138 PHOTOCONTROL OF GROWTH 



treatment, and further, the path by which it is Hnked, if at all, to the 

 eventual growth reaction. 



Each of these approaches has certain advantages, and each, in the 

 hands of competent investigators, has yielded some useful information 

 about the physiology of photomorphogenesis. For a variety of reasons, 

 some carefully conceived and others entirely accidental or without 

 reason, we have concentrated on the third approach. As it happens, 

 this is the approach that has been least followed in the past, owing un- 

 doubtedly to the fact that very few specific chemical changes can as 

 yet be directly related to the reversible photoreaction. However, theo- 

 retically at least, this approach should be quite rewarding, in that very 

 specific chemical questions can be asked of the organism in relation to 

 photomorphogenesis. 



To the best of our knowledge, only four specific chemical changes 

 have thus far been fairly directly linked with the red-far red photore- 

 ceptor system: 



1. The production of a yellow pigment in the cuticle of Rutgers 

 tomato fruits is controlled by light (Piringer and Heinze, 1954). This 

 pigment, presumably a flavonoid, is synthesized by a fruit given an 

 inductive red light treatment. The red light effect, which has an action 

 spectrum very similar to that for the inhibition of flowering in 

 Xanthium, is negated by subsequently applied far-red light. 



2. Anthocyanin synthesis in cabbage and turnip seedlings and in 

 apple hypodermis is enhanced by preirradiation with red light, this 

 enhancement also being reversible by subsequently applied far red 

 (Siegelman and Hendricks, 1956, 1957; Mohr, 1957). 



3. The lag phase in chlorophyll synthesis in dark-grown bean leaves 

 is eliminated by pretreatment with red light, the effect again being 

 reversible by far red (Withrow et ah, 1956). The same photoreaction 

 apparently also controls protochlorophyll synthesis (Wolff et al., 

 1957). 



4. The activity of the enzyme indoleacetic acid oxidase found in 

 etiolated pea buds is sharply decreased several hours after an exposure 

 of the plant to low irradiances of red light (Hillman and Galston, 

 1957). This effect, which is also readily reversible by subsequently 

 applied far-red irradiation, is due to an increased level in the bud of a 

 substance which inhibits the enzyme (Hillman and Galston, 1957^ 



