Auxins, Growth Regulators ■ 107 



indoleacetic acid (IAA) under conditions in which the controls 

 remained vegetative. These, however, were "threshold" conditions- 

 supplementary light of intensities not quite sufficient to cause 

 flowering by itself was used to extend the photoperiod beyond its 

 critical value. No auxin promotions were observed under strict 

 short-day conditions. Promotion of flowering in another LDP, 

 Wintex barley, has been observed by Leopold and Thimann (1949). 

 This effect was obtained under inductive conditions and appears to 

 be simply a promotion of later inflorescence development. Note 

 that in the same experiments (see Chapter Five) x-irradiation, 

 which may reduce the auxin level, also increased flowering. 



In the SDP Xanthium, Bonner and Thurlow (1949) reported 

 that application of the auxins IAA, naphthaleneacetic acid, or 2,4- 

 dichlorophenoxyacetic acid (2,4-D) to cuttings or to leaves of intact 

 plants prevented the flowering response to short days. This effect 

 was opposed by the auxin antagonists 2,4-dichloroanisole and 

 2,3,5-triiodobenzoic acid (TIBA). The antagonists themselves, under 

 threshold conditions— night interruptions barely sufficient to keep 

 the controls vegetative— caused the initiation of "flowerlike buds," 

 which, however, did not develop into flowers (Bonner, 1949). 



Auxin inhibitions of flowering in Xanthium have been studied 

 further by Lockhart and Hamner (1954) who showed that IAA 

 increased both the magnitude and consistency of the inhibition 

 caused by a second dark period following the inductive night 

 (Chapter Two). Additional data on auxin inhibition in both 

 Xanthium and Biloxi soybean are provided in Hamner and Nanda 

 (1956). Salisbury (1955), again with Xanthium, found that auxin 

 inhibited flowering only if applied before translocation of the 

 "flowering stimulus" appeared to be completed— that is, before the 

 end of the period during which removal of the induced leaves 

 could reduce the flowering response. If applied later, it promoted 

 flower development, particularly under reduced light intensities 

 or in the absence of actively growing buds. Inhibitions by IAA 

 applied before and during the inductive dark period have also 

 been reported in the SDP Pharbitis (Nakayama, 1958), although 

 earlier work showed promotions under similar conditions (Naka- 

 yama and Kikuchi, 1956). 



One of the few plants in which auxins have a major effect on 

 flowering is the pineapple {Ananas comosus). As noted in Chapter 



