

410 Morpho genetic Factors 



In a disease of rice produced by the fungus Gibberella fujikuroi, it 

 was observed some years ago that many of the infected plants grew 

 taller than uninfected ones (Kurosawa, 1926). Young and uninfected 

 rice plants treated with culture filtrates of this fungus grew unusually 

 tall. A similar increase in growth was observed when this filtrate was ap- 

 plied to some other plants, both monocotyledons and dicotyledons. Sev- 

 eral different but related substances were purified from Gibberella and 

 are now commonly termed the gibberellins. Their nomenclature is still 

 somewhat confused but they may be named gibberellin A ls A 2 , and A :i , 

 the last being the best known and often called gibberellic acid. 



Gibberellin commonly produces a very marked increase in stem 

 elongation ( Fig. 18-27 ) . This is particularly conspicuous in certain dwarf 





600 



500 



400" 



Sprayed with 20 ppm. 

 Gibberel lie Acid 



E 

 « 

 10 



« 



> 

 4 



300- 



200 



weeks after spraying 

 Fig. 18-27. Relation between concentration of gibberellic acid and plant height in 

 bean plants. ( From Gray. ) 



plants, notably peas. Brian and Hemming (1955) induced a fivefold 

 increase in height in such plants, bringing them up to the size of tall 

 races, by applying a little of this substance to one of the leaves. It had 

 no effect on plants of the tall races. The length but not the number of 

 internodes was increased. The so-called "slender" mutants of peas, 

 which are tall but spindly, showed no effects of gibberellin treatment. 

 Phinney ( 1956 ) found that gibberellin caused some dwarf mutants in 

 maize to grow as tall as the normal plants from which they had been 

 derived but some other dwarf races showed little response. Tall plants 

 were unaffected. The relation of gibberellin to dwarfing is evidently a 

 complex one. Most of the elongating effect is caused by increase in cell 

 length rather than in cell number. There are a few cases, however, where 

 cell division as well as cell elongation has been stimulated. 



