1438 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



photosynthetic apparatus could produce more if it were not for the slow- 

 ness of certain enzymatic reactions — and these can be completed during 

 the dark intervals. (Here, again, we neglect the fact that under certain 

 conditions "bursts" of gas exchange have also been observed in weak light, 

 where induction losses are negligible.) 



Padoa and Vita (1928) repeated Warburg's experiments with the water plant 

 Elodea canadensis. They found ijE factors up to 2.71, with not less than five maxima 

 (at 16, 80, 406, 650 and 887 alternations per second). Between these peaks, ijE values 

 declined to 1. The reality of these several maxima and minima, not noted by other 

 investigators, is very doubtful. 



The next step in the elucidation of the shape of Ije = f{t) curve was 

 made in the well-known studies of Garner and Allard (1931) on the effect 

 of intermittent light on the growth of plants. Figure 34.2, taken from 

 their work, shows a striking minimum of the growth of potted plants of yel- 

 low cosmos when the alternations occur about once a minute. The growth 

 curve rises steeply on both sides of the minimum; 5 second intervals are 

 almost as favorable as the "natural" intervals of 12 hours. 



It was often suggested that the basis of some intermittency effects in 

 plant physiology may lie in the influence of intermittent light on photosyn- 

 thesis. In confirmation of this, Portsmouth (1937) found that the growth 

 curve of Garner and Allard runs closely parallel to the curve representing 

 the yield of photosynthesis in relation to the frequency of light alternations. 

 We recall that Willstatter and Stoll tried to attribute the intermittency ef- 

 fects, described by Brown and Escombe, to an inadequate supply of carbon 

 dioxide. Gregory and Pearse (1937) suggested a similar explanation for 

 the growth curve of Garner and Allard. They thought that it may be 

 caused by incomplete opening of stomata in intermittent hght, leading to 

 carbon dioxide starvation. (Both the opening of stomata in light and 

 their closure in the dark are not instantaneous; the ratio of their velocities 

 determines the average aperture of the stomata in intermittent hght.) 

 Portsmouth thought that the sluggishness of the stomata may provide a 

 clue also to the decline of photosynthesis in alternating light. 



Gregory and Pearse measured the apertures of the stomata of Pelar- 

 gonium zonale in alternating light, and found the slits to be particularly 

 narrow when the periods of light and darkness were 5 seconds each. They 

 thought this period to be sufficiently close to the minimum of the Garner- 

 Allard growth curve to warrant the attribution of the latter to stomatal in- 

 fluences. However, figure 34.2 shows that, at i = 5 seconds, the plant 

 development proceeded quite satisfactorily. Furthermore, we mentioned 

 above that Warburg had observed intermittency effects in Chlorella cells, 

 where no stomatal effects are possible. More recently, Iggena (1938) 

 found that a growth curve similar to that observed by Garner and Allard 



