FREDERICK G. SMITH 115 



had reached complete germination. The suggestion was made that it is 

 the aerobic phase of respiration which is most sensitive to 2,4-D inhibi- 

 tion. Taylor (36) has made a similar study of the respiratory changes 

 following treatment of wheat and mustard seedlings but at lower levels 

 of 2,4-D. His data also show some evidence that CO2 evolution is less 

 affected than oxygen uptake expecially in wheat and in the earlier 

 stages of development. It is interesting to note further that aerobic 

 bacteria seem to be much more sensitive to 2,4-D inhibition than 

 anaerobic types (39). The aerobic species were inhibited from 2,000 to 

 as low 0.2 ppm. in some cases while facultative and obligate anaerobes 

 were unaffected or even slightly stimulated. Further comparisons of this 

 type would be valuable, especially if organisms were selected which were 

 more similar metabolically except for oxygen requirement. 



Respiratory Changes in Larger, Intact Plants 



Most work specifically on toxic or herbicidal effects of growth sub- 

 stances has been on larger intact plants with a single dose treatment of 

 the aerial parts. Morphological effects vary widely with the conditions 

 from a stunted or suppressed development to accelerated, abnormal 

 growth. Metabolic changes may be equally profound. In general altera- 

 tions in gross carbohydrate and nitrogen metabolism are found with 

 both lAA and 2,4-D treatment at levels as low as 25 to 50 ppm. in the 

 applied solutions. It is obviously difficult to compare such dosage levels 

 with those in the work on isolated tissues or seedlings since the effect 

 may be at some distance from the point of application and the amount 

 absorbed is uncertain. However, there is characteristically a marked 

 acceleration in hydrolysis, translocation, and utilization of the carbo- 

 hydrate reserves (2,22,23,24,27,31,32) accompanied by increased protein 

 synthesis (29,30), water content (10), and mineral content (11), and 

 by varying degrees of cell division and enlargement. Further evidence 

 of accelerated metabolism was shown in Luecke, Hamner, and Sell's 

 recent report (19) of large increases in treated bean stems of the B 

 vitamins which are coenzymes in intermediary carbohydrate metabolism. 

 In some of these cases there was also a marked increase in respiratory 

 activity which may have accounted for most if not all of the carbohydrate 

 utilized (27), while in others the over-all respiratory level seemed to be 

 unchanged (23). 



A more detailed examination has been reported by the author (30) 



