CHAPTER VI 



Factors Altering Effectiveness 

 of Auxin Applications 



The effectiveness of an auxin material applied to a plant is a func- 

 tion of the distribution of the auxin through the plant and the 

 responsiveness of the plant parts into which the auxin moves. The 

 former of these is determined by two separate steps — the absorption of 

 the auxin into the plant and the translocation from one plant part to 

 another. The latter function is determined in turn by the status of 

 the plant and its nutritional condition. Any environmental factor 

 influencing the effectiveness of auxin application may have its in- 

 fluence through one or several of these particular phases of auxin 

 action. 



ABSORPTION 



The absorption of auxin sprays applied to foliage is strongly 

 affected by the physical structure of the foliage itself. Surfaces to which 

 the spray adheres poorly permit less effective absorption than surfaces 

 which wet more easily. For example, garden pea leaves have a waxy 

 surface which make them rather difficult to wet. As a result foliar 

 sprays of a given concentration may have less effect on this plant than 

 they do on tomato, the leaves of which wet very easily. Not only does 

 the waxiness of the surface influence this function, but the physical 

 form of the leaf may influence absorption as v*'ell. For example, an 

 onion leaf presents a small surface per unit volume and its position 

 is such as to encourage run-off of the spray droplets. 



Several investigators have studied the possible role of stomata in 

 the absorption of auxin materials applied to the leaf, but in each case 

 evidence brought forth suggests that stomata do not have much bear- 

 ing on such absorption (Weaver and DeRose, 1946). This is in 

 contrast to volatile spray materials which may enter leaves principally 

 through stomatal openings (van Overbeek, unpublished). 



Temperature conditions profoundly influence absorption. Rice 

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