ii8 



PLANT GROWTH SUBSTANCES 



inhibition and the other physiological effects. Toxic action in more 

 mature plants may actually result in increased total respiration, with or 

 without accelerated growth and development. Probably the fundamental 

 effect, however, is the tendency to a change in type of respiration. 



What evidence do we have from these toxic effects of the way growth 

 substances affect respiratory metabolism.? One of the most interesting 

 possibilities is the greater sensitivity of the aerobic than of the anaerobic 

 phase of respiration in seedlings to 2,4-D inhibition. This may also be 

 involved in the differing sensitivities of roots and shoots if Taylor's 

 observations on rice and wheat (35) on the relative abilities of these 

 organs to grow anaerobically are of general occurrence. Mitchell, 

 Burris, and Riker's (21) failure to find any change in respiratory 

 quotient in bean stem tissue after short term in vitro treatment may 

 indicate only that this aerobic-anaerobic difference is confined to less 

 mature tissues. There is, in fact, considerable evidence for a change in 

 respiratory mechanism from seedling to mature leaf tissue (1,20). It 

 would be interesting to know as well whether the difference in monocot 

 and dicot sensitivity to 2,4-D at emergence might be explainable in 

 these terms. The special sensitivity of aerobic phases of respiration to 

 growth substance inhibition is at least consistent with the fact that plant 

 growth in general is aerobic, and with the present views of Thimann, 

 Bonner, and others (7,38) that lAA affects growth through a sulfhydryl 

 enzyme system and is involved in aerobic phosphate transfer. 



In the light of recent work of Thimann and W. D. Bonner (38) and 

 J. Bonner (7) it is interesting to compare the action of known respiratory 

 inhibitors with that of the growth substances, themselves. At appropriate 

 concentrations most of the respiratory inhibitors like the growth sub- 

 stances will either accelerate or inhibit growth. In some cases the concen- 

 trations for growth inhibition and respiratory inhibition are similar, for 

 example, the action of cyanide and lAA on Avena coleoptile (6). In 

 other cases growth may be more sensitive than respiration, as in iodo- 

 acetate (13) and fluoride action (9) and the effects of growth substances 

 on elongation and water uptake in roots. Furthermore, at a given 

 concentration growth may be inhibited and respiration stimulated as in 

 the case of 2,4-dinitrophenol (7) and in some types of 2,4-D treatment. 

 There are also some parallel changes in the inhibitive effects of respiratory 

 inhibitors and growth substances with the age of tissues (38). These com- 

 parisons are not meant to imply that lAA or 2,4-D act necessarily by the 



