the inactive 4-(2,4-DB) and are not killed (62). The highly selective heribicide 4-(2,4-DB) 
was developed directly as a result of these investigations. This herbicide has shown 
excellent promise for the post-emergence control of broadleaved weeds in (a) cereals 
underseeded with certain forage legumes; (b) the establishment of pure stands of forage 
legumes; (c) forage legume seed-production fields; (d) flax; (e) tolerant crop plants such 
as corn or rice grown in the vicinity of crops highly susceptible to the phenoxyacetic and 
alpha-phenoxypropionic acids, such as cotton and grapes; and (f) certain other weed-crop 
situations (50). The new chemical should prove valuable for the control of weeds in cereal 
crops underseeded with legumes. The herbicides available prior to the introduction of 
4-(2,4-DB) could not be used safely for the control of weeds in cereal crops underseeded 
with alfalfa or other legumes inthis important phase of the corn-wheat-alfalfa -alfalfa type 
rotation, 
Recent studies have also shown that 2-chloro-4,6-bis(ethylamino)-s-triazine [sima- 
zine], a very promising pre-emergence herbicide for weed control in corn, when absorbed 
by the roots of the corn plant was converted to a nonphytotoxic form. Therefore, corn 
shows considerable tolerance to simazine, On the other hand, many important weeds do 
not effectively metabolize simazine to an inactive chemical. These weeds are highly 
susceptible to injury and are easily controlled by simazine. This is an excellent example 
of a herbicidally active chemical being metabolized in the plant to an inactive chemical. 
Thus, the metabolic fate of simazine is opposite to that of 4-(2,4-DB) when it is converted. 
Research scientists have also found recently that simazine inhibits the efficiency of 
photosynthesis in plants (43), It was theorized that if simazine inhibited the efficiency of 
photosynthesis in plants, this action should be reversible if simazine-treated plants were 
supplied with an available source of carbohydrate. By feeding glucose through the leaves 
of simazine-treated plants, it has been possible to protect plants against the phytotoxic 
effects of simazine (43). Similar results have been obtained with some of the phenylurea 
herbicides (29). 
A third fundamental study indicates another breakthrough in weed control research, 
In a series of investigations involving the response of bacteria (Escherichia coli), yeast, 
germinating seedlings, and intact plants to 2,2-dichloropropionic acid |dalapon|], it has 
been shown that dalapon and several other chloro-aliphatic acids inhibit the enzymatic 
synthesis of pantothenic acid, a B-vitamin essential to plant growth, Pantothenic acid is 
made by an enzyme which couples beta-alanine to pantoic acid, This enzyme has been 
isolated, The herbicides were shown to be competing with pantoic acid. The response 
of plants to dalapon may be due to the temporary inhibition of pantothenic acid synthesis. 
To examine this hypothesis, pantothenic acid was supplied to barley plants through their 
leaves, The exogenous source of pantothenic acid partly protected dalapon-treated plants 
against the herbicidal action of dalapon (34), 
The N-phenylcarbamate herbicides at low concentrations stimulate respirationin root 
tips of treated plants while higher concentrations markedly decrease respiration (57). 
These herbicides are also potent inhibitors of chlorophyll synthesis and cause striking 
mutagenic effects in plants (51). 
The phenoxyalkylcarboxylic acids may cause striking temporary changes in the 
chemical composition of plants (56), Numerous other instances of important breakthroughs 
resulting from fundamental research could be cited, but I believe these are adequate to 
indicate the critical need for additional fundamental research in weed control, 
METABOLISM OF HERBICIDES 
The various studies conducted during the last 10 years on the fate of herbicides in 
plants show that most herbicides are metabolized in varying degrees by plants. Some 
herbicides are metabolized so rapidly and so extensively that we are not certain of the 
toxic form of the chemical actually involved in the control of growth, Species may differ 
greatly in either (a) their ability to metabolizea specific herbicide and (b) the biochemical 
pathways by which the structure of the original molecule is altered. For some chemicals, 
these differences seem to be the determining factor in herbicidal selectivity. 
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