Paper No. 20 

 EFFECTS OF LIGHT AND PHOTOPERIOD ON MORPHOGENESIS 



By Frank B. Salisbury * 



Abstract 



Photomorphogenesis in plants includes virtual- 

 ly all aspects of plant development : germination, 

 etiolation, growth via photosynthesis, phototrop- 

 ism, phototaxis of chloroplasts, opening and clos- 

 ing of leaflets, entrainment of circadian rhythms, 

 pigment formation, plant form, light damage (in- 

 cluding ultraviolet), flower formation, bud dor- 

 mancy, and many other responses. Action spectra 

 implicate protochlorophyll, chlorophyll, caro- 

 tenoids or flavins, phytochrome, and unknown 

 pigments. The High Energy Reaction (HER) 

 may be phytochrome-mediated and is of interest 

 to those dealing with natural ecosystems. Photo- 

 morphogenetic responses may be direct photocon- 

 versions (photosynthesis), triggered (germina- 

 tion), triggered amplified quantitative (most 

 phytochrome responses), or triggered amplified 

 quantitative time-related (primarily photoperiod- 

 ism). Phototropism involves a redistribution of 

 auxin, but the mechanism is not yet understood. 

 Complications include failure of reciprocity for 

 negative and second positive curvatures, and ef- 

 fects of red light on subsequent sensitivity to blue 

 light. Photoperiodism is a widespread phenom- 

 enon in plants (and animals), including both veg- 

 etative and reproductive responses. There are 

 numerous flowering response types and inter- 

 actions with temperature. The night interruption 

 phenomenon is a phytochrome response, but time 

 measurement (the central feature of photoperiod- 

 ism) cannot be explained in terms of phyto- 

 chrome conversion. Although characteristics of 

 the photoperiodism clock are similar to those of 

 the circadian clock, current evidence indicates 

 that the two clocks are not identical. 



Additional key roords: Photomorphogenesis. 



1 Professor of Plant Physiology, Plant Science Depart- 

 ment, Utah State University, Logan, Utah 84321. 



phototropism, phytochrome, high energy reaction, 

 biological clocks, circadian rhythms. 



Introduction 



A plant growing on the range, be it grass or 

 forb, is the product of the morphogenetic or de- 

 velopmental events that produced it. Virtually 

 all of these are sensitive to light. Photomorpho- 

 genesis is a highly complex process involving nu- 

 merous integrated events, each of which may be 

 influenced or even dependent upon light. Hence, 

 the initial task of cataloging morphogenetic re- 

 sponses to light is a formidable one. I have tried 

 to organize the principal responses into 14 cate- 

 gories, beginning with germination and going to 

 reproduction and dormancy. Some are broad 

 while others are more specific. It is not possible in 

 the limited space to review the entire topic of 

 photomorphogenesis and still reach the depth of 

 specific research papers, but the subject has been 

 broadly reviewed in several books (2. 9, 14, 22. 28. 

 30, 35. 36. ^2. U. 48, 53. 54). and numerous re- 

 views have appeared in the Annual Review of 

 Plant Physiology (see References at end of this 

 paper). A few technical papers are cited in re- 

 lation to some topics, although most citations are 

 to review articles and books. 



Photomorphojjenetic Phenomena 



Germination Of Many Seeds And Spores (33) 



It has long been known that germination of 

 many seeds, fern spores, and so forth, is pro- 

 moted, or inhibited, or not influenced by light. 

 Study of seeds promoted by red light (some are 

 promoted by blue) led to discovery of the phyto- 

 chrome pigment system (see below). Some seeds 

 are influenced in their germination by photo- 

 period rather than by exposure to a given quality 

 and quantity of light (13). There are long-day 



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