336 PLANT GROWTH SUBSTANCES 



maize caryopses. What is equally important, they throw considerable 

 light on the results observed when NAA is used for the reduction of 

 fruit set of the apple and some other fruit crops. 



Green beans. — Abnormally hot weather at the time when string beans 

 are setting fruit may reduce the yield of pods considerably. This can 

 be overcome to an appreciable extent by either spraying or dusting the 

 plants with a desirable growth regulator. It may be combined with a 

 necessary insecticide. Maturity of the pods, as judged by size is also 

 increased by the treatment (59,96,26). The beneficial effect apparently 

 is brought about by stimulation in growth of the carpels (pods) and an 

 increase in chlorophyll content of the leaves. Probably the abscission 

 layer between fruit and pedicel is also strengthened. The seed number in 

 some cases may be reduced but, where the proper concentration is 

 employed, it is usually about the same. 



Fruit Thinning 



Although we have tried repeatedly to increase the set of stone and 

 pome fruits by the use of synthetic growth regulators, the results have 

 invariably been negative. Instead of an increase, there has commonly 

 been a reduction. This response, too, has been found under certain 

 circumstances to have practical value as will be seen forthwith. 



Burkholder and McCown (12) reported that reduction in the crop 

 of apples, when the set is excessive, may be brought about by spraying 

 the trees in full bloom with NAA. This has been confirmed by others 

 (18,75,79), some of whom found that exact timing of the apphcation, 

 as on the first day of full bloom, is not required. 



During the past four years we have conducted extensive field investiga- 

 tions on thinning apples and peaches with NAA, with special reference 

 to the time of application and concentration to be used (60,64). The 

 results, still largely unreported, show that this growth regulator is an 

 efficient substance for reduction of the number of fruits on an apple or 

 peach tree. For apples, the best time of application seems to be one to 

 two weeks after full bloom. The concentration of the spray material to be 

 used is 10-30 ppm. depending on the variety and whether it is an annual 

 or biennial bearer. Varieties that are difficult to thin chemically may 

 require two applications of the spray. Peaches may be thinned success- 

 fully with NAA, applied about one month after bloom, at a concentra- 

 tion of 30-40 ppm. for the light setting varieties and 40-60 ppm. for 



