33 

 The physiological response of plants to 2,4-D has been attributed to 

 the inhibition and/or stimulation of respiration, blockage of pro- 

 toplasmic streaming, alteration of DNA transcription and/or RNA 

 translocation, and interference with enzyme regulatory systems (Brian, 

 1964; Wort, 1964a; Ashton and Crafts, 1973; Yamada et al . , 1974; 

 Mullison, 1982). During the mid-1950' s numerous papers appeared which 

 linked auxin action with nucleic acids (Ashton and Crafts, 1973). West 

 et al . (1960) reported an increase in the protein and RNA content of 

 cucumber stem tissue when treated with herbicidal concentrations of 

 2,4-D. This led to the assumption that the cytochemical basis of 2,4-D 

 action was a stimulation of nuclear activity and a reversion to meriste- 

 matic metabolism (Chrispeels and Hanson, 1962). It was later shown that 

 this increase in RNA was primarily in the ribosomal fraction which was 

 also accompanied by an increase in a messenger type RNA (Key and 

 Shannon, 1964; Key, 1964). Shannon et al . (1964) suggested that 2,4-D 

 induced protein synthesis and excess nucleic acids would preclude normal 

 cell function and that this was the biochemical basis for the herbicidal 

 action of 2,4-D. Chen et al . (1972) experimented with a 2,4-D tolerant 

 wheat species and a 2,4-D sensitive cucumber species and demonstrated 

 that as 2,4-D concentrations increased, the RNA content of wheat showed 

 a net decrease, whereas the cucumber RNA content increased by over 200 

 percent. Chen et al . (1972) postulated that the ability to resist 

 alteration of RNA species by a plant was the basis for the selectivity 

 of 2,4-D. Ashton and Crafts (1973) summarized the effects of 2,4-D on 

 nucleic acid and protein content of suscep table species and indicated 

 that this was the main biochemical reaction to 2,4-D. This increase in 

 RNA was attributed to an inhibition of the synthesis of ribosomal RNase 



