224 Auxins in Agriculture 



tion begins before dehiscence of the anther sacs occurs, and conse- 

 quently the capacity of the flower to pollinate itself is lost. In those 

 cases where the pollen fails to reach the style, the forcing of tomato 

 flowers with auxins can be very beneficial. 



We know that sterile pollen is produced on tomato plants grown 

 in weak light. Also, the temperature range for optimal pollen function- 

 ing is rather limited and when high or low temperatures are experi- 

 enced, viable pollen becomes sterile (Smith, 1935). With regard to the 

 influence of temperature, it is interesting to notice that the optimal 

 temperature for pollen germination is around 85° F, whereas optimal 

 fruit-set without pollen is obtained as much as twenty degrees lower, 

 viz., 65° F. The effects of temperature on pollen germination and on the 

 ability of ovaries to set fruit are plotted together in figure 95, where this 

 difference in optima can be seen. In field tomatoes, auxins are very 

 useful in forcing fruit-set when temperatures fall below 60° F (Wittwer 

 et al, 1948; Odlund and Chan, 1950). This often permits the setting of 

 the earliest flowers in the spring when low temperatures would normally 

 prohibit fruit-set. These earliest fruits have the highest market value of 

 the entire field crop. Curiously enough, when temperatures are exces- 

 sively high, auxins again increase fruit-set (Mullison and Mullison, 

 1948). In the southern United States where temperatures become very 

 high and light intensities are also high it has been found that shading 

 tomato plants considerably improves the responsiveness to auxin (Moore 



100 R 



TEMPERATURE 





^<. 



Fig. 95. A comparison of the effects of temperature upon pollen germination 

 (Smith, 1935) and upon the capacity of tomato ovaries to set after forcing with 

 auxin, p-chlorophenoxyacetic acid (Leopold and Scott, 1952). 



