414 PLANT PHOTOPERIODISM 



as well as auxins, induce flowering in long-day plants under threshold 

 conditions. The reason for this phenomenal behavior is unknown. 

 Nonetheless antiauxin cannot be the actual floral hormone even of 

 short-day plants because antiauxins are effective only if applied to the 

 leaf and are ineffective if applied to the bud. It seems quite clear that 

 the antiauxins must exert their effect on flowering by somehow pro- 

 moting production of flowering hormone in the leaf. 



The production of flowering stimulus during the dark period of a 

 short-day leaf is not inhibited by anti-amino acids such as thionyla- 

 lanine, methionine, and canavanine (Labouriau, 1956). Neither is it 

 inhibited by materials which antidote the synthesis of nucleic acids 

 such as thiouracil (Labouriau, 1956). It would appear, therefore, 

 that the synthesis of flowering hormone does not specifically involve 

 the synthesis of either proteins or nucleic acids or, by implication, 

 nucleoprotein. Synthesis of flowering hormone is inhibited by cyanide, 

 azide, fluoride, and dinitrophenol. The synthesis of the material would 

 therefore appear to be an active process. Interestingly enough it has 

 been shown by Nakayama et al. (1956) that synthesis of flowering 

 hormone, i.e., processes taking place during the second half of a 16-hr 

 dark period, are not inhibited by malonate. It seems probable that 

 further investigations with specific metabolite antagonists might well 

 define the pathways of metabolism involved in the production of 

 flowering hormone. Such investigations should be carried out with 

 clear understanding of the time intervals involved. If we are to inhibit 

 synthesis of flowering hormone and to minimize effects on time 

 measuring, etc., the inhibitors should be applied only during that part 

 of the dark period which exceeds the critical night length. But for the 

 time being it appears that we cannot as yet conclude from inhibitor 

 studies anything positive about the nature of the flowering stimulus. 



The flowering impulse, once formed in the leaf during a dark period 

 of length longer than 8.5 hr, may be again destroyed by (a) high-tem- 

 perature treatment (40°C, 2 hr) (Lockhart and Hamner, 1954) or 

 {b) the presence of applied auxin. The leaf may be pasteurized, as it 

 were, of the floral stimulus which it has accumulated. This process has 

 been characterized in physiological detail by Lockhart (1955). The 

 pasteurization process is not dependent upon the presence of oxygen; 

 it is independent of light; antioxidants added to the leaf do not preserve 



