300 Auxins in Agriculture 



auxin, is reported inactive against tobacco mosaic virus; and maleic 

 hydrazide, which can act as an auxin antagonist, actually promotes 

 growth of wheat rust lesions (Livingston, 1953). This latter effect may 

 be more appropriately assigned to the alteration of carbohydrate con- 

 tent than to auxin antagonism. 



The fungus, Cladosporium herbarum, which parasitizes plants by 

 mycelial growth through cell walls, has been found to be stimulated 

 in groAvth by auxins, apparently in part at least by stimulation of 

 the exoenzyme pectin esterase produced by the pathogen (Waygood 

 et al, 1954). Waygood (1954) has suggested that the resistance or 

 susceptibility of plants to this pathogen may be related to their 

 natural auxin contents. 



Another therapeutic property of auxin is its ability greatly to 

 enhance the action of antibiotics. Streptomycin has been used to 

 prevent fire-blight, a bacterial disease of apple trees. The simulta- 

 neous application of indoleacetic acid with the antibiotic increased the 

 disease protection up to 80 per cent (Goodman and Hemphill, 1954). 

 The suggestion has been made by Iyengar and Starkey (1953) that 

 there is a synergistic action between some antibiotics and auxin, 

 though no data were given to support the proposal. Some evidence for 

 such a synergism is given in chapter VII. The synergistic interaction 

 appears to apply both to the growth effects and to the antibiotic 

 effects. 



Mention has been made in chapter XV of the effectiveness of 

 auxins in controlling mold on nursery tree stock and on citrus fruits. 



An interesting observation has been made by LaBaw and Des- 

 rosier (1954) that auxins can reduce the heat resistance of mold spores 

 in canned products, which suggests an interesting possible use of 

 auxins in food processing. The effect was not limited to auxins, but 

 TIBA and some of the chlorinated benzoic acids which are not auxins 

 have similar effects. These workers found that British Anti-Lewisite 

 (1,2-dithiopropanol, a sulfhydryl protective agent) reversed the effects, 

 suggesting a role of sulfhydryl substances in the spore survival system. 



CONTROL OF APICAL DOMINANCE 



Since the classic work of Thimann and Skoog (1934) which 

 showed that auxins produced at the tip of the plant inhibit the 

 growth of lateral buds, it has become more and more evident that 

 the auxin system controls the branching habit of all higher plants. 

 Thus plants with an abundant supply of natural auxin maintain a 

 strong apical dominance and, conversely, plants with a relatively low 

 auxin supply develop a more branched habit (Delisle, 1937). 



