406 



PLANT METABOLISM 



PLANT GROWTH SUBSTANCES 



The growth of plants is regulated by a variety of chemical entities 

 included under the broad heading, plant growth substances.^ These com- 

 pounds by being transported within the plant may effect changes at a 

 distance from their point of formation. The most studied and versatile, 

 naturally occurring plant growth substance is 3-indoleacetic acid: 



HC 



^ 



HC 



^ 



3-Indoleacetic acid 



H 

 UC^ ^C 



HC 



'^ 



H 



C— CHoCN 



II 

 CH 



N 

 H 



3-Indoleacetonitrile 



This compound, variously known as p-indoleacetic acid (lAA) and hetero- 

 auxin, and the structurally similar 3-indoleacetonitrile, have been isolated 

 directly from plants, as has traumatic acid (HOOC — CH=CH — (CHo)s- 

 — COOH), but evidence for most of the other growth substances reported 

 to occur in plants lacks substantial chemical verification. Indoleacetic 

 acid at a concentration of 0.01 mg. per liter will cause oat coleoptiles - to 

 bend and will increase streaming of their prott)plasm; 2.0 mg, per liter 

 will inhibit the growth of oat roots by 50 per cent and will stimulate 

 the respiration of oat seedlings; higher concentrations will inhibit both 

 respiration and stem growth. 



In addition to indoleacetic acid, many chemically synthesized substances 

 will influence the growth of plants. For a compound to be effective it 

 must as a rule contain an unsaturated ring and a side chain on the ring 

 carrying a carboxyl group, or a group readily converted to a carboxyl; 

 the carboxyl must be at least one carbon atom removed from the ring 

 and must have a proper spatial relationship with the ring. 



To the activity of growth substances are attributed such responses of 

 plants as phototropism (bending toward light) , geotropism (bending of 

 roots toward and stems away from gravity), bending resulting from in- 

 jury (traumatic acid is active), initiation of flowering, and epinasty 

 (downward bending of leaves without wilting) . Plant growth substances 

 have been used in a large number of practical applications. They are 

 employed to prevent premature dropping of blossoms and fruits. The 



^ "Plant growth substances" is a broad term covering the compounds often referred 

 to as plant hormones, phijtohoymones, groirth regulating suhatances, and auxins. 

 ^ The sheath around the first leaf sent out by the germinating oat seed. 



