228 Auxins in Agriculture 



be obtained by the use of 25 mg./l. of PCA on the flower chister (figure 

 96). The use of higher concentrations does not result in any further 

 gain in fruit size, but reduces the gain instead. 



Many workers have reported immense gains in yield of tomatoes, as 

 great as 400 or 500 per cent. These large gains are always cases in which 

 the yield is an expression of the first pickings or the early yield. Auxin 

 sprays can increase yields significantly only when limitations of fruit- 

 set exist. Such conditions usually occur in the field primarily during 

 the early stages of growth. A striking example of this fact is found in 

 the report by Mann and Minges (1949), who followed the yields of field 

 tomatoes through the picking season (figure 97). They found that while 

 untreated vines produced fruit beginning on July 12, the vines treated 

 with auxin were producing fruit more than a month earlier. The big 

 increases in yield attributable to auxin treatment were restricted to 

 the first month of production, for it is in the first month of flowering 

 that field-grown tomatoes are most generally limited in their fruit-set 

 by conditions which can be alleviated with auxin. 



A limitation of the beneficial effects on tomato yield of auxins has 

 been pointed out by the work of Hemphill (1949) who found that 

 flower buds in the early stages of enlargement are inhibited in develop- 

 ment by auxin sprays. As a consequence of this inhibition there may 

 be a reduction in the number of flowers reaching maturity a week or 

 two after auxin applications. 



Large increases in the early yield of snap beans have been obtained 

 through the use of auxin sprays to increase setting (Wittwer and Mur- 

 neek, 1949). These sprays were beneficial only at high temperatures 

 (above 90° F), presumably because pollination is limited under such 

 conditions. 



DIFFICULTIES RESULTING FROM FORCED PARTHENOCARPY 



There are, of course, certain drawbacks to the use of auxin sprays 

 to force fruit-set. One of these is damage to the foliage caused by the 

 epinastic properties of the auxins which are commonly used for forcing 

 fruit-set. Considerable damage can be done to the foliage and this can 

 largely decrease yield if the damage is excessive. Not only epinastic 

 curling of the leaves may be produced, but also abnormal development 

 of leaves which are in the process of expansion at the time of treat- 

 ment. Such damaged leaves fail to expand normally and give the ap- 

 pearance of leaves infested with cucimiber or tobacco mosaic. Such 

 distortions become evident about two weeks after spraying the flower 

 clusters with PCA concentrations as high as 50 mg./l. or higher. 



Another type of difficulty experienced is the production of de- 



