20 

 Interaction with Other Plant Growth Substances 



Interaction or synergism between gibberellins and other plant 

 growth substances, auxins in particular, has been evaluated in a variety 

 of plant species and experimental conditions. Nitsch and Nitsch (1956) 

 evaluated effects of various combinations of IAA and GAo on oat 

 col eop tiles and the first internode and demonstrated less than additive 

 growth responses in all but one combination of the two substances. Kato 

 (1958b) noted increased growth response of GA and IAA in cucumber over 

 IAA alone; however, since the response was less than additive, no 

 synergism was implied. Marth et al . (1956) commented that wounding 

 of several species prior to application of GA3 increased the 

 GA2 response, suggesting synergism with wound hormones. Stowe and 

 Yamaki (1957) cited increased responses to combinations of IAA and gib- 

 berellin which were more than additive in pea epicotyls. Ng and Audus 

 (1964 and 1965) demonstrated the requirement for an unidentified endoge- 

 nous substance from Avena internodes in order to induce a synergistic 

 interaction between GA., and applied IAA or 2,4-D on Avena stem segments. 

 Audus (1972) reported a three-fold increase in sensitivity of rice 

 leaves to IAA when also treated with GA.,. Pieterse et al . (1980) and 

 Pieterse and Roorda (1982) reported a tenfold increase in the activity 

 of 2,4-D on Eichhornia crassipes when applied in combination with GA^. 



The observed interactions between gibberellins and applied natural 

 and synthetic auxins have revolved around two possible mechanisms (1) 

 an alteration of the endogenous auxin levels, or (2) an alteration of 

 the rate of translocation of auxins. Numerous studies have reported 

 enhanced biosynthesis of IAA gibberellin (Galston and Purves, 1960; 



