YIELD OF PHOTOSYNTHESIS IN ALTERNATING LIGHT 1435 



t* 

 (34.2) lEt = Piit = PiiEj^-T:ji 



Here, /S designates the increase in rate of photosynthesis brought about 

 by an increase in the intensity of continuous hght by a factor of {ta + t*)/t. 

 Equation (34.2) follows from the consideration that, if we first raise the 

 intensity of continuous illumination by this factor, and then use a rotating 

 sector to produce intermittent illumination with light periods of t* seconds 

 duration, the result must be the same as that obtained by concentrating 

 the whole energy of the original illumination in exposures of t* seconds each. 



Equation (34.2) shows that the factor Iei depends on the shape of the 

 light curve, P = /(/), in continuous light. 



Section A of this chapter will deal with phenomena observed in inter- 

 mittent light with equal light and dark periods, U = t*, which we will desig- 

 nate as alternating light. In this case, its = 2 in (cf. equation 34.1), 

 and one of these two factors (rather than the factor ist) is commonly used 

 for the characterization of the intermittency effect. 



Section B will deal with flashing light {t* <C to). For very short flashes 

 (t* < 0.001 second), the true momentary hght intensity during the flashes 

 seems to become unimportant. (About Burk's disagreement with this 

 view, see p. 1475.) Plants react to such short flashes as if they were in- 

 stantaneous, i. e., the yield per flash depends only on the total energy of the 

 flash (time integral of its intensity) . 



In photography — and in photochemistry in general — one is accustomed 

 to consider the reduction of average light intensity by rotating sectors equiva- 

 lent to the reduction cf true hght intensity by filters (reciprocity law) . This, 

 however, does not apply to reactions which are subject to light satura- 

 tion — i. e., reactions that contain, in addition to a photochemical stage, 

 a "dark" process of limited efl&ciency. In the saturation region, the two 

 ways of reducing light intensity can have the same effect only if the dura- 

 tion of the dark periods is much shorter than the time required for the 

 completion of all dark reactions. 



A. Alternating Light* 



1. Yield of Photosynthesis in Relation to Frequency of Alternations 



In alternating light, the factor ijE can be expected to be larger than 

 unity {iit > 0.5) if the periods ta and t* are very long or very short, and less 

 than unity in the intermediate region. Long intervals (of the order of 

 several hours) can improve the utilization of light energy because during 

 the dark "rest periods" the plant can recuperate from the injury or ex- 

 haustion that often follows a period of intense photosynthesis. Some 

 phenomena involved in the natural adaptation of plants to the alternation 



* Bibliography, page 1483. 



