THEORY OF ALTERNATING LIGHT EFFECTS 1441 



Aufdemgarten (1939) measured the photosynthesis of Hormidium 

 flaccidum in intermittent hght, using van der Paauw's gas flow method. 

 His results are shown in figure 34.4a, the shape of which resembles closely 

 that anticipated in figin-e 34.1 The minimum lies someAvhat above 1 min- 

 ute (at t = 2.5 minutes); in some experiments, a second minimum was 

 found at ^ = 10 minutes. 



In a Stichococcus bacillaris suspension, a flat minimum stretched from 

 1 to 2.5 minutes (fig. 34.4b). The factor Z/g rises sharply on the side of 

 the short intervals as t declines to 0.06 second, then more slowly; at 

 t = 0.0085 second, I/e reaches 1.7 (cf. fig. 34.4c). On the side of long 

 intervals, z'/g rem.ains below unity up to ^ = 1.5 hour (cf. fig. 34. 4d). 



Experiments with excised leaves (Impatie7is parvijlora, Vitis vinifera, 

 etc.) gave less regular curves, but they, too, showed a distinct minimum 

 in the region of ^ = 5 minutes. 



2. Theoretical Discussion of the Effect of Alternating Light 



There seems to be little doubt that the general shape of the Ije = f{t) 

 curves in the region between 0.01 and 1000 seconds is strongly influenced 

 by the interplay of two factors: the "Emerson-Arnold period," which 

 causes the intermittency factor to be highest at alternations of the order 

 of lOO/sec; and the induction period, which produces a minimum in the 

 region between ^ = 1 and 5 minutes. The course of the curve above 5 

 minutes seems to reveal the influence of the "long" induction period (cf. 

 fig. 34.4c). Whether "exhaustion" or "fatigue" effects produce a second 

 hump, somewhere between t = 1 hour and oo (tentatively indicated in fig. 

 34.1), is not certain, and probably depends on special circumstances. 



A quantitative interpretation of the intermittency factors in the region 

 where the induction period is the decisive factor is complicated by the fact 

 that, in alternating light, a change of frequency affects both the light and 

 the dark periods, and thus produces two antagonistic effects. A longer 

 dark interval means a more complete preparation of the induction phenom- 

 ena, while a longer light interval means more time for overcoming the 

 initial inhibition. The first effect can be expected to prevail at alternation 

 frequencies of more than 1/min. (where induction phenomena blanket prac- 

 tically the whole illumination period), the second at alternation frequencies 

 of less than 1/5 min. (where a further lengthening of the dark period can add 

 only little to the induction losses; cf. fig. 33.6). The exact position of the 

 minimum must depend on kinetic equations that govern deactivation of the 

 photosynthetic apparatus in the dark and reactivation in light (for a dis- 

 cussion of the kinetics of these reactions, see chapter 33, part C). 



The disappearance of photosynthetic production losses due to induction 



