INFLUENCE OF GIBBERELLIN AND AUXIN 335 



auxin level in the leaf would change in the course of an inductive cycle, 

 particularly in the course of its dark period. If the second alternative 

 were right, no such change need be expected, but there should be a 

 relation between the auxin content of the leaf and its effectiveness in 

 photoinduction. 



A number of authors have analyzed the auxin content of induced 

 and noninduced short-day plants. The results were rather heter- 

 ogeneous. Von Witsch (1941) reported that plants in short day had 

 less auxin than plants in long day. Cooke (1954) found that, after 

 transfer to short days, auxin content first increases and later decreases. 

 Vlitos and Meudt (1954) reported that induced soybean plants con- 

 tain about 100 times more auxin than noninduced ones; other short- 

 day plants did not exhibit similar differences, but induced individuals 

 seemed to contain more indolepyruvic acid than vegetative ones. 

 Except for that of Vlitos and Meudt, who used paper chromatographic 

 separation, most of this work is open to question, since the possibility 

 of the presence of inhibitory materials does not seem to have received 

 due attention. But the main shortcoming of all this work is that the 

 analyses were done either at certain intervals during the course of the 

 inductive treatment, or — more frequently — long after photoinduction 

 had been consummated. However, as stated before, if auxin is a part 

 of the inductive mechanism, the crucial fluctuations should occur in 

 the course of the inductive dark period. Therefore, the available 

 results of the analytical approach must be termed inconclusive. 



The fluctuations in the auxin content of leaves of different age, on 

 the other hand, argue against a direct role of auxin in photoinduction. 

 No measurements of the age changes of auxin content seem to have 

 been made in the leaves of any of our photoperiodic war horses, such 

 as Xanthium or Biloxi soybean. But if one may judge from such deter- 

 minations in other plants, it appears that leaves with a high auxin 

 content may be highly sensitive to photoinduction. The auxin content 

 of a leaf is at a peak when the leaf undergoes most rapid expansion; 

 it drops off sharply as the leaf reaches full size, and then may stay at 

 a fairly uniform level until the onset of senescence (Goodwin, 1937; 

 Jacobs, 1952; Shoji et al, 1951). In Xanthium, however, highest 

 photoinduction sensitivity is reached in a leaf which has attained about 



