Metabolism and mode of action 



internal diffusion pressure deficit. It has long been known that sugars or 

 potassium salts act as cofactors in auxin-induced elongation of coleoptile 

 sections. The role of these substances is apparently in part an osmotic one. 

 They contribute to the maintenance of internal osmotic concentration. 



lAA af 1\t , 



Figure 7. The presence of auxin 

 (lAA, 5 mgjl.) is without effect on the 

 length of the lag period in elongation 

 of coleoptile sections in a medium com- 

 posed of mannitol and sucrose {total 

 concn. 0-3 M). After Ordin et al. 

 (1955). 



T/me 



The presence of sucrose in the medium extends the period over which elonga- 

 tion is linear with time. The effect of sucrose on rate of water uptake may be 

 in part due to the utilization of the material in respiration and support of 

 cellular syntheses. In addition, however, sucrose clearly provides internal 

 osmotically active material. 



1-2 



1-0 



0-8 

 % 



o D-H- 



E 



0-Z 



/ 



■ 9m\n 



•-^?Jy2=<?niin 



\ 



A 



J L 



'I t 



60 



120 180 ZfO 



Time min 



Figure 8. Time course of water movement into and out o/Avena coleoptile sections as followed with 

 deuterium-labelled water, DHO. The ascending curve represents course of DHO movement into tissue 

 from DHO labelled external solution. The descending curve represents the course of DHO loss by 

 labelled tissue to unlabelled solution. After Ordin (1955). 



Further information concerning the role of auxin in the Avena coleoptile has 

 been obtained by the study of absolute rate of water movement in this 

 tissue (Ordin, 1955). This has been done with the help of dilute solutions of 

 deuterium-labelled water, DHO. Groups of sections were placed in 1 per cent 

 DHO solutions for various periods of time. After each desired time period a 

 group of sections was dried superficially with filter paper, placed in a ground 

 glass stoppered vial and water removed by lyophillization in a micro 

 distillation apparatus. The water thus removed was analysed directly in a 



266 



