VI CELL ENLARGEMENT 779 



ticularly if in oil solution, some 2,4,5-T can remain for months or into the next 

 growing season to reenter the circulation in spring and kill or distort the young 

 growth. Thus the herbicidal action of auxins is quite different from that of other 

 toxic compounds (see the brief review by Crafts, 1956). 



Auxins with long aliphatic side chains, such as 2,4-dichlorophenoxy-butyric 

 acid and higher homologues, are subject to [B-oxidation in the side chain and thus 

 give rise to acids with two less carbon atoms (Wain and Wightman, 1954; Faw- 

 cett et al., 1956a, b). In this way acids with an odd number of methylene groups 

 eventually yield acetic derivatives and consequently have relatively high herbicidal 

 activity, while those with an even number of methylene groups yield the phenol, 

 which is essentially inactive or only weakly toxic :- 



R-0-(CH2)5 COOH ^ R O (CH2)3 COOH -^ R-O-CH.COOH 

 R-0-(CH2)4 COOH -^ R-0-(CH2)2 COOH -» R-OH 



The ability to carry out the oxidation is not present in all plants and the enzymes 

 concerned appear to differ for difTerent R groups. It is hoped, therefore, that this 

 behavior may be the basis for still more selectivity among auxin herbicides. 



Some plants, especially the cereals and grasses, are quite resistant to high auxin 

 concentrations and in this way dicotyledonous weeds can be killed in wheat, oats 

 or sugar-cane without harm to the crop. Yet young seedlings of these same grasses 

 are very sensitive, and within a given grass species, such as maize, varieties of at 

 least moderate sensitivity hav^e been observed. Some dicotyledons, too, including 

 thistles and strawberries, are fairly resistant (Crafts, 1956). The basis for the 

 resistance is still unknown. No attempt can be made to summarize here or even 

 mention the flood of literature on synthetic auxins as herbicides; Audus (1953, 

 Chap. 9) and Leopold (1955, Chap. 16) have given extensive treatments, while 

 European practice is reviewed by Woodford and Kasasian (1956). 



(c) Control by oxygen 



The promotion of cell enlargement by auxin is aerobic. Sections of coleoptile 

 or pea stem used in bioassays grow at a greatly reduced rate if immersed only a 

 millimeter below the surface of the solution. Tuber slices grow best when breaking 

 the surface of the solution or when shaken vigorously in shallow layers. Using such 

 potato slices, the dependence of cell enlargement (in auxin solution) on oxygen 

 tension has been determined (Hackett et al., 1953). Enlargement increases linearly 

 up to nearly 20% oxygen — an entirely different relationship from that shown by 

 most oxidases which are saturated at oxygen tensions far below that of air. Since 

 the oxidase controlling growth has the highest oxygen affinity of any of them (see 

 p. 786) it follows that this curve is probably only measuring the rate of difTusion of 

 oxygen into the tissue. 



After a not too long period of partial anaerobiosis, e.g. in 7% O2 and 93% N2, 

 the sections recovered their previous growth rate completely on transference to 

 air (Hackett and Thimann, 1952a). More complete, or more prolonged anaerobio- 

 sis usually has irreversible damaging effects and often causes outflow of cell con- 

 tents ("exosmosis") like that due to supraoptimum auxin concentrations. These 

 observations confer great importance on the intercellular spaces, stomata and 



Literature p. 8i6 



