Parthenocarpy 225 



and Thomas, 1952). Whether this improvement is owing to the lowered 

 temperatures of the plant or to reduced light intensities is not clear. 

 It is evident, then, that fruit-set may be limited by excessively high or 

 low temperatures and light intensities, and that such limitations can be 

 partly overcome by auxin treatment. 



Lack of nutrient materials for growth may be another physiolog- 

 ical factor limiting natural fruit-set. Since a number of mature leaves 

 is essential for successful pollination, it is not surprising that tomato 

 seedlings possessing only a few small leaves have difficulty in success- 

 fully pollinating the first cluster of flowers. As figure 92 shows, ovaries 

 treated with PCA will set fruit with fewer leaves than those supplied 

 with pollen. One of the most effective uses of auxins in commercial pro- 

 duction of field tomatoes is to obtain fruit-set in the first clusters when 

 there are few leaves on the young plants. 



Plants in which carbohydrate availability is limited by weak light 

 conditions, by disease, by high nitrogen levels or by the presence of a 

 large proportion of fruits to leaves, may be unable to set fruit by 

 normal pollination. In some of these instances where the inability to 

 set fruit is caused by limitation of nutrients, a small beneficial effect 

 may be obtained by forcing fruit-set with auxins (Murneek et al, 1944; 

 Leopold and Scott, 1952). Low^ carbohydrate and high nitrogen situa- 

 tions frequently occur in greenhouses in winter when light intensities 

 are low. 



The Prevention of Abscission 



It is well known that auxins applied to various plant organs pre- 

 vent or at least delay abscission. The formation of an abscission layer 

 in various organs is associated with the presence of weak auxin gra- 

 dients or to a rather abrupt decrease in the auxin concentration (Shoji 

 et al, 1951; Addicott and Lynch, 1951). Auxin production in flowers is 

 largely centered in the maturing stamens (e.g. Wittwer, 1943) and 

 ovaries (Katunskij, 1936). Once these have matured the auxin level in 

 the flower may become quite low. Unless more auxin is added the level 

 is frequently insufficient to hold the flower on the plant for any length 

 of time. Flower abscission is encouraged by high night temperatures — 

 above 78° F (Went, 1945), and by low nitrogen levels (Kraus and Kray- 

 bill, 1918). Since the flowers of tomato which are abscissed are still 

 capable of fruit-set, the prevention of abscission would permit setting 

 over a longer time period under conditions where abscission is prone to 

 occur. It should be noted, how'ever, that some workers have not ob- 

 tained increases in fruit-set of tomatoes with auxin w^hen abscission was 

 active (Roberts and Struckmeyer, 1944). ^ — r7~~ - 



