778 PLANT GROWTH lO 



tate (see section Vlf, p. 782) increases the toxicity. Similar toxic effects at high con- 

 centrations occur with tuber sUces, tissue cukures and other material. The optimum 

 curve has been explained as follows: the molecule has to combine with two re- 

 ceptors, e.g. an enzyme and a substrate, which it thus brings together. (The idea 

 is somewhat similar to that o^ complement in immunology,) If there is an excess of 

 auxin molecviles present, one group will combine with all of the enzymes and an- 

 other group with all of the substrates, so that, instead of bringing these two mate- 

 rials together, the auxin actually keeps them apart (Skoog et al., 1942). Recently 

 this proposal has been revived and, though it has been claimed to be consistent 

 with the kinetics of growth in coleoptile sections, there is disagreement in this 

 regard. Several groups of workers find that high auxin concentrations actually 

 accelerate the initial growth rate, for short times, and that thereafter a toxic effect 

 becomes superimposed, while McRae, Foster and Bonner (1953) believe that 

 there is true growth inhibition from the start, and ascribe considerable theoretical 

 significance to this (see Bennet-Clark, 1956b, for a critical discussion). 



Another view of the optimum curve is that every auxin molecule has in it growth- 

 promoting and growth-inhibiting properties (Linser, 1954). As a result, the 

 optimum curve is analysable into two parts, of which that at the lower concentra- 

 tions shows simple growth promotion, while that at higher concentrations can 

 be imitated by a non-auxinic growth inhibitor such as eosin. Both these concepts 

 rest upon mathematical analysis of growth curves, treating the growth phenome- 

 non like a simple one-step enzymatic reaction. The extent to which they correspond 

 with the undoubtedly complex biological facts cannot yet be deduced, pending 

 detailed knowledge of the mode of action of auxins in causing growth, but it seems 

 inherently unlikely that the simple treatment can be pressed very far. The action 

 curves of many drugs and poisons show optima (see Thimann, 1 956c, for discussion) . 



The toxic effect is in any case of great practical importance, since it is — as 

 already mentioned — the basis of the popular weed-killing chemicals. Although 

 naphthalene-acetic acid (p. 763) was used in the early experiments (Templeman 

 and Sexton, 1946) the chlorinated acids, 2,4-D, 2,4,5-T and Methoxone (XI, 

 XII and XIII) were subsequently found more effective and are now universally 

 used. Sprayed on leaves, they are rapidly absorbed, killing the leaf and the 

 young shoot; they are also transported both upwards in the transpiration stream 

 and downwards in the bark or phloem to kill most or all of the above-ground 

 parts. Smaller quantities are transported into the roots, which often survive, 

 especially in the case of trees and other woody plants. The latter are more effec- 

 tively killed by treating the stems externally with an oil solution of 2,4,5-T (XII) 

 which is absorbed through the bark and which creeps down below ground level over 

 the bark surface to be absorbed into the upper parts of the roots (see Hay, 1956a). 



The translocation of a toxic substance, in toxic amounts, through living tissues 

 is a phenomenon that seems to require explanation, as pointed out by Crafts 

 (1956), but it is clear that it takes place only for a very short time, and that there- 

 after the conducting system becomes immobilized. There is considerable destruc- 

 tion in the living tissues also, about 50% of the internally present 2,4-D being 

 destroyed in bean seedlings in 24 h. {HoWey etai, 1950; Hay and Thimann, 1956, 

 and literature there cited). Nevertheless, once in the xylem of woody plants, par- 



