150 Fundamentals of Auxin Action 



experiments. Kelly (1949) has found, however, that the transfer of 

 crab grass and bean seedlings from low temperatures into higher 

 temperatures after the time of spraying immensely increased the ef- 

 fectiveness of the spray. For example, bean plants sprayed at a temper- 

 ature of 15° C with 5,000 mg./l. 2,4-D showed no killing for a period 

 of 4 days. Transfer of the plants into 25° temperature then produced 

 a very strong killing effect (85 per cent). However, plants sprayed at 

 5° and then subsequently moved into temperatures of 15° showed a 

 strong killing effect (60 per cent) even though the initial temperature 

 of 15° had not given any kill. Increase in killing associated with the 

 change of temperature apparently cannot be attributed to renewed 

 absorption of the 2,4-D. Experiments were made in which the leaves 

 were washed before being transferred to a higher temperature, and 

 still the increase in kill was obtained. Greater respiratory stimulations 

 follow auxin applications at high temperatures than at low (Brown, 

 1946). 



Temperature and light conditions bear on the nutritional status 

 of the plant. These two factors strongly influence the carbohydrate 

 level of the plant, and accordingly they not only affect absorption 

 and translocation of the auxin but they simultaneously alter the sensi- 

 tivity of the plant to the auxin after absorption and translocation 

 have been effected. 



In occasional instances it has been found that relative humidities 

 may influence auxin effectiveness (e.g. Hitchcock and Zimmerman, 

 1935). One can deduce that humidity may have an effect on auxin 

 activity in the unusual situations where translocation of the auxin is 

 taking place in the xylem, as when the auxin has been introduced into 

 the vascular system. In addition, relative humidity may alter the rate 

 of drying of droplets of auxin solution on the leaves. In this way, low 

 humidities may somewhat impair absorption of auxin. There is some 

 evidence that water solutions enter leaves more readily from large 

 droplets than from small (Smith, 1946), presumably because of slower 

 drying action. 



The effects of environmental factors on the absorption, transloca- 

 tion, and final effectiveness of the auxin at the cellular level may be 

 rather complex. Thus, for example, light retards absorption of aque- 

 ous auxin sprays, light stimulates translocation, and light may alter 

 the final effectiveness of the auxin at the cellular level. Because of the 

 compounded effects of light on the various phases of auxin action, it 

 is evident that differences in light conditions at the time of auxin 

 application may have large effects on the net result, though the direc- 



