220 PLANT GROWTH SUBSTANCES 



are, however, a number of important problems that would appear to 

 be amenable to application of growth substances, on which little or no 

 progress has been made. Some of these should be mentioned here. 



Parthenocarpy. — The subject of parthenocarpy is ably reviewed in a 

 separate paper in this symposium, and it is not the intent to encroach on 

 that discussion here other than to point out that for our most important 

 tree fruits, such as apples, peaches, pears, and the citrus varieties, no 

 practical progress has been made in stimulating parthenocarpy. The 

 work of Crane and Blondeau (9,11) with the Calimyrna fig may consti- 

 tute a partial exception to this general statement. While most figs 

 are naturally parthenocarpic, this variety is almost completely non- 

 parthenocarpic in the sense that it does not hold its fruit to maturity 

 unless pollinated, although it should be noted that considerable de- 

 velopment of the syconia takes place without pollination. In the work 

 of these investigators sprays of indolebutyric acid at 1,500 ppm. (9) 

 and, of more practical significance, 2,4,5-trichlorophenoxyacetic acid 

 at concentrations as low as 10 ppm. (11) caused the fruit to complete 

 its development and at a greatly accelerated rate. The need for more 

 work on parthenocarpic stimulation in tree fruits is real, in view of the 

 number of self-sterile varieties and frequent unfavorable pollination con- 

 ditions which result in poor set. There is also the possibihty that by 

 proper stimulation parthenocarpic fruit might be superior in some 

 respects to fertilized fruit. The problem is intimately tied in with the 

 general problem of improving fruit set. It should be pointed out, 

 however, that the parthenocarpic response is a different reaction from 

 simply the prevention of abscission, for it is possible to hold ffowers 

 and young fruits on the tree for appreciable periods, but without 

 stimulation of ovary development. 



Delay in flowering. — The greatest hazard in most sections in the pro- 

 duction of many deciduous fruits, particularly those that tend to bloom 

 early in the spring, is freezing temperature at flowering time. A delay 

 of even 10 days in blossoming might save many crops from freezing 

 damage. The inhibiting effect of many of the hormone chemicals on 

 lateral leaf bud development is well known. Unfortunately flower buds 

 are not as subject to inhibition by these compounds as are leaf buds. 

 Winklepeck (51) applied a spray of NAA at 125 ppm. to peach trees 

 just as the flower buds were swelling and beginning to break and reported 

 that the sprayed trees arrived at full bloom two weeks later than the 



