108 • Chemical Control of Flowering 



Five, one variety flowers in response to geotropic stimulation, an 

 effect that has been ascribed to a change in native auxin distribu- 

 tion. In addition, a number of varieties can be made to flower by 

 applications of synthetic auxins such as naphthaleneacetic acid 

 (NAA) or 2,4-D. IAA appears to be a native auxin in pineapple, 

 and, paradoxically, it has been suggested that NAA may act in this 

 situation as an auxin antagonist— an antiauxin, in fact, competing 

 with the native IAA— and that flowering may result from a lowering 

 of the effective level of the native auxin (Bonner and Liverman, 

 1953; Gowing, 1956). Whatever the explanation, the phenomenon 

 itself is easily repeatable and of considerable economic importance; 

 sprays of synthetic auxins are used to schedule flowering, and hence 

 fruiting, in commercial plantations (see van Overbeek, 1952; 

 Leopold, 1958). 



Flowering in pineapple can be brought about also by several 

 (< unpounds structurally unrelated to auxins, including /?-hydroxy- 

 ethylhydrazine, acetylene, and ethylene (see Leopold, 1958). Indeed, 

 pineapple is not the only plant in which ethylene can cause 

 flowering. Khudairi and Hamner (1954b) found that ethylene 

 chlorohydrin vapor caused flower initiation in Xantliiinn plants 

 under 16-hour photoperiods. As with the auxin-antagonist results 

 mentioned previously, the experiments were carried out under 

 threshold conditions, with supplementary light of low intensities. 



The mechanism of ethylene action on flowering or any other 

 plant process is unknown, but there is some evidence that it acts 

 as an auxin antagonist, possibly reducing the native auxin content. 

 If this is so, then its effects on both Xanthium and pineapple are 

 in accord with the hypothesis that synthetic auxins act as anti- 

 auxins for the pineapple, and the whole set of observations can be 

 used to support the hypothesis that, at least under certain condi- 

 tions, flowering may occur after the lowering of a superoptimal 

 auxin level. However, with the bits of evidence discussed in Chapter 

 Five, this hypothesis remains highly speculative. 



Auxin antagonists have provided another major difficulty in 

 analyzing the auxin relationships to flower initiation. Certain com- 

 pounds believed to be true antiauxins (such as 2.1-dichloro- 

 phenoxyisobutyric acid or 2,4-6-trichlorophenoxyacetic acid) and 

 others that may rather inhibit auxin transport (such as 2,3,5- 

 triiodobenzoic acid) promote flowering in annual Hyoscyamus under 



