Soybean (Glycine max (L.) Merr., var. Chippewa 64) plants were grown in culture solutions containing 14C ring-labeled 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine). Culture solution temperatures were varied but ambient temperature about the shoots was constant. The amount of atrazine absorbed increased with increases in herbicide concentration, absorption time, and root temperature. Root temperature had more influence on atrazine accumulation in stems and leaves than in roots. Transpiration rates of atrazine-treated plants were reduced at least 50%.

Effects of three diphenylether herbicides [2,4-dichlorophenyl-p-nitrophenyl ether (nitrofen); 2,4,6-trichlorophenyl-4′-nitrophenyl ether (hereinafter referred to as MC-1478); and 2,4′-dinitro-4-trifluoromethyl-diphenylether (hereinafter referred to as C-6989)] were measured on phosphorylation and electron transport in spinach (Spinacia oleracea L.) chloroplasts, and mung bean (Phaseolus aureus L., var. Jumbo) and white potato tuber (Solarium tuberosum L.) mitochondria. All of the diphenylethers acted primarily as inhibitors of chloroplast noncyclic electron transport, and the coupled photophosphorylation. The compounds ranked in the following decreasing order of inhibitory effectiveness: MC-1478 ≥ C-6989 >> nitrofen. A site of action close to light reaction II was suggested. At high molar concentrations, marginal interference with cyclic electron transport or phosphorylation was obtained. In mitochondria, MC-1478 and nitrofen acted primarily as electron transport inhibitors with malate, NADH, and succinate as substrates. MC-1478 was a slightly stronger inhibitor than nitrofen. Only slight stimulation of ADP-limited oxygen uptake was obtained during the oxidation of NADH and succinate; whereas, strong inhibition of oxygen uptake was obtained with malate. C-6989 also weakly stimulated ADP-limited oxygen uptake with NADH and succinate but differed from the two chlorinated diphenylethers in that electron transport was not inhibited when ADP was present in excess. Interference with ATP generation could be one of the mechanisms through which the phytotoxicity of diphenylether herbicides is expressed.

Available literature indicates that relatively few agricultural leaders and farmers became interested in weeds as a problem before 1200 A.D. or even 1500 A.D. For many centuries, weed control was mostly incidental to tillage for seedbed preparation and growing of crops and to growing and cutting or pasturing of thickly planted crops. Occasional references in literature previous to 1900 mentioned use of mechanical devices and a few inorganic herbicides specifically for weed control.

State weed laws directed at control of plant diseases were enacted during 1721 to 1766, but weed and seed laws involving weeds directly were not enacted until 100 to 200 years later. Only a few extension-type publications on weeds were issued in the United States and Canada between 1860 and 1900. There was a rapid increase in such publications after 1900. Research with inorganic chemicals as herbicides was begun in the 1890's in Europe and in a few states and provinces, and was increased at a rapid pace until the early 1940's. New developments in mechanical and biological control of weeds increased steadily during the same period. However, weed control remained a relatively minor phase of agronomy, botany, horticulture, agricultural engineering, and plant physiology until the early 1950's.

About 10 years after the discovery of (2,4-dichlorophenoxy)acetic acid (2,4-D) in 1942–1944, the much increased interest of scientists, federal and state governments, industrial companies, and the general public had begun to bear fruit. The word “weed” or “weeds” began to appear in the titles of college courses and extension specialists. Weed conferences had been organized in six regions of the United States and Canada and in 10 states.

The first meeting of the Weed Science Society of America was held in 1956 and Weed Science was adopted as its official journal. The number of herbicides in general use in the United States and Canada increased from 15 in 1940 to 25 in 1950, and to 100 in 1969. The total support for weed research in 1962 in the United States was six times that in 1950. The number of full-time research and extension workers or their equivalents in part-time workers had increased 20-fold and 13-fold, respectively, over the number in 1940.

The rate of advancement in the art and science of weed control has increased so rapidly that the progress in each of the recent brief periods 1941 to 1968, 1901 to 1940, and 1800 to 1900 is considered greater than that in all previous periods, beginning about 6000 B.C.

Canada thistle (Cirsium arvense (L.) Scop.) on roadsides was controlled by 4-amino-3,5,6-trichloropicolinic acid (picloram), (2,4-dichlorophenoxy)acetic acid (2,4-D), and (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), alone and in combination at various rates. There was no synergistic or additive effect, as determined by Canada thistle response, due to the addition of 2,4-D or 2,4,5-T to picloram. Control with ¾ lb/A picloram was not significantly less than that obtained with higher rates 1½ and 2 lb/A. Picloram reduced Canada thistle stands 80 to 100% and 2,4-D generally reduced the plant population 50% or less during the year of application and the year after. Late summer appeared to be an opportune time for spraying Canada thistle with ¾ to 1 lb/A picloram.

Field-harvested johnsongrass (Sorghum halepense (L.) Pers.) rhizomes were assayed for bud dormancy and growth potential under a number of experimental conditions. Natural dormancy was not detected in single-node rhizome pieces harvested at any time of the year. Shoot growth rates were highly variable. Kinetin, benzyl adenine, gibberellic acid, and 2-chloroethylphosphonic acid were ineffective in enhancing germination or normalizing shoot growth. Scale leaves of rhizomes had no effect on bud growth. Rhizome buds were strongly influenced by apical dominance and emerged shoots partially suppressed germination of adjacent buds. The optimum temperature for bud germination and shoot growth was approximately 30 C. Bud germination was suppressed at 15 C. Johnsongrass rhizomes failed to exhibit cold hardiness at any time in their life cycle.

Uptake of herbicides by excised roots of soybean (Glycine max (L.) Merr.) plants was studied over periods of 30 min, 8 hr, and 48 hr. In the 48-hr experiment, initial rate of herbicide uptake was rapid and was greater at higher temperatures and external herbicide concentrations. Rates of initial uptake varied among herbicides. Over 30-min periods, rapid herbicide uptake by excised roots for 2.5 to 5 min was followed by much slower uptake. Total uptake for 30 min and 1 hr was in the order of 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea(linuron) > isopropyl-m-chlorocarbanilate (chlorpropham) > S-ethyl dipropylthiocarbamate (EPTC) > 3-amino-2,5-dichlorobenzoic acid (amiben) > 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine). However, total uptake after 48 hr was in the order of chloropropham > linuron > EPTC > atrazine > amiben, indicating that rates of uptake for the herbicides changed over 48 hr. Values of Q10 for herbicide uptake were always less than 1.85, indicating that physical processes were important in uptake of herbicides by excised soybean roots. Uptake of herbicides over 8 hr by roots of intact plants was similar at 5, 10, and 20 C to uptake by excised roots, but uptake at 30 C was less for roots of intact plants than for excised roots. Transpiration increased as temperature increased, but herbicide uptake and transpiration were unrelated.

The metabolic fate of root-applied, carbonyl-labeled, 14C-3-(3-chloro-4-bromophenyl)-1-methoxy-1-methylurea (hereinafter referred to as chlorbromuron) was investigated in tolerant corn (Zea mays L., var. N.J. 9) and susceptible cucumber (Cucurbita sativus L., var. Marketer) in a time-course study. The major metabolite found in corn shoots and roots was the nonphytotoxic 3-(3-chloro-4-bromophenyl)-1-methoxyurea. Evidence of “binding” was found in both shoots and roots of corn. No evidence was found for the metabolism of chlorbromuron in cucumber; however, some binding occurred in both shoots and roots.

Weed control performance of several herbicides, their effects on tobacco (Nicotiana tabacum L.), and two methods of soil incorporation were studied in North Carolina in 1967 and 1968. Herbicides which gave acceptable weed control without injuring tobacco or lowering its quality were 4-(methylsulfonyl)-2,6-dinitro-N,Ndipropylaniline (nitralin),1,1,4-trimethyl-6-isopropyl-5-indanyl ethyl ketone (D-497), methyl ester of 3-amino-2,5-dichlorobenzoic acid (amiben), N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine (benefin), S-propyl butylethylthiocarbamate (pebulate), S-propyl dipropylthiiocarbamate (vernolate), N,N-dimethyl-2,2-diphenylacetamide (diphenamid), dimethyl tetrachloroterephthalate (DCPA), 2,3,5-trichloro-4-pyridinol (pyriclor), and 2-(α-naphthoxy)-N,N-diethylpropionamide (R-7465). Soil incorporation of benefin, pebulate, and benefin + vernolate with a power-driven rotary cultivator gave superior weed control compared to incorporation by discing four times. Significant early-season stunting of tobacco plants was observed both years in plots where benefin (1.12 or 2.25 lb/A), benefin + vernolate (0.75 + 1.5 lb/A), or benefin + pebulate (1.12 + 4.0 lb/A) was incorporated by discing. No stunting was observed where incorporation was with the rotary cultivator except for benefin at 2.25 lb/A. Neither yield nor quality of the crop was reduced.

Total foliar fatty acid and hydrocarbon content of sicklepod (Cassia obtusifolia L.) treated with 0, 0.14, 0.28, 0.56, 1.12, 2.24, or 4.48 kg/ha S-ethyl dipropylthiocarbamate (EPTC) was measured by gas-liquid chromatography. Total leaflet fatty acid content increased at sublethal rates of the herbicide, but qualitative composition of the fatty acids was not affected. Percent of long chain alkane (≥ C28) content decreased as EPTC application rates increased. Palmitic (23.9%), stearic (9.1%), oleic (3.8%), linoleic (13.6%), and linolenic (33.2%) acids accounted for 83.6% of the total leaflet fatty acid content of sicklepod. Leaflet cuticle thickness of upper and lower epidermal layers decreased 15 and 20%, respectively, as herbicide concentration increased.

Absorption of (2,4-dichlorophenoxy)acetic acid (2,4-D) by honeyvine milkweed [Ampelamus albidus (Nutt.) Britt.] was 7.2, 9.3, and 10.9% of the applied herbicide at 1, 4, and 8 days after treatment, respectively. Addition of 1.0% v/v Tween 80 to the treatments increased absorption to 55.8, 71.3, and 78.7% at the same sampling dates. Metabolic alteration of the herbicide occurred only in the aerial portions of the plant, and this was not great enough to be considered as a means of resistance. Translocation of 2,4-D into the roots was more closely related to new root growth than to the amount of top growth. Plants with six leaves and no new root growth did not translocate 2,4-D into the root zone. Plants with the same number of leaves but with an average of five and 20 new roots translocated 2.5 and 7.7%, respectively, of the applied herbicide to the roots.