Heavy Metals and Flowering • 113 



either nitrogen or phosphate to the lowest level used delayed 

 flowering to node 12 or 13 at the earliest. The authors proposed 

 the interesting generalization that flowering in many tropical 

 daylength-indifferent plants is far more dependent upon nutrition 

 than it is in photoperiodic or vernalizable plants in which the 

 environmental requirements have been satisfied. In this connection, 

 note that Gott et al. (1955) found that a low nitrogen level delayed 

 flowering in unvernalized or partially vernalized winter rye but 

 hardly affected vernalized plants. 



Although the literature on nutrition and flowering is more 

 extensive than that presented here, these examples serve to indicate 

 that, at least at present, there is no good evidence for a close rela- 

 tionship between a particular major element and flower initiation 

 in most plants. 



HEAVY METALS AND FLOWERING 



There is some indication that iron nutrition may be more 

 critically involved in photoperiodic induction. In a preliminary 

 survey to see whether any of a large number of different mineral 

 deficiencies would reduce the capacity of Xanthium to respond to 

 short-day treatment, Smith et al. (1957) noted that iron, and possi- 

 bly boron and magnesium deficiencies, had some effect. In further 

 experiments they found that plants suffering from iron-deficiency 

 symptoms failed to flower or flowered abnormally even when trans- 

 ferred to a high-iron medium after photoinduction. Such results 

 are suggestive, although the inhibition of vegetative growth as well 

 as the response to short-day leave them somewhat equivocal. Any 

 special significance for iron in flower initiation has been questioned 

 by Shibata (1959) in a brief investigation on rice (Oryza sativa). 



A more clear-cut result was obtained by the writer (Hillman, 

 1961a), using a clone of the duckweed Lemna perpusilla growing 

 in a well-chelated medium (see below). The plants were pretreated 

 by growing them in media with various levels of iron for several 

 days, given one (inductive) long night, and then all returned to a 

 high-iron medium. Under these conditions, the flowering response 

 to the single long night was essentially abolished by pretreatment 

 with a level of iron not low enough to affect vegetative growth. 

 In other words, the iron requirement for induction appeared to 



