EFFECT OF NATURAL VARIATIONS OF CHLOROPHYLL CONTENT 1263 



From the kinetic point of view, a more relevant value than the assimila- 

 tion number is the assimilation tvne, Ia- Willstatter and Stoll designated by 

 this term the minimum time required by a molecule of chlorophyll for the re- 

 duction of one molecule of carbon dioxide: 



(32.3) tA = (44 X 3600)7(900 X >'a) = 156/i^a (seconds) 



(44 is the molecular weight of carbon dioxide, and 900 the approximate 

 average molecular weight of chlorophyll) . 



For "normal" green leaves, the ^a values of Willstatter and Stoll were 

 between 20 and 25 seconds. Values of this order of magnitude have been 

 found for leaves the chlorophyll content of which differed by a factor of 

 three or even more. The absorption of light by these leaves probably did 

 not vary by more than 10 or 20% (even though some of them were "dark 

 green" and others "light green"). In the saturation region, such a varia- 

 tion of absorption could not in itself cause a marked change in the rate of 

 photosynthesis. Therefore, the fact that the saturation rate did change 

 approximately proportionately with [Chi ] is an indication that the velocity 

 of the dark, rate-limiting process was in these plants proportional to the 

 content of chlorophyll. Since the maximum rate of an enzymatic reaction 

 usually is proportional to the available amount of the enzyme, we conclude 

 that in this case the "limiting" catalyst was either chlorophyll itself, or a 

 compound the quantity of which in "normal" plants is (approximately) pro- 

 portional to that of chlorophyll. The possibility of identifying the rate- 

 limiting catalyst with chlorophyll was discussed in chapter 28 (p. 1030) . 

 The alternatives, presented there, were either to attribute saturation (in 

 continuous light) to a catalyst the concentration of which is roughly equiva- 

 lent to that of chlorophyll (one such possibility being chlorophyll itself), 

 and the working period of which is of the order of 10 seconds, or to attribute 

 it to a catalyst whose concentration is a thousand times .smaller and working 

 time a thousand times shorter, i. e., of the order of 0.01 sec. Flashing light 

 experiments, to be described in chapter 34, were quoted there as demon- 

 strating directly the existence of a catalyst of the second type, with respect 

 to both concentration and working period. Chlorophyll could be a "com- 

 peting" Hmiting factor, imposing a "ceihng" only shghtly higher than that 

 imposed by the catalyst revealed by flashing light experiments; but the 

 deviations from constancy of va, shown by the "abnormal" objects of Will- 

 statter and Stoll (Table 28. V) , do not support this hypothesis. These devia- 

 tions can be as wide as 100% (va = 15) in green leaves and very much larger 

 in aurea leaves. In Table 28. V we find, for three aurea varieties, the values 

 Pa = 78, 82 and 117, respectively. If the photosynthesis of these plants 

 were limited, in strong light, by the requirement that, after having partici- 

 pated in the primary photochemical act, each chlorophyll molecule has to 



