1048 



THE LIGHT FACTOR. I. INTENSITY 



CHAP. 28 



for F and P the two curves shown in figvire 28.24: The fluorescence curve 

 remained Hnear up to 16 kerg/cm.^ sec, while photosynthesis became Ught- 

 saturated at about 10 kerg. Subsequent investigations have sho^vn, how- 

 ever, that such an absence of correlation between tp and 7 is by no means 

 the general rule. More often, a definite relation — usually, antiparallelism 

 — is found between the two jnelds {i. e., the jaeld of fluorescence increases 



12 3 4 5 6 



INCIDENT INTENSITY, arbitrary units 



Fig. 28.24. Photosynthesis light 

 saturated, fluorescence unchanged in 

 Chlorella suspension (after Wassink, 

 Vermeulen, Reman and Katz 1938). 



200 



400 



600 



LIGHT INTENSITY, kerg /cm'^ sec. 



Fig. 28.25. Yield of fluorescence in- 

 creases in wheat at high light intensity 

 when CO2 supply is low and remains con- 

 stant when [CO2] is high (after McAlister 

 and Myers 1940). 



when the yield of photosynthesis becomes smaller, and vice versa). An in- 

 crease in the yield of fluorescence at high light intensities was first observed 

 by McAlister and Myers (1940) in young wheat plants. It began at about 

 200 kerg/cm.2 sec. However, this increase occurred only when the carbon 

 dioxide supply was low (0.03%) ; no change of <p was noticeable even up to 

 700 kerg/cm.2 sec, when this supply was ample (in 4% CO2) (c/. fig. 

 28.25). It will be noted that McAlister and Myers used much higher light 

 intensities than Wassink and co-workers; but in their experiments {of. 

 Table 28.1), saturation was not quite reached, in the presence of 4% CO2, 

 even at 600 kerg/cm.^ sec. 



Franck, French and Puck (1941), working with Hydrangea leaves, found 

 an increase in fluorescence yield above 20 kerg/cm.^ sec, even with ample 



