8o2 



PLANT GROWTH 



400 



300 



?P200 



_E 



"5 100 



u 



20(1 



_ 150 



^ 8 100 

 E^ 

 ?° 50 



ment somewhat simpler than that of shoots and has led to considerable use of 

 roots in varied physiological studies. It may be worth while to point out, though 

 the work will not be discussed in detail here, that it was with roots that the im- 

 portant phenomenon of resistance to radiation damage was discovered. X-irra- 

 diation of roots in low oxygen tensions caused much less damage than in air 

 (Thoday and Read, 1 947-49) and the 

 subsequent study of this phenomenon 

 with bacteria has inade it possible 

 greatly to reduce radiation damage 

 by treatment with ethanol, cysteine 

 or other potential hydrogen donors 

 (Hollaender and Stapleton, 1953). 



For present purposes, the main 

 growth study to which roots have 

 been put concerns the effect of auxins. 

 In the last few years this work has 

 been extensively developed in Sweden 

 by Aberg, Burstrom and their colla- 

 borators. As stated, root elongation is 

 inhibited by concentrations of auxin 

 which promote the elongation of 

 shoots. In the case of wheat roots, 

 inhibition is exerted by io~^M lAA, 

 while NAA, 2,4-D or 6-chloro- 

 lAA have about the same activity 

 (Burstrom, 1951; Hansen, 1954). 

 Parallel with the inhibition of elonga- 

 tion of the root as a whole goes in- 

 hibited elongation of the epidermal 

 cells, and in general also inhibition 

 of their multiplication. Each of a 

 large variety of substances operates 

 over a different range of concentra- 

 tions and the dose-response curve 

 has to some extent different and 

 characteristic shapes. Examples of 

 the behavior of wheat roots, ex- 

 pressed in length and number of the 

 epidermal cells, are collected in Fig. 1 1 . Each curve represents one substance. 



The left-hand side of the figure shows a series of auxins (except nos. 3 and 10). 

 They inhibit both elongation and multiplication, and their curves show rough 

 parallelism. The right-hand side of the figure shows the effect of a series of aryl- 

 isobutyric acids. These compounds were selected because they have very little 

 auxin activity on shoot tissues, (though No. 39 is quite active in the slit pea stem 

 test). In all cases these substances promote root cell extension over a wide range 

 of concentrations (commonly three logarithmic units) and inhibit it sharply at 



-8 -7 -6 -5 -4 -8 -7 -6 -5 -4 

 Log concentration M 



Fig. 1 1 . Elongation of wheat root epidermal 

 cells, and cell numbers (calculated from root 

 length divided by cell length). All data after 

 3 days in solutions. 

 Left: arylacetic acids: 

 Phenylacetic 

 Phenoxyacetic 

 2-methylphenoxyacetic 

 2,4-dimethylphenoxyacetic 

 2,4-dichlorophenoxyacetic 

 10 3,5-dichlorophenoxyacetic 

 12 2-methyl, 4-chloro-phenoxyacetic 

 15 i-naphthaleneacetic (NAA) 

 Right: aryl-isobutyric acids: 

 29 a-phenoxyisobutyric 

 31 a- ( 2 ,4-dimethylphenoxy) -isobutyr ic 

 36 a- ( 2 ,4-dichlorophenoxy) -isobutyric 



38 a-(2-methyl, 4,6-dichlorophenoxy)- 

 isobutyric 



39 a-3-indoleisobutyric 

 Selected from Hansen, 1954. 



(2,4-D) 



