Il6 PLANT GROWTH SUBSTANCES 



of the changes in respiratory capacity, chemical composition, and gross 

 histology in bean stems after treating the plant with i,ooo ppm. of 

 2,4-D. Within 24 hours there was a significant difference between treated 

 and control tissues in oxygen uptake on a dry weight basis, though not 

 on total nitrogen basis, and before any signs of abnormal meristematic 

 activity. Maximum differences in oxygen uptake and anaerobic carbon 

 dioxide evolution occurred by the seventh day again with the treated 

 tissue higher on a dry weight basis and lower on a nitrogen or protein 

 basis. Anaerobic glycolysis on a nitrogen basis showed the largest dif- 

 ference, with the treated slices less than one-third the control value. 

 This difference was also reflected in the characteristically lower res- 

 piratory quotient of treated tissue. While this type of analysis gives a 

 more complete picture of the respiratory changes following treatment 

 it is not sufficient. For example, it does not distinguish between direct 

 action of the growth substances on the respiratory mechanism per se and 

 indirect effects due to changes in the available substrate. The best we can 

 conclude at present from the toxic effects on larger intact plants is that 

 there is a pronounced alteration in metabohsm that seems to be closely 

 associated with respiratory processes. 



Respiratory Changes in Tissue Slices and Enzyme Effects 



Another type of approach is found in the in vitro treatment of tissue 

 slices which has the advantage of limiting the variability in the plant 

 material and affording better control of treatment. Bean stem sHces 

 similar to those described above (30) were treated with o.i to 100 ppm. 

 of 2,4-D in aerated aqueous media and the changes in oxygen uptake 

 measured after 24 to 48 hours. No significant effects were found at o.i 

 ppm.; at i and 10 ppm. results ranged from slight inhibition to marked 

 acceleration but still without clear evidence of stimulated meristematic 

 activity; and at 100 ppm. the treated slices were inhibited about 80 

 per cent. Mitchell, Burris, and Riker (21) have made a more extensive 

 study of this kind in which the inhibition of oxygen uptake by 0.002 

 M. 2,4-D and lAA (350 and 410 ppm. respectively) in root and stem 

 slices of several species was measured over shorter intervals. The per- 

 centage inhibition was greater in roots than in stems, except for lAA 

 on tomato, and greater with 2,4-D than lAA, except for tomato roots. 

 They also found that lAA of about 100 ppm. caused no change in res- 

 piratory quotient although there was 47 per cent inhibition of oxygen 



