334 GROWTH FACTORS AND FLOWERING 



auxin on photoinduction in Xanthium is very much greater if the Hght 

 period preceding the dark period is short. They also observed (in 

 Biloxi soybean) a simple, linear relation between the logarithm of 

 auxin concentration and the effect (reduction of the flowering re- 

 sponse) and assume that auxin reacts directly with some high-intensity- 

 light product and renders it unavailable for the subsequent reactions 

 of photoinduction. If it can be assumed that the first and the second 

 high-intensity-light reactions are identical, and if it is furthermore 

 assumed that the product of these reactions is necessary for the stabil- 

 ization of the floral stimulus, it becomes possible to interpret auxin 

 inhibition of photoinduction in short-day plants in a unified and 

 specific manner. 



Question 4. Does their native or endogenous auxin — or, as we can 

 now say more specifically, the native auxin in their leaves — play a 

 part in photoinduction of short-day plants? Several studies suggest 

 that the efficiency of photoinduction can be increased by measures 

 which result in a decrease of the endogenous auxin level in the plant. 

 Bonner (1949) showed that application of antiauxin caused a weak 

 flowering response in Xanthium plants which were kept just below 

 the threshold of flower formation (by irradiation with weak red light 

 during the night periods of inductive cycles). Khudairi and Hamner 

 (1954a) obtained a similar effect by applying ethylene chlorohydrin, 

 a substance that seems to lower the auxin content of plant tissues. 

 Lona and Bocchi (1955) found that Perilla plants which had been 

 treated with eosin required fewer photoinductive cycles for flower 

 formation than untreated plants, and it may be assumed that eosin 

 lowers the auxin content of the plants because of photosensitized auxin 

 destruction. All these results, even though they can hardly be con- 

 sidered as definite, still make it likely that a relatively high auxin 

 level in a short-day plant reduces the effectiveness of photoinduction. 



Question 5. What, exactly, is the role of the native auxin in photo- 

 induction? Two possibilities can be envisaged: either that auxin is 

 directly involved in photoinduction, that it is, in other words, an 

 integral part of the inductive mechanism itself; or that it modifies the 

 operation of this mechanism, without being an actual part of the 

 latter. If the first alternative were true, one might expect that the 



