PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



221 



bending toward the light and roots of many 

 species away from it. Leaves may be oriented so 

 that their surfaces are normal to the incoming 

 light rays, or in a few cases (for example, the 

 compass plant, SilpMum sp.), leaf surfaces are 

 mostly parallel to the sun's rays at noon. Flowers 

 may also be oriented in relation to light. 



Phototaxis Of Chloroplasts 



Chloroplasts tend to lie along cell walls that 

 are parallel to the sun's rays when intensities are 

 high, and along walls normal to the sun's rays 

 when intensities are low. 



Opening And Closing Of Leaflets (22) 



Leaflets of many species, particularly Albiz- 

 zia and Mimosa, fold up when plants are trans- 

 ferred to the dark — depending upon the quality 

 of light preceding the darkness. The phytochromc 

 system is again involved. 



Entrainment Of Circadian Rhythms (2, 9, 15) 



Many plants (and animals) exhibit circadian 

 rhythms, in which functions vary with a peri- 

 odicity approximating 24 hours. These include 

 growth rates, petal opening and closing, concen- 

 trations of various substances such as starch, and 

 leaf positions. The sleep movements of leaves 

 have been most intensively studied. Typically, 

 leaves are in a vertical position dm'ing the night 

 and a horizontal position during the day. When 

 plants are placed under constant conditions of 

 temperature or light (total darkness or light of 

 some constant intensity), the sleep movements 

 continue. Under normal conditions, the move- 

 ments are entrained to the daily cycle of light and 

 darkness, but under constant conditions the 

 rhythms become free - running, having periods 

 typically shorter or longer than 24 hours. Entrain- 

 ment is a response to the change from darkness to 

 light (dawn) or light to darkness (dusk), so it is 

 another example of photomorphogenesis. Under 

 constant conditions, the rhythms can be phase- 

 shifted by light perturbations. 



Pigment Formation (27, 30) 



In addition to the synthesis of chlorophyll re- 

 ferred to above, many other pigments depend 



upon the presence of light for their synthesis. 

 Good examples are the anthocyanins of apple or 

 turnip skins and the carotenoids of tomatoes. A 

 high-energy phase, depending typically upon blue 

 light, is often followed by a low-energy phyto- 

 chromc phase dependent upon red light. 



Plant Form, Branching, And Elongation Of 

 Stems (31, 32) 



Plant form is often strongly influenced by light 

 quality, intensity, or both. Cocklebur plants, for 

 example, have a single unbranched stem when 

 grown in the greenhouse but branch profusely in 

 the field. Plants grown under fluorescent light are 

 typically shorter and more branched than those 

 grown under incandescent light (rich in far-red) 

 or under a mixture of fluorescent and incadescent 

 light. There is an interesting interaction between 

 intensity and light quality. At low intensities, red 

 light is more effective in inhibition of stem elon- 

 gation than blue light, but at high intensities. 

 blue light is more effective (31, 54). Plant form 

 is also strongly influenced by photoperiod (44. 

 48). Strawberries on long days, for example, form 

 numerous runners but do not form these on short 

 days, flowering profusely instead. Even a tomato 

 plant, day-neutral in its flowering response, is 

 strongly influenced in its form by day length, 

 as shown in figure 2. 



Sun And Shade Leaves 



Leaves developing under high light intensities 

 frequently have more than one layer of palisade 

 tissue, and their photosynthetic process is satu- 

 rated at higher light intensities than leaves de- 

 veloping in the shade (4- 18). (This response, 

 could easily be a subcategory under Plant Form.) 



Damage Due To High-Intensity Light 



It is known that chlorophylls tend to bleach 

 out when light intensities reach very high levels 

 (for example. 12.000 foot-candles), a process 

 known as photooxidation. Ultraviolet light will 

 also damage plants, typically causing a bronzing 

 of surfaces or death with sufficient exposure 

 (48). The damage caused by ultraviolet light 

 maybe prevented if plants are subsequently ex- 

 posed to high intensities of blue light, a process 

 known as photoreactivation or photoreversal. 



