VI CELL ENLARGEMENT 785 



parative studies and in respiration. Cell enlargement is inhibited 50% at about 

 5-10"^ Af in many tissues (Thimann and Bonner, 1949). Although it is believed to 

 act by combining with dithio compounds such as lipoic acid, 



HSCH2CH2CH(SH)(CH2)4GOOH 



the arsenite inhibition cannot be reversed or protected against by adding lipoic 

 or other dithio acids. 



Some years ago it was found that germination of seeds is inhibited by unsaturated 

 lactones, such as parasorbic acid, coumarin, XXXI, and protoanemonin, XXXII. 



CH^=CH 

 CH2=C /C=0 



o 



Protoanemonin 

 XXXII 



Both of these inhibit cell enlargement, XXXII being the more potent, and both 

 cause very marked promotion at low concentrations. Although it has not been 

 rigidly proved that these lactones do react with -SH groups in the plant (Price and 

 Leopold, 1957) it is clear that the growth inhibition of coleoptiles and pea stems 

 can be prevented by treatment with certain sulfhydryl compounds such as 

 2,3-dimercaptopropanol (BAL). At about 3-io"'*Af this dithiol appears to have 

 few secondary effects and almost completely protects the plant tissue against both 

 lactones (Thimann and W. Bonner, i94gb). Elongation of roots is inhibited by 

 coumarin and other lactones too, the inhibition by coumarin being freely revers- 

 ible on removing the coumarin (Audus, 1948). The inhibition caused by certain 

 other lactones could not, however, be prevented by BAL (see section VI lb, p. 801). 

 An unknown germination inhibitor in oat hulls, on the other hand, had its inhibition 

 largely removed by BAL or by glutathione (Elliott and Leopold, 1953). 



Perhaps comparable to the lactones are the maleimides. Maleic acid is known 

 to be a powerful inhibitor of the -SH enzyme succinic dehydrogenase, and it 

 does inhibit growth of sections. But the maleimides, perhaps due to higher lipoid 

 solubility, are more effective than the acid, and N-phenyl maleimide inhibits 

 growth 50% at 5- lo'^ Af (Van Overbeek et al., 1955). Cysteine prevents the inhibi- 

 tion. 



A second group of inhibitors comprises the compounds interfering with phosphor- 

 ylation and phosphate transfer. 2,4-Dinitrophenol (DNP) is the most effective 

 of these, acting powerfully on coleoptiles, pea stems, potato and artichoke tubers 

 etc. It also inhibits protoplasmic streaming, a process which is correlated with cell 

 enlargement (see section VIj, p. 793). Although it may increase respiration by its 

 uncoupling action, DNP was not found to increase growth at any level. At 5 mg/1, 

 respiration was promoted by 38% while elongation was inhibited 88% (J. Bonner, 

 1949). Arsenate, which competes for phosphate in various metabolic systems, 

 inhibits cell elongation clearly though more weakly, and the inhibition is de- 

 pendent on the phosphate supply (J. Bonner, 1950). It inhibits elongation of roots 

 about as powerfully as that of shoots (Audus, 1952; see Fig. 9D). 



Literature p. 816 



