LIGHT AND PLANTS 



A Series of Experiments Demonstrating 



Light Effects on Seed Germination, 



Plant Growth, and Plant Development 



By R. J. Downs, formerly plant physiologist, and H. A. Borthwicjk and A. A. Piringer, plant physiologists, 



Crops Research Division, Agricultural Research Service 



INTRODUCTION 



Each year scientists in the U. S. Department of 

 Agriculture receive many inquiries from students, 

 teachers, and other interested biologists for details 

 of simple but dramatic experiments to demonstrate 

 the formative effects of light on plants (photomor- 

 phogenic effects). To answer these requests for spe- 

 cialized information, detailed and systematic ex- 

 periments and demonstrations on effects of light on 

 seed germination, growth, flowering, and fruiting are 

 outlined herein. References accompanying each ex- 

 periment provide supplementary reading and addi- 

 tional details. Certain references will not be readily 

 available to all interested persons, but, in general, 

 the cited papers can be obtained from college and 

 other school libraries of most metropolitan areas, as 

 well as the personal libraries of local plant scientists. 



LIGHT AND SEED GERMINATION 



Seeds of many kinds of plants germinate poorly or 

 not at all when planted and covered with soil. In 

 many instances these are seeds that require light for 

 germination. Some seeds, such as those of pepper- 

 grass (Lepidium virginicum) , do not germinate at 

 all in darkness. Others, such as seeds of Grand Rapids 

 lettuce (Lactuca sativa), often germinate as much as 

 30 percent in darkness, and some lots of lettuce even 

 higher. All the seeds of both peppergrass and Grand 

 Rapids lettuce germinate following a single brief ex- 

 posure to light. A single exposure to light, neverthe- 

 less, is not adequate to promote germination of all 

 kinds of light-sensitive seeds. Seeds of the Empress- 

 tree (Paulownia tomentosa), for example, require one 

 or more periods of light each day for several days. 



Germination of still other kinds of seeds, such as 

 those of henbit {Lamium amplexicaule), appears to 

 be inhibited by light. 



A favorable temperature is one of the require- 

 ments for germination and often a change or alterna- 



tion of temperatures is more effective than a constant 

 one in obtaining maximum germination. For example, 

 only about 30 percent of peppergrass seeds, imbibed 

 in water and placed on blotters in petri dishes at a 

 constant temperature of 70° F., may germinate in the 

 light. If the temperature is alternated, more & 

 germinate. If the seeds are imbibed in a solution 

 containing 0. 02-percent potassium nitrate KNO 

 and the temperature alternated, maximum germina- 

 tion is attained. Such an alternation of temper:: | 

 might be 77° F. for 8 hours per day and 60° for 16 

 hours per day. 



Germination studies are often made on blotters in 

 petri dishes in order to facilitate handling, planting, 

 and counting the seeds. If petri dishes are not avail- 

 able, plastic sandwich boxes with lids and filter paper 

 or paper towels are good substitutes. Studies of the 

 effect of light on germination of seeds imply that 

 some of the seeds must be kept in the dark to act as a 

 check or control. The term "dark" means "total dark- 

 ness," a complete absence of light. (See p. 2.) Bags 

 of at least two layers of black sateen cloth provide 

 the darkness required for dark controls. These bags 

 must be large enough to contain the dishes, with 

 enough slack at the opening so that a flap may be 

 folded back to prevent entrance of light. An alter- 

 nate method would be to cover the dishes with two 

 or more layers ^( aluminum foil. 



Studies on the effect of light on various plant 

 responses can he made in greater detail, using red and 

 far-red' radiant energy. These wavelengths are the 

 most effective ones for regulating many plant 

 responses to light and the> can be obtained by using 

 colored filters in conjunction with the proper light 

 source. The fluorescent lamp emits considerable red 

 but almost no far-red and is, therefore, used as a 

 source of red light. A filter o\ two layers oi red cello- 

 phane removes all visible light except red and since 



In Europe tins would 



- near infrared. 



