INDUCTION AFTER CHANGE TO LOWER LIGHT INTENSITY 1359 



pable of giving the highest initial rate in weaker light. It is noteworthy that, even in the 

 maximum of both curves, the initial rate is about 25% below the steady rate at 2300 lux. 

 Steemann-Nielsen saw in this an indication that these inhibition effects are due to the 

 competing action of two independent factors, one activating and the other inhibiting 

 the photosynthetic apparatus, rather than to variation in the intensity of a single acti- 

 vating factor, the optimum value of which increases with light intensity. (In the latter 

 case, the maxima of the cui-ves in figures 33.13N and O should be 100%.) By further 

 analysis of the curves on the basis of these concepts, Steemann-Nielsen constructed a 

 curve showing the time development of the inhibiting factor at 23,000 lux; the curve 

 was sigmoid and showed a saturation after about 20 minutes (at 5.6 ° C). 



In section 7, we will describe observations of induction losses after a 

 period of photoxidation, caused by various factors. One of them is ex- 

 cessive light. It may be asked whether a relation exists between these ex- 

 periments and the observations of Steemann-Nielsen. Offhand, it seems 

 unlikely that significant photoxidation could occur at intensities as low as 

 7000 lux, even at temperatures close to 0° C. (In the experiments de- 

 scribed in section 7, light intensities of the order of 100,000 lux were used.) 

 We will see below, however, that Steemann-Nielsen tends to consider the 

 analogy as significant. 



Steemann-Nielsen (1949) resumed the investigation of induction after 

 reduction of light intensity, using this time Cladophora insignis. In this 

 fresh-water green alga, induction losses of this kind could be observed only 

 on rare occasions, despite an improvement in the technique of the Winkler 

 method which permitted one-minute oxygen determinations. A number 

 of curves were secured, however, which showed the effect. In one experi- 

 ment, a change from 31 to 2.6 klux was followed by 14 minutes induction 

 (at 10° C), and in another, a change from 10.7 to 2.2 klux, by 15 minutes 

 induction (at 18° C). The effect was more pronounced after a change 

 from 41 to 2.2 klux than after a change from 10.7 to 2.2 klux — although 

 photosynthesis was saturated at both initial intensities; in agreement 

 with this result, an induction loss was noted also upon transition from a 

 higher to a lower intensity in the saturating range. Figure 33.13P shows 

 a set of measurements illustrating these findings. 



In new experiments with two species of Fucus, the induction after light 

 reduction could not be found (in contrast to earlier results with Fucus 

 serratus) . 



Steemann-Nielsen concluded that photosynthesis includes one dark re- 

 action, which can produce light saturation, but is not usually limiting and 

 which involves reactivation of chlorophyll (somehow changed in the pri- 

 mary photochemical process). Whenever this reaction becomes Umiting 

 in the light-saturated state (because of some special metabolic conditions), 

 a sudden reduction of light intensity finds a part of chlorophyll in the inac- 

 tive state incapable of contributing to oxygen production. The initial 



