INFLUENCE OF ARTIFICIAL CHANGES OF CHLOROPHYLL CONTENT 1267 



(since it can be inactivated without noticeable damage to chlorophyll), is 

 provided by the experiments of Davis (1948, 1952), who obtained Chlorella 

 mutants containing normal chlorophyll, but incapable of photosynthesis. 



3. Influence of Artificial Changes of Chlorophyll Content 



on Photosynthesis 



The experiment of Emerson, described at the conclusion of the preced- 

 ing section, forms a transition from the study of natural to that of artificial 

 changes in chlorophyll content. Since we cannot extract or destroy a part 

 of chlorophyll present in the cells without killing the cells, the desired 

 artificial changes in chlorophyll content have to be obtained indirectly, by 

 varying the conditions of culturing, so as to induce the plants to produce 

 less (or more) than their normal complement of pigments. 



As the first experiments of this kind, we may consider those in which 

 Willstatter and Stoll (1918) found abnormally high va values in etiolated 

 plants, i. e., seedlings grown in the dark, just in process of becoming green. 

 For example, the assimilation number of yellowish-green etiolated Phaseo- 

 lus vulgaris plantules was as high as 133 (as compared with 9.4 in a green 

 control plant). The chlorophyll concentration was 0.7 mg. in 10 g. fresh 

 leaves of the etiolated plant, and 18.6 mg. in the control specimen. 



These results of Willstatter and Stoll disagreed with the earlier conclusions of 

 Irving (1910) (who worked in Blackman's laboratory); the latter had found that etiolated 

 seedlings do not photosynthesize at all until they have acquired a considerable amount of 

 chlorophyll. Inman (1935) found that the photosynthesis of etiolated plantules begins 

 simultaneously with the appearance of green color. 



Beber and Burr (1937) reported that, in etiolated oat seedlings, photosynthesis did 

 not begin until some chlorophyll h was formed (not confirmed bj' Smith, cf. p. 1766). 



Smith (1949) plotted the rate of photosynthesis (at 45 klux) of etiolated bean and 

 corn seedlings in the process of greening, as observed by Willstatter and Stoll, as function 

 of (1 — e~ <'°'^s*- [Chi] ^^ g^j^jj obtained a straight line. He interpreted this as an indication 

 that the increase of P with [Chi] was a trivial consequence of increasing absorption. 

 However, this kind of relationship is to be expected only in the light-limited state, and 

 even if etiolated seedlings might not have been completely light-saturated at 45 klux, 

 they are not likely to have been in the light-limited state in such intense illumination — 

 particularly after the chlorophyll content had increased to 10-15 mg. in 10 g. (fresh 

 weight). Furthermore, Smith's interpretation disregards the \).\^\ absolute assimilation 

 numbers obtained by Willstatter and Stoll for etiolated leaves. 



Smith (1949) also made observations of the rate of dyestuff reduction (Hill reaction) 

 by chloroplasts extracted from etiolated barley seedlings at different stages of greening, 

 and found a systematic increase in activity, similar to that observed for photosynthesis. 



More recent studies of greening, indicating transient formation of a photosyntheti- 

 cally inactive chlorophyll form, will be described in chapter 37B (p. 1767). 



