PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



225 



These responses are certainly unusual, since one 

 can almost always find some quantitative response 

 to a light exposure. Even in these cases, careful 

 studies might reveal seeds that began to germinate 

 with threshold exposure but then stopped, or 

 anomalous rhythms caused by threshold ex- 

 posures. 



3. Triggered, amplified, quantitative responses. 

 — These are much more common. Light triggers 

 the response, but exposure determines the level of 

 response. The energy required for the response is 

 usually much greater than the energy of the light 

 absorbed, so the light rechannels metabolic en- 

 ergy, rather than providing energy as it does in 

 photosynthesis. Examples are etiolation, photo- 

 tropism, phototaxis. opening and closing of leaf- 

 lets, pigment formation, plant form, and devel- 

 opment of sun or shade leaves. 



4. Triggered, amplified, quantitative, time-re- 

 lated responses. — In these cases, not only exposure 

 is important, but the time that the exposure is 

 given. In photoperiodism, an interruption of the 

 dark period inhibits flowering in short-day plants, 

 and the extent of inhibition is strongly a function 

 of the exact time when the light interruption is 

 given. Some of the responses known to be con- 

 trolled by photoperiodism are germination (a few 

 species), plant form, flower formation, and bud 

 dormancy. Entrainment of circadian rhythms is 

 also strongly time-dependent. 



considered the possibility that auxin was destroy- 

 ed on the light side, but recent research has con- 

 firmed Went's original measurements, indicating 

 that as much auxin exists in plants exposed to 

 light as in plants kept in the dark, but there is 

 a redistribution to the dark side (8). How does 

 this happen? The actual mechanism remains al- 

 most a complete mystery. Figure 4 compares an 

 excellent action spectrum for phototropism with 

 the absorption spectra of carotene and riboflavin. 

 The match is very good for riboflavin in the 

 ultraviolet part of the spectrum, and perhaps a 

 little better for carotene in the visible part of 

 spectrum. Rather recently, mutants were found 

 with 80 percent less carotenoid than normal 

 plants (1). These respond phototropically as well 

 as their normal counterparts. Nevertheless, it has 

 not yet been possible to completely eliminate caro- 

 tenoids as the photoreceptor pigments in photo- 

 tropism. 



The picture is complicated by the fact that cur- 

 vature does not always increase with increasing 

 light exposure (fig. 5). Above a certain exposure 

 level, coleoptiles bend less and finally bend in the 

 opposite direction, giving a negative curvature. 

 With increasing exposure, this is followed by a 

 second positive curvature. 



Figure 5 assumes that the law of reciprocity 

 holds. Long exposures at low intensities should be 



Phototropism 



Virtually all of the processes mentioned so far 

 have been investigated in considerable detail. To 

 provide examples, we shall introduce phototrop- 

 ism and flower initiation. Plants in the field are 

 responding to light in all the ways mentioned 

 above, but for most of these this is of little con- 

 sequence to the range manager. Phototropism is 

 a good example, but flower initiation in response 

 to photoperiod is important in several ways. 



A coleoptile (or true stem of either monocots 

 or dicots — although less work has been done with 

 stems) bending toward a light source does so 

 due to differential cell elongation rates : the cells 

 on the dark side elongate most rapidly. Darwin 

 found that the cleoptile tip responds to the light, 

 although bending occurs well below. Went later 

 showed that there is a lateral transport of auxin 

 toward the dark side. For a while, investigators 



100%1 



m • . 



5 ' 



340 360 380 400 420 440 

 m ultraviolet ^~|^hk violet a^aa 



460 480 



\mm blue — 



500 520 540 

 ■»|""""» green ■■■ 



WAVELENGTH mm) 



Figure 4. — Action spectrum for phototropism and absorp- 

 tion spectra of riboflavin and carotene (50). 



