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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



Figure 2. — Morphological response of tomato to photo- 

 period. Both plants have flowers and young fruit. 



Flower Formation (14, 29, 44, 49) 



In a very few cases, flowers form in response 

 to light intensity (52), but in many cases, flower 

 formation is controlled by photoperiod. Short-day 

 plants flower in response to decreasing day 

 lengths, while long-day plants flower in response 

 to increasing day lengths. Some plants are day- 

 neutral, flowering independently of day length. 



Bud Dormancy (55) 



Many deciduous trees become dormant in the 

 fall in response to shortening days. A few of these 

 resume activity in the spring in response to 

 lengthening days. Some trees also exhibit a period 

 of dormancy in midsummer in response to long 

 days. These responses are invariably strongly 

 modified by temperature. Winter dormancy, for 

 example, can often be induced by exposure to low 

 temperatures, regardless of day length, and it 

 is frequently broken by prolonged exposure to 

 cold. The interaction of light and temperature, 

 though particularly striking in this case, is typ- 

 ical of virtually all photomorphogenetic responses. 



Other Phenomena 



This category is included to emphasize the 

 point that the above categories are only represen- 

 tative and not exhaustive. Numerous examples 

 could be mentioned, such as the production of 

 foliar embryos on Bryophyllum under long days 

 (54-), the replication of chloroplasts, and the con- 

 version of fern protonemata into prothallia. 



Pigments 



The first law of photochemistry states that 

 light, to be effective, must be absorbed. Hence, in 

 any photomorphogenetic investigation, we might 

 attempt to discover the pigment absorbing the 

 light in a given process. The approach is to de- 

 termine an action spectrum, in which response 

 is plotted as a function of wavelength, and then 

 to compare this action spectrum with the absorp- 

 tion spectra of suspected pigments (10). This 

 has been done for many of the processes listed 

 above, sometimes with satisfying and important 

 results. We will now reclassify the above re- 

 sponses according to the pigments known or sus- 

 pected to be in control. 



1. Protochlorophyll.- — The conversion of proto- 

 chlorophyll to chlorophyll is a straightforward 

 photochemical reaction, and the action spectrum 

 for the process closely matches the absorption 

 spectrum of protochlorophyll. 



2. Chlorophyll. — The action spectrum of photo- 

 synthesis also matches, rather closely, the absorp- 

 tion spectrum of chlorophyll, but as anyone 

 knows who has attempted to keep up with the 

 burgeoning literature, the situation is far from 

 straightforward. For example, there is the Emer- 

 son Enhancement Effect, brought about by the 

 interaction of photo-processes I and II and a 

 collection of energy to a reactive center of the 

 photosynthetic unit (3, 21). 



3. Carotenoids or flavins. — These pigments 

 absorb blue light, and phototropism is known to 

 be a response to blue light. Nevertheless, as shown 

 below, this is an unsatisfying example. Although 

 most evidence implicates riboflavin as the absorb- 

 ing pigment, conclusions remain ambiguous. 

 There are also other plant responses to blue light 

 for which pigments have not been determined. 



4. Phytochrome (11, 28, 51).— In 1937. Flint 

 and McAllister (20) determined an action spec- 



