Heavy Metals and Flowering ■ 115 



ical reactivity. Many compounds of biological importance (for 

 example, amino acids) are chelating agents in addition to their 

 other properties. Especially effective chelating agents, such as 

 ethylenediaminetetraacetic acid (EDTA, "versene"), bring about 

 considerable changes in plant metabolism, probably by affecting 

 processes involving metals. 



When EDTA is added in sufficient quantity to a mineral 

 medium supporting good growth, it profoundly modifies the photo- 

 periodic responses of a clone of Lemna perpusilla and a clone of 

 Lemna gibba. Lemna perpusilla, previously daylength-indifferent, 

 now responds as a typical SDP; Lemna gibba, unable to flower 

 under any photoperiod on the first medium, now flowers rapidly 

 as an LDP in the medium with EDTA. The effects of EDTA on 

 vegetative growth are quite minor and not related to photoperiod. 

 It seems obvious that the major effect of EDTA here is not directly 

 on flowering itself but on flowering through its sensitivity to photo- 

 period, since in Lemna perpusilla EDTA permits a long-day inhibi- 

 tion of flowering whereas in Lemna gibba it permits a long-day 

 promotion. These effects are related to a report by Kandeler (1955) 

 —the first in which the control of flowering in any duckweed was 

 observed— that Lemna gibba flowered under long photoperiods 

 given with fluorescent light only in "aged" medium, in which the 

 plants had grown for some time. It now appears that EDTA substi- 

 tutes for this "aged" medium effect and vice versa. Since, at least in. 

 Lemna perpusilla, chelating agents other than EDTA are effective, 

 the action is not specific to EDTA alone and is probably a conse- 

 quence of chelation (Hillman, 1959a, 1959b, 1961a, 1961b). 



It has recently appeared that in more purified media, these 

 two plants show their photoperiodic responses even in the absence 

 of EDTA. Under these conditions, very low levels of cupric or 

 mercuric ions promote Lemna perpusilla flowering in long days, 

 have no effect in short days, and inhibit Lemna gibba flowering 

 in long days. Thus these ions, by the reasoning above, appear to be 

 relatively specific inhibitors of the response to long days; the action 

 of the chelating agents observed earlier probably represents preven- 

 tion of the effects of contaminants (undoubtedly copper) in the 

 medium. Such results may provide new tools for the analysis of 

 photoperiodism; however, much further work will be required to 

 explore such a complex and sensitive experimental system (Hillman, 

 1961c). 



