WEED control: applied botany 419 



Translocation. In the transport of certain systemic herbicides we have a 

 curious anomaly, namely, the ability of the phloem to carry toxic chemicals 

 through living tissues at concentrations that will kill meristematic cells. Thus 

 there must be a sort of selectivity between tissue systems within the plant. 

 And we are able to utilize this selectivity for killing root systems of perennial 

 weeds, providing we keep the concentration within limits and move the 

 chemical rapidly. This demands proper formulation of the chemicals to facili- 

 tate absorption and active synthesis of foods to aid in moving them into and 

 through the plant. Some chemicals that are so moved in plants are phenyl 

 acetic acid, benzoic acid, and the herbicides amino triazole, maleic hydrazide, 

 and the phenoxy compounds. 



In plant physiology there is a long-standing controversy concerning the 

 nature of the sieve tubes of the phloem; some picture them as highly active 

 cells that move solutes by means of energy derived from respiration; others 

 view them as passive conduits through which a stream of food materials in 

 solution moves along a hydrostatic gradient developed osmotically as a result 

 of the separation of regions of synthesis from those of utilization. The observa- 

 tions on herbicide transport seem to substantiate the latter mechanism. It is 

 difficult to reconcile the translocation of toxic chemicals with the high meta- 

 bolic state postulated for the sieve tubes by advocates of the first mechanism. 

 The lack of specificity and the prolonged functioning of this transport system 

 do not fit the concept of high activity required; rather, a simple physical 

 system of a more passive nature would seem better to fit the facts. 



Mode of action. Search for the mechanisms of action of the various newer 

 herbicides is occupying the interest of many researchers. Considering first the 

 auxin-like materials, there is a large literature stemming from work on growth 

 regulators. Taking 2,4-D as an example, many enzyme systems have been 

 found to be unaffected by this toxicant. Definition of the requirements for 

 activity have been liberalized. The strict two-point and three-point concepts 

 seem not to hold; for stimulatory effects plus toxicity it seems that a ring 

 nucleus plus a side chain with an acid group and certain structural relations 

 are all that are required. Possibly chemicals lacking the stimulatory action 

 may involve simply the chain structure with the acid group, for example, 

 Dalapon. 



When we view the number of chemicals that are active, the variety of plants 

 that respond, and the drastic modifications that are found, it seems obvious 

 that processes of a basic nature, essential to the very life of the plant, are 

 being altered. Study of these processes should start early in the series of reac- 

 tions that make up the total metabolic machinery of the plant. Selectivity of 

 herbicides has been shown to depend on many differences between species and 

 their many modes of life. Some, at least, relate back to specific enzyme systems 

 and their roles in plant metabolism. It seems logical that most of these should 

 be initial, or at least intermediate, in the chain of metabolism. It seems un- 



