Most herbicides can be recovered from plant tissue as the original molecule even 
though a major percentage of the compound may have undergone a change in chemical 
structure. Consequently, we are not certainofthe actual chemical structure through which 
many of the herbicides exert their growth-controlling actions, Conceivably, some herbi- 
cides are applied in one form, translocated in a degraded or metabolized form, and 
express inhibition at the site of action as still another chemical structure, Other chemi- 
cals seem to be absorbed, translocated, and accumulated at the site of action as the 
original molecule. 
Whether herbicides arrive at the site(s) of action in plant cells as the originally 
applied molecule, or as a degraded toxic metabolite, the accumulative evidence available 
today suggests that morphological, soil, translocation, physiological, and biochemical 
‘*obstacles’’, and their interaction with environmental influences determine the selective 
action of herbicides by influencing the concentration ofthe herbicide that reaches the site 
of action at any one time (figure 5). It is also conceivable that the site(s) of action in 
different species possesses differential susceptibility to the toxicant. 
LEAF AND SOIL 
a TRAN ATION 
OBSTACLES SLOG 
OBSTACLES 
ir SPRAY RETENTION 
PENETRATION BIOCHEMICAL 
ie i are 
ABSORPTION OBSTACLES 
MORPHOLOGICAL 
PHYSIOLOGICAL 
ADSORPTION 
DETOXIFICATION 
CHEMICAL 
APPLICATION 
AS 
CH AE SAA 
Ay Soa GOS 
otf Ot Seca e 8% 
DEGRADATION 
ACTIVATION 
MICRO ORGANISMS 
ADSORPTION 
VOLATILITY 
DECOMPOSITION 
OBSTACLES WHICH DETERMINE THE CONCENTRATION OF TOXIC MATERIAL AT THE SITE OF ACTION. 
BN-10803-X 
Figure 5 
CONCLUSION 
The fate of chemicals in plants is a relatively recent subject for scientific investi- 
gation. In view of the complexity of the subject, it is not surprising that after 10 years of 
study we still know comparatively little about the metabolism of herbicides in plants, 
Future research designed to uncover the principles which determine how and whether 
chemicals reach the growth site(s) involved in herbicidal action will undoubtedly aid 
greatly in the solution of the more difficult weed control problems. 
If the vast, versatile, efficient, and economical source of energy in herbicide mole- 
cules is to be used effectively and safely to control weeds, fundamental research must be 
conducted to develop techniques to study the effects of herbicides and their breakdown 
products on plants, soils, animals, and man, 
As new herbicides are developed their behaviorin plants and their effects on animals, 
soils, and man must also be studied before safe, effective recommendations can be made 
to farmers, 
129 
