DISCUSSION 



Results of this investigation are in conflict with findings of 

 Pieterse (1979) and Pieterse and Roorda (1982) who reported a ten-fold 

 enhancement of the effects of 2,4-D when simultaneously treated with 

 gibberellic acid. However, differences in experimental design and 

 environmental controls probably account for the lack of agreement. 

 Pieterse and Roorda (1982) conducted their studies in concrete reser- 

 voirs in a heated greenhouse, utilized an atomi zing-type spray apparatus, 

 and applied spray volumes of 40 1/ha or 4 ml/sq m. This study was con- 

 ducted outdoors in plastic-lined metal containers using a hand-held non- 

 atomizing sprayer and in an undisturbed infestation of waterhyacinths in 

 the St. Johns River using an airboat spray system. Spray volumes were 

 934 1/ha or 93.4 ml/sq m. 



Hitchcock et al . (1949) reported quicker killing of individual 

 waterhyacinths when sprayed with an atomizer as compared to similar 

 rates of 2,4-D sprayed on undisturbed plants in the field. Hitchcock 

 et al . (1949) and Koch et al . (1978) reported increased 2,4-D efficacy 

 under greenhouse conditions due to more effective wetting and the fact 

 that spray solution which does not fall directly on the target plants 

 may be trapped on the water surface of the experimental containers, 

 allowing greater absorption by plant roots than would be the case under 

 natural conditions. These observations also provide a partial explana- 

 tion for the field trials in Lake Dexter (Part 3) not producing the same 

 level of efficacy observed in the small plot evaluations (Part 1). 



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