224 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



contrasted to the low intensities for brief periods 

 effective in phytochrome responses. Blue and 

 far-red wavelengths were most effective, and no 

 sign of photoreversability could be detected. The 

 control system, whatever it is, was referred to as 

 the High Energy Reaction (HEE). 



For several years, it was thought that the HER 

 must be controlled by a separate pigment system, 

 but many workers now feel that it is a special 

 manifestation of the phytochrome system. 

 There are three principle lines of reasoning: 

 First, absorption spectra of the two forms of 

 phvtochrome overlap (fig. 3), so when a plant is 

 illuminated with far-red light, although most of 

 the phytochrome is immediately converted to the 

 red-absorbing form (Pr). some of this pigment 

 absorbs far-red light (although much less effici- 

 ently), being converted in the process back to the 

 far-red-absorbing form (Pfr). Pfr then absorbs 

 far-red light and is converted back to Pr. but 

 Pfr is continually being supplied. The end result 

 is that when plants are illuminated with far-red 

 light, most of the pigment exists as Pr, but some 

 (on the order of 10 percent, depending upon the 

 exact wavelength of far-red light) always exists 

 as Pfr. Most physiological studies have indicated 

 that Pfr is biologically active. If a brief ex- 

 posure to far-red is given, the small percentage 

 of Pfr will be metabolically destroyed in a fairly 

 short time, but if long exposures to far-red are 

 given, the small quantity of Pfr is maintained 

 over a long interval of time. It can be as effective 

 as a large quantity present for only a short time. 



Second, the various forms of phytochrome are 

 undergoing continual metabolic synthesis and de- 

 struction, this itself being influenced to varying 

 degrees by the high-intensity light environment. 

 Third, it is now known that Pr and Pfr have a 

 number of intermediate forms. Some of these may 

 exhibit photomorphogenetic activity. 



Typically, the responses controllable by the 

 HER are similar or identical to those controllable 1 

 by the phytochrome system, which is strong 

 evidence that the HER is really a special case 

 of phytochrome action. Examples include ger- 

 mination of some seeds, etiolation effects, forma- 

 tion of anthocyanins and carotenoids, and per- 

 haps effects upon plant form (including differ- 

 ences between sun and shade leaves) and flower 

 formation. Much study on the HER has been 



carried out by Hans Mohr and his colleagues at 

 the University of Freiburg (36, 37). They have 

 documented numerous interesting examples, in- 

 cluding many changes in metabolites. In one 

 case, the plumular hook on a lettuce seedling may 

 be caused to form by exposure to low intensities 

 of red light (phytochrome system) and caused 

 to straighten by long exposures to high-intensity 

 far-red light (the HER). These responses are 

 especially important to plants in nature (for ex- 

 ample, range plants), since they are typically 

 exposed to high-intensity illumination for long 

 durations, but seldom to the conditions typical of 

 phytochrome experiments. 



6. Unknown pigments. — We emphasized the 

 difficulty of determining specific pigments in 

 phototropism. It is easy to find many other am- 

 biguous examples. "We are not certain of the pig- 

 ment system involved in the HER, although phy- 

 tochrome provides the most fashionable current 

 explanation. Entrainment of circadian rhythms is 

 influenced by red and far-red light, but in some 

 experiments, other wavelengths were equally ef- 

 fective. Probably DXA absorbs the ultraviolet 

 light damaging to leaves, but protein may also 

 absorb ultraviolet, and the photo-reactivation 

 brought about by blue light is quite ambiguous. 

 Apparently, in bacteria an enzyme is directly ac- 

 tivated by the blue light, but this remains to be 

 studied in higher plants. 



Response Mechanisms 



Once the pigments have been identified, then 

 we are interested in how they act. What are the 

 further mechanisms of a photomorphogenetic re- 

 sponse? Most of this remains to be learned, but 

 we can classify the responses into at least four 

 broad categories : 



1. Direct energy transformations. — Protochlo- 

 rophyll conversion, photosynthesis, light damage, 

 and probably photoreactivation provide examples. 

 The illuminating energy drives the photochem- 

 ical process directly. 



2. Triggered responses : On-off. — In a few cases, 

 exposure to a suitable quantity of light triggers 

 a response, which is then of an all-or-none char- 

 acter. Usually we think of a seed as either germ- 

 inating or not germinating, so this may provide a 

 good example. Circadian rhythms are probably 

 either bein<r exhibited or not being exhibited. 



