408 8. INHIBITOR DISTRIBUTION IN LIVING ORGANISMS 



Crafts (1951) and van Overbeek (1956) are recommended. The following 

 is a cursory discussion of those aspects of absorption and translocation per- 

 tinent to the distribution of inhibitors; the attempt has been made to in- 

 corporate recent work with radioactive substances, herbicides, and anti- 

 biotics. 



The upward movement of an inhibitor that is absorbed by the root system 

 would occur mainly in the xylem with the transpiration stream. The xylem 

 is basically dead tissue, the unit cell being the long hollow tracheid, com- 

 munication between adjacent tracheids being made through perforations 

 in the walls. The rise of fluid is due mainly to the loss of water from the leaves 

 by transpiration and to a lesser extent to root pressure, the active movement 

 of fluid by the root cells (evident in the flow of liquid from the cut surface 

 of a stem). The rate of the transpiration stream through the xylem thus 

 depends on many factors, both in leaves and roots; some of these proc- 

 esses can be affected by inhibitors as they move through the plant. A sub- 

 stance can be distributed to all the tissues of a plant by this means. Thus 

 when P^2 is applied to the roots in the form of inorganic phosphate it is 

 absorbed and moves upward rapidly, being found in the growing tips 

 within an hour, indicating rates of 50-100 em/hr (Frazier et al., 1956). 

 As pointed out many years ago (Stout and Hoagland, 1939), the diffusion 

 out of the xylem into adjacent living tissue is very rapid during transloca- 

 tion of phosphate. Little is known about the differential uptake by the 

 various tissues but differences do occur as shown in radioautographs of 

 wheat kernels during uptake of P^-. Large ionic molecules, such as penicillin 

 and streptomycin, are distributed throughout plants much more slowly, 

 due probably to the slow uptake by the roots, although it has been postu- 

 lated that a negative charge on the tracheid walls might account in part 

 for the very slow translocation of substances such as streptomycin and Mn'^"^ 

 (Pramer, 1954; Single, 1958). Within 19 hr after application of chloramphen- 

 icol to the roots of bean plants, the antibiotic was distributed throughout 

 the plant, but it required several days for the distribution to become uni- 

 form, whereas after 18 hr in streptomycin there was antibiotic only in the 

 lower parts of the plant. Lidication that uptake by roots is limiting here 

 was shown by the more rapid accumulation of these antibiotics in shoots 

 from which the roots had been cut (Pramer, 1954). 



The downward movement of carbohydrates, amino acids, auxins, and 

 herbicides from the leaves is mainly in the phloem tissue and it is likely that 

 this is the route taken after application of inhil^itors. This type of translo- 

 cation is complex because phloem is a living tissue and movement of a 

 substance occurs through protoplasm; indeed, the mechanisms involved 

 are not at present well understood. The unit cell is the sieve tube and the 

 protoplasm of each sieve cell is in contact with the protoplasm of adjacent 

 sieve cells through the perforations in the walls. It is not known if the pro- 



