218 Auxins in Agriculture 



of mature leaves permits a quantitative increase in the amount of 

 fruit-set obtained (figure 92). It is interesting to note that the par- 

 thenocarpic fruit-set resulting from the application of auxin has a 

 lower leaf requirement than fruit-set by pollination. 



Investigations to determine the nature of the nutrients provided 

 by leaves which are apparently essential for tomato fruit-set have not 

 indicated that a specific substance or a specific group of substances 

 is necessary, but rather that many different types of organic compounds 

 can take the place of the mature leaves in this function. Experiments 

 with excised flowers indicate that many sugars, organic acids, organic 

 nitrogenous compounds and reducing substances can serve as the 

 substrates for auxin-induced parthenocarpy. A possible reason for the 

 non-specificity of this function appears in the suggestion by Leopold 

 and Guernsey (1952) that the stimulation of some energy-producing 

 reactions may account in part at least for the requirement for organic 

 nutrients, and that a wide variety of organic materials stimulate 

 such reactions in the metabolic scheme. 



METHODS OF TREATMENT 



Selection of Auxins 



One of the species most readily set by auxins is the tomato, and 

 in view of the economic importance of this crop the bulk of the in- 

 formation available on the induction of parthenocarpy is concerned 

 with the tomato. Among the many auxins which have been used, PCA 

 (p-chlorophenoxyacetic acid) and BNOA (^-naphthoxyacetic acid) 

 have emerged as the most effective and most widely used. These are 

 both fairly weak auxins and both produce relatively little deformation 

 of tomato leaves. The search is still being carried on for other auxins 

 which will be more effective in setting fruit, have less epinastic effects 

 on the leaves, or bring about systemic fruit-setting. 



OCHgCOOH /^ Y^NoCHgCOOH 



\A/ 



p-chlorophenoxyacetic Acid jS-naphthoxyacetic Acid 



Indoleacetic acid is fairly ineffective in inducing parthenocarpy 

 in tomatoes, presumably because of the ease with which it is inacti- 

 vated by the natural enzymes of the plant. Redemann et al (1951) 

 have shown that the relatively high effectiveness of pollen extracts in 

 causing tomato fruit-set is due to the presence of the methyl ester of 

 indoleacetic acid. They find that the ester is approximately one 



