214 Auxins in Agriculture 



medium can entirely eliminate the capacity of the cutting to form 

 roots (Hemberg, 1951). Although deficiencies of other inorganic 

 nutrients such as phosphorus, potassium, calcium, and magnesium can 

 lower the rooting response, there does not appear to be as marked a 

 sensitivity to these ions as there is to nitrogen and boron. 



or 



UJ 

 GO 



16- 



Number of 

 Roots 



Root Length 



200 



400 



600 



800 MG/L 



CONG. OF AUXIN 



Fig. 90. The effects of auxin (indoleacetic acid) in stimulating the number 

 of roots and the concomitant inhibition of root length in citrus cuttings (Biale and 

 Halma, 1937). 



It is well known that root growth is extremely sensitive to auxin 

 levels, and that it is inhibited by even very dilute auxin applications. 

 It is not surprising, then, that auxins applied to cuttings to improve 

 root formation should sometimes retard the growth of the roots so 

 formed. A particularly clear case of inhibition of root length asso- 

 ciated with the stimulation of rooting with auxin can be seen in 

 figure 90. 



