410 8. INHIBITOR DISTRIBUTION IN LIVING ORGANISMS 



1957). It is interesting that reducing the oxygen tension of the roots slowed 

 down the loss of a-methoxyphenylacetate, indicating an active process in 

 the movement. The translocation of the herbicide 2,4-D from leaves is 

 also dependent on either photosynthesis or added sugar. Although the bear- 

 ing of these results on inhibitor translocation may be tenuous, the author 

 believes that similar mechanisms will be operative and identical pathways 

 demonstrable when experiments with inhibitors are done. 



There is, however, one complication in the transport of inhibitors. In 

 phloem tissue the inhibitor may depress the metabolic components of the 

 process and thereby suppress the mechanism for its own movement and 

 dissemination. Destruction of phloem is a characteristic result of the appli- 

 cation of 2,4-D and if the applied concentration is too high, it cannot reach 

 other parts of the plant because the transport system is inactivated. If 

 enzyme inhibitors are to be distributed throughout a plant by foliar appli- 

 cation, their concentration in the phloem cells must be below the level 

 required to produce appreciable inhibition of exergonic metabolism or 

 to otherwise interfere with the translocation mechanisms. 



It is likely that some upward transport can occur in the phloem and 

 that occasionally downward movement may be observed in the xylem. The 

 importance of the various conducting systems depends on the type of plant, 

 the condition of the plant, the substance under study, and the region of the 

 plant. Circulation of a substance may also occur in the following way. 

 The material passes out of the leaf into the stem phloem and as it descends 

 it diffuses out into the xylem and is transported upwards. This has been 

 shown with P^- in cotton plants (Biddulph and Markle, 1944). Such a cir- 

 culation ensures more rapid and complete distribution of the substance. 

 For our present purpose, the important aspect of the problem is that the 

 distribution of an inhibitor in various plant tissues depends on quite com- 

 plex mechanisms of transport, processes which can easily be aifected by the 

 inhibitor. The amount of uptake of an inhibitor by various tissues will 

 vary, just as in animals, with the concentrations presented to the tissues, 

 and this will depend on transport pathways. Accurate analyses of inhibitors 

 in plant tissues are much needed for a better understanding of the differ- 

 ential effects produced. 



DISTRIBUTION OF INHIBITORS WITHIN CELLS 



There are two primary problems: the total concentration of the inhibitor 

 in the cell relative to the concentration outside, and the distribution of 

 the intracellular inhibitoi between the various structures or compartments 

 making up the cell. One would particularly like to know in any case the 

 concentration of inhibitor at the site of the enzyme it is acting upon. We 

 have seen that analyses of organs for inhibitors are not always reliable as 



