0141733 



respectively. Zinc phytotoxici ty to barley seedlings was reported 

 | in the range of 160 to 320 ppm (Davis et al . 1978). It is 



apparent that the suggested plant tissue phytotoxic level of 500 

 ppm zinc will produce phytotoxici ty in most plants. Only two 

 values in excess of the suggested 500 ppm plant tissue phytotoxic 

 level were found not to be phytotoxic (508 ppm for corn forage and 

 527 ppm for lettuce shoots) (Mortvedt and Giordano 1975, Mitchell 

 et al. 1978). Phytotoxic criteria levels reported in the litera- 

 ture ranged from 100 to 400 ppm zinc (Kabata-Pendias and Pendias 

 1984) . 



The suggested 50 ppm tolerable zinc level in vegetation is 

 based on the lowest phytotoxic tissue level found for crops of 

 interest (barley, oats, wheat, alfalfa and other forage crops). 

 The value 51 ppm was reported for a 20 percent yield reduction in 

 wheat (Boawn and Rasmussen 1971). These authors also reported a 

 20 percent yield reduction for sweet corn and sorghum at zinc 

 tissue levels of 41 and 34 ppm respectively. These values were 

 the only occurrences of phytotoxici ty found in the reviewed 

 I literature at levels less than the 50 ppm suggested tolerable 

 concentration. 



133 



