684 LIGHT AND LIFE 



and the latter from a plant unable to flower to a long-day plant. 

 The action of EDTA in either case is decreased by lowering the pU, 

 at least in the range 3.6-5.0. With L. perpiisilla, the EDTA effect 

 is imitated by a considerable number of other chelating agents; with 

 L. gibba, by medium which has become "old" from supporting the 

 growth of plants in long day. Whether the possible reciprocal rela- 

 tions hold is unknown at present. 



If only one or the other of the EDTA effects were known, the 

 simplest conclusion would be that the compound merely inhibited 

 or promoted flowering under the specified conditions, with no neces- 

 sary relation to photoperiodism. Since EDTA can also modify vegeta- 

 tive growth, particularly in L. gibba, where it increases both root 

 length and the gibbous character of the fronds, the effect might 

 appear to be unspecific. Taking both systems together, however, it 

 is difficult to avoid the conclusion that the photoperiodic mechanism 

 itself must be intimately affected; what the effects on both species 

 have in common is not a promotion or inhibition of flowering but 

 the potentiation of a response to long days. 



The question of how EDTA might affect the photoperiodic mechan- 

 ism cannot be answered as yet. The results with L. perpiisilla indi- 

 cate an effect through metal-chelating action, and the pH-activity 

 relationships for both species (Figs. 1 and 2) are in accord with this 

 conclusion (2) . Nevertheless, there are further problems of inter- 

 pretation. The pH-activity relationship might suggest that EDTA 

 acts in the medium itself, not within the plant; attempts to reverse 

 the effect with increased levels of various metals, however, fail con- 

 sistently, at least in L. perpiisilla. This could indicate that it acts 

 in the medium by increasing, rather than decreasing, the availability 

 of one or more metals, since both are well-known effects of chelating 

 agents (3) . Further observations on L. perpiisilla do not support 

 this view. In a medium completely devoid of chelating agents (e.g., 

 M omitting tartrate) growth is poor, and iron-deficient in appearance. 

 If tartrate or EDTA are used to restore iron availability and normal 

 growth, the former is active at about 10-' M, but the latter is required 

 at at least 10— "' M for the same effect. Yet EDTA is far more active 

 than tartrate in affecting the photoperiodic behavior (8) , so the effec- 

 tiveness of the two agents in restoring iron availability does not 

 parallel their activity on daylength requirements for flowering. 



It is well known that EDTA and probably other chelators arc 

 taken up by plant tissues and have profound effects on internal metal 

 relationships and also, perhaps indirectly, on enzyme activities of vari- 



