1264 THE PIGMENT FACTOR CHAP. 32 



spend 10 or 20 seconds in a photochemically inactive state, assimilation 

 times of less than 10 seconds would be impossible; in fact, however, the 

 above-quoted va values of yellow leaves correspond to assimilation times 

 of 1.5 to 2 seconds. Apparently, nature had deprived these leaves of nine 

 tenths of the normal chlorophyll content, without a corresponding reduc- 

 tion in the concentration of the rate-limiting enzyme. 



One can conclude, from the behavior of aurea leaves, that, intrinsically 

 chlorophyll molecules do not require periods of the order of 10 sec. to com- 

 plete the part of photosynthesis in which they are directly involved, but are 

 available for a new photochemical act within a much shorter time. 



Related to the same subject are the experimental results of Shcheglova (1940). 

 She found that the light-green parts of leaves of Polygonum sacchalinense have a higher 

 natural rate of photosynthesis in the first half of the summer than the dark-green ones, 

 while in the second half of the summer (when the average light intensity is lower) the 

 relation is reversed. In laboratory experiments with the same plant at different tem- 

 peratures, she found that at 20-30° C, the light-green sections of the leaves (1.2-1.8 mg. 

 chlorophyll per 100 sq. cm.) give a higher yield in strong light than the dark-green ones 

 (2.2-3.1 mg. chlorophyll per 100 sq. cm.), and thus have a much higher assimilation 

 number (6.6-5.5 vs. 3.7-2.5); while at 13-17° C, the relation is reversed, with the result 

 that the assimilation numbers are about equal (2.2 and 2.4). 



In interpreting differences in assimilation numbers as indicative of varia- 

 tions in the content of a rate-limiting enzyme, one has to keep in mind that 

 the maximum rate of an enzymatic reaction may be affected, in addition to 

 the concentration of the enzyme, also by the properties of the medium in 

 which the enzyme has to act. Not only the reaction (the pH) of the 

 medium, but also its colloidal properties may influence this rate. This 

 is particularly true if the rate-limiting step is bimolecular (e. g., if it involves 

 the encounter of a substrate molecule with an enzyme molecule) ; but even 

 if the rate-limiting step is monomolecular (transformation of the reaction 

 complex {enzyme -f- substrate! ), its rate nevertheless may be affected 

 by association with colloidal particles. 



Dastur (1924, 1925) and Dastur and Buhari walla (1928) noted that in aging leaves 

 the water content declined more rapidly than the chlorophyll content, and suggested 

 that variations in water content could provide an explanation for the drop in the as- 

 similation number, reported by Willstatter and StoU for aging leaves. The state of 

 hydration of the chloroplasts could perhaps affect the rate constant of the limiting en- 

 zyme, even if the concentration of the latter remains unchanged. 



Conditions similar to those in aurea Avere found in some autumn leaves. 

 According to Willstatter and Stoll, the autumnal decrease in photo- 

 synthesis usually parallels the disappearance of chlorophyll, so that the 

 assimilation numbers remain approximately constant. Sometimes, how- 

 ever, the decrease of P'^^''- lags behind that of [Chi], so that a transient 

 increase in va to 12-20 occurs in early autumn, before the final dechne has 



