966 7. MERCURIALS 



curials act may often be unexpected, is the zinc-deficiency disease of coffee 

 trees in Kenya due to spraying with mercurial fungicides (Bock et al., 1958). 

 Not only do the plants exhibit typical signs of zinc deficiency — chlorosis, 

 abnormal growth of shoots and leaves, and short internodes — but the zinc 

 content is reduced to 25% of normal. In the promotion program for mer- 

 bromin, Macht (1931 a) purported to show that organic mercurials are less 

 toxic than Hg++ to plants, but his data are equivocal, since Hg++ is toxic 

 around 0.1 mM, while merbromin inhibits growth slightly at 0.0013 mM, 

 50% at 0.043 mM, and 81% at 1.29 mM. The growth of Avena coleoptiles 

 and of pea stems is readily inhibited by the mercurials, PM being much 

 more potent than p-MB (see accompanying tabulation) (Thimann and Bon- 



ner, 1949). It is likely that the carboxylate group prevents the p-MB from 

 penetrating as well as PM. These results will suffice to demonstrate growth 

 inhibition by the mercurials, and we shall turn to what little evidence is 

 available for the mechanisms involved. 



Onion roots exposed to 0.0075 mM PM develop terminal swellings and 

 growth is immediately stopped; however, after a day new growth starts 

 distal to the swelling (Macfarlane and Nadeau, 1948). Doubling the con- 

 centration leads to 100% mortality of the roots. Hg++ even at 0.05 mM 

 does not cause terminal swelling, inhibits growth only 15%, and does not 

 kill any of the roots. Macfarlane (1951) pointed out that PM acts on onion 

 roots cytologically like colchicine, in that spindles are abnormal, chromo- 

 some movement is impeded, and polyploidy results in the zone of cell en- 

 largement proximal to the meristem. In addition, there is chromosome 

 stickiness, fragmentation, and aggregation. Although mitotic and chromo- 

 somal disturbances certainly occur, there may be some question as to the 

 validity of terming these effects mutagenic or radiomimetic (Macfarlane, 

 1953). Meyer (1948) also observed such changes in the root tips of Crepis 

 capillaris exposed to 0.013 mM Hg++, the sister telophase nuclei often being 

 connected by chromatin bridges, with some breakage and recombination, 

 leading to 80% diploid metaphases and 1.5% tetraploid metaphases. The 

 formation of cell wall material in the microspores of excised Lilium henryi 

 anthers is reversibly blocked by 0.01 mM p-MB and the progress of meiosis 



