• Chlorophyll formation requires light and the 

 light must be of higher intensity than that which 

 controls stem length. 



• The red, far-red reversible; photoreaction that 

 controls seed germination also controls stem 

 length and leaf size. 



Additional experiments can be designed to answer 

 many other questions relating to the manner by 

 which light controls plant growth. Examples of such 

 questions are as follows : Are bending (phototropism) 

 and growth of internodes controlled by the same 

 photoreaction? This question can be answered by 

 using different regions of the spectrum (colors of 

 light) and testing to see if bending and growth are 

 controlled by the same colors. 



Does the duration of darkness following the far- 

 red irradiation of light-grown plants affect the ulti- 

 mate length of the internodes? What is the optimum 

 period of darkness and why is it optimum? 



How concentrated is the pigment that controls 

 growth? How do we know it is not chlorophyll? 

 These questions can be answered by comparing the 

 growth responses of albino and green corn or barley 

 seedlings. 



LIGHT AND PLANT PIGMENTS 



The autumn coloration of leaves and stems of 

 woody plants is in part caused by the formation of a 

 red pigment called anthocyanin. The formation of 

 anthocyanin is also responsible for the red color of 

 apple fruits and for the red to purple color of milo, 

 turnip, and cabbage seedlings. 



A common observation is that apples often do not 

 turn red uniformly but that one side of the fruit is 

 green or at least a lighter shade of red than the other 

 side. The reddest side of the apple is usually facing 

 outward from the tree. The formation of the red 

 color (anthocyanin) in apple fruits is controlled by 

 light. Detailed studies have shown that anthocyanin 

 formation in milo, turnip, and cabbage seedlings and 

 in leaves of red maple and other trees is also regulated 

 by light. 



Unlike many other light-controlled plant re- 

 sponses, anthocyanin formation requires high-inten- 

 sity light for a relatively long time. However, at the 

 close of the high-intensity light period the low-inten- 

 sity-red, far-red photoreaction may exert final control 

 on anthocyanin synthesis. Thus, if the plant material 

 is irradiated for a few minutes with far-red at the 

 close of the high-intensity light period, the potential 

 anthocyanin synthesis is inhibited and very little is 

 formed. If a brief irradiation with red follows the 



far-red, then anthocyanin is formed m an amount 



equal to that produced by the hijrh-. 



alone. 



An example of a low-intensity light-controlled 

 coloration is the yellow color of the skin of the tomato 

 fruit. Plant breeders recognize differences; in the color 

 of the skins of fruits of certain tomato variel 

 have classified the skins as yellow or clear. The red 

 flesh and a transparent or white skin give the fruit a 

 translucent pink color, whereas the red flesh and a 

 yellow skin give the fruit an orange-red appearance. 

 In many tomato varieties the formation of this yellow 

 pigment is controlled by light. Moreover, the same 

 reversible red, far-red photoreaction that controls 

 flowering of photoperiodically sensitive plants, 

 germination of light-sensitive seeds, and many other 

 plant responses also controls the formation of the 

 yellow pigment in the skins of tomato fruit. 



Demonstrations C-l through C-4 concern light 

 and its control of plant coloration. From these 

 demonstrations we know: 



• That light is required for the formation of the 

 red color (anthocyanin) of certain seedlings and 

 apple fruits. 



• Light is required for the formation of a yellow 

 pigment in the skin of tomato fruit. 



• The coloration occurs only in the areas that re- 

 ceived light — there is no translocation of the 

 stimulus. 



Additional experiments can be designed to learn 

 more about the light reaction and about the chemical 

 processes that result in pigment formation. Questions 

 that one might ask are: How much energy is required 

 to induce the formation of anthocyanin.' As light 

 energy is increased, does the amount of anthocyanin 

 increase proportionately? Once the light require- 

 ment is fulfilled, what is the rate of anthocyanin 

 formation? What is the role of temperature.' What is 

 the role of sugar? Does the red. far-red reversible 

 photoreaction operate in the control of coloration' 



EFFECT OF DURATION OF LIGHT 

 ON PLANTS 



Flowering of many kinds of plants is controlled by 

 the relative length of the daily light and dark periods. 

 This phenomenon is called pbotoperiodism. Some 

 plants, such as certain varieties oi chrysanthemum. 

 poinsettia. morning-glory, cocklebur, and lambs- 

 quarters, are short-day plants and flower in nature 

 only when the days are short and the nights are long. 

 Certain varieties of spinach, beet, barley, and tuber- 





