PHOTOPERIODISM, CHELATING AGENTS, AND 



FLOWERING OF LEMNA PERPUSILLA AND 



L. GIBBA IN ASEPTIC CULTURE 



William S. Hillman 



Yale University, 

 New Haven, Connecticut 



The photoperiodic control of flowering has been known for ahiiost 

 40 years, but its mechanism remains obscure. Recent in-vitro and 

 in-vivo work on the red, far-red reversible pigment (1) should help in 

 understanding the role of light, but little is known about how this 

 is linked to flower initiation. A possible reason for this, other than 

 the obvious complexity of the problem, is that students of flowering 

 physiology and photoperiodism have dealt with singularly unwieldy 

 experimental systems. The size and slow growth of most higher plants 

 severely limit the number and size of experiments possible in a given 

 space and time; while plants such as Xanthiiim have been used for 

 all the fundamental work so far, and have obvious advantages for 

 the study of hormone movements, it is difficult to keep them under 

 precisely controlled conditions. The use of duckweeds (Lemnaceae) 

 as experimental organisms avoids many of these difficulties and pro- 

 vides almost unique opportunities for specifying the physical and 

 chemical environment. Unfortunately, Saeger's (14) suggestion to 

 this effect, made over 30 years ago, has been largely ignored; it is 

 only within the past five or six years that photoperiodism has been 

 demonstrated in at least two species of Lemna. The scanty work to 

 date, however, suggests a useful new approach to photoperiodism, 

 since it appears to indicate a close relationship between this process 

 and metal — probably trace-metal — nutrition or metabolism. This 

 paper provides a brief review of earlier work by others and by the 

 writer, a report on recent work to be published in greater detail else- 

 where, and a discussion of its implications for future research in this 

 field. 



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