often serve as enzymes - -transformers of cell tissue that activate life 

 processes without themselves being used up as nutrients or cell-building 

 materials. 



For a substance so sensitive to light differences, phytochrome is 

 remarkably durable and resistant to cold, although falling temperature 

 retards its reversibility. Even at -70° C., phytochrome has retained a 

 tenth of its reversible effectiveness. Cold as extrenne as -180° C. has 

 put it out of commission. However, warnaing restores full reversibility. 

 Phytochrome has remained reversible after 9 months' storage at -15° C. 

 (5° F.) 



Fitting Phytochrome to Photoperiods 



Ever since phytochrome has been separated from plants, one question 

 of crucial importance has been: Does this pigment inside living plants 

 shift its form at night at a rate in keeping with all that has been learned 

 in 40 years about plants' dark photoperiod timing? 



The light beam instruments alone did not answer this. They demon- 

 strate a rapid reversibility of phytochrome, in living plants or as an 

 extract, under successive irradiation with red and far-red light--but not 

 the natural rate of change toward inactive form in darkness. 



As a way of measuring this rate of change in darkness, the photo- 

 periodic team used a light beam instrument to check the state of phyto- 

 chrome in corn seedlings and other living plant tissues at different stages 

 of each dark period. After Z, 4, and 6 hours of a dark period, living plant 

 materials were put under far-red light to learn how much concentration 

 of the inactive form the phytochronne would show. 



This check test has tied phytochrome, as it is known today, firmly 

 to the photoperiodic law of nature discovered by Garner and Allard. 

 The instrument recordings show phytochrome shifting to inactive form 

 at orderly rates during dark photoperiods - -confirming that the blue 

 pigment accounts for the clocking of darkness in fields, forests, and 

 gardens . 



A shift from active to predominantly inactive form in darkness takes 

 on the average about 12 hours. But the ratio differs with plant species, 

 and slows when temperature falls. 



Concentration of the pigment changes faster at the onset of darkness 

 than during the later hours of the night. After only 2 hours of darkness, 

 a corn seedling has reached the half-point of total change. The corn 

 plant's rate is fairly typical, judging by tests on many plants. 



In nature itself, seedling tree experiments show impressively how 

 the reversibility of phytochrome works photoperiodically , and how it can 

 be managed. For example, a red maple springing from seed grows scarcely 

 more than half a foot its first year, in New England where long nights 

 prevail. In one experiment, a red maple grown from seed stood half a 

 foot tall at 1 months when given this normal light ration. But a companion 

 red maple grew to 8-foot height from seed in the same 10 months, and 

 kept leaf production in pace with stenn growth, because its schedule was 

 a continuous "sumnner" of long days and short nights. The fast-growing 

 tree's light allowance was adjusted so that the total energy, spread over 



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