold Hmiting period (again in 

 agreement with Tamiya's results). The main content of the work of 

 Gilmour et al. was the analysis of the flash yield, P, vs. dark time, ta, curves 

 under different conditions. In contrast to Tamiya, their interpretation 

 was based on the assumption that the flash saturation curves of the Emer- 

 son-Arnold type are real, and that a kinetic theory must account for them, 

 as well as for the "Tamiya type" curves (and, more generally, for the effect 

 of dark periods > 0.01 sec. on the energy utilization). 



At "low" flash energies (50-100 lux sec), the plots of A log P vs. Ata 

 (increment in flash yield vs. increment of dark period) were simple straight 

 lines, indicating, as mentioned above, a single rate-hmiting first-order dark 

 reaction, with a half-time of about 0.016 sec at 6.9° C, and 0.0113 sec. 

 at 15.6° C. — in good agreement with the constants of the Emerson-Arnold 

 reaction in photosynthesis as reported by various observers (c/. Table 

 34.11). 



In some runs, a zero-order reaction was noticeable at the very beginning 

 of the dark period. It was followed by an approximately temperature 

 independent first-order reaction with a half-time of about 0.02 sec. (6.9- 

 33.3° C), and another zero-order reaction, with a rate constant of 4.3 X 

 10-3 at 6.9°, 4.7 X 10"^ at 15.6°, 2.5 X 10"' at 23.6° and 0.83 X lO"'' 

 at 33.3° C. (mole Fe^Vmole chlorophyfl/sec). At the highest tempera- 

 ture used, 33.3° C, the plot showed two sharply separated linear segments, 

 the above-mentioned, almost temperature independent first-order reaction 

 being preceded by a faster one with a half-time of 0.007 sec The latter 

 may be identical with the temperature dependent first-order reaction 

 (Emerson-Arnold reaction) which dominates the picture at the lower flash 

 energies. 



The superposition of a zero-order limiting reaction upon a second-order 

 one recalls Weller and Franck's suggestion that a zero-order process, by 

 which the reductant is supphed to the photosynthetic system, may be the 



