126 Fundamentals of Auxin Action 



If auxins are applied to flowers, they sometimes cause the flowers 

 to set fruit and in other cases cause the flowers ultimately to abscise. 

 The species which can be made to set fruit are consistently among the 

 types with many-seeded fruits — the tomato, pepper and the cucurbits 

 (Nitsch, 1952). In many species, flowers can be made inviable or young 

 fruits can be aborted by auxin treatments. The physiological means 

 by which these results are obtained are discussed in chapter XII. 



Auxins applied to developing fruits may increase the rate of 

 fruit growth, or prolong the period over which fruit growth occurs. 

 If the auxins are applied or are still retained when fruit is nearly 

 mature, they may in some instances cause early ripening or pigmen- 

 tation of the fruit. The first of these effects, increase in rate of fruit 

 growth, is strikingly illustrated in the report of Crane and Blondeau 

 (1949) using 2,4,5-T on figs (figure 122). In the case of the tomato the 

 greater growth rate may be restricted to only a few days following 

 treatment (Singletary, 1950). The second effect, the prolongation of 

 growth of fruits, is typified by the response of pineapple reported by 

 Krauss et al (1948). These workers found that spraying pineapple 

 fruits with yg-naphthoxyacetic acid seven weeks before maturation 

 would bring about a continuation of fruit growth for two weeks 

 longer than the controls, resulting in larger fruits. The third effect, 

 the hastening of pigmentation of fruits was originally noted by Gard- 

 ner et al (1940). Subsequent reports indicated that 2,4-D could induce 

 ripening of fruits in storage (Marth and Mitchell, 1949), but this 

 effect is very probably attributable to impurities in the auxin material. 

 Compounds like the phenols which cause metabolic uncoupling (dis- 

 cussed in the next section) may in fact cause ripening, and such 

 phenols are commonly found as impurities in 2,4-D preparations 

 (Hansen, 1951; Goldacre et al, 1953). The intensification of pigmenta- 

 tion seems to be a genuine auxin effect and is used commercially in 

 apple production which will be discussed in chapter XIII. 



METABOLIC EFFECTS 



Soon after the discovery of the profound effects on growth ob- 

 tained with auxins, Bonner (1933) carried out the first experiments 

 on respiratory responses to auxins. The possibility that an increase in 

 respiration rate might be associated with the increase in growth fol- 

 lowing auxin treatment was pointed out and good experimental data 

 were presented to show tliis correlation. Not only did the respiration 

 rate increase with growth-promoting concentrations of auxin but it 

 decreased with growth-inhibiting concentrations. In a subsequent 

 study, Bonner (1936) found that when a preparation presumed to be 



