The theme for our meetings this year is “A Positive Concern for Society.” It is in this context that I will talk about “Weed Science in Perspective.” In doing so, we need to consider where we are, where we have been, and where we are going.

Many speakers preface their remarks to a group such as this with the statement, “I am most happy to be here, and have the opportunity to discuss with you…”. I have deliberately avoided that opening remark in most talks, but in this case that bit of rhetoric is appropriate. I am indeed most happy to once again address that professional society with which I have been most closely and most happily associated.

When the topic of this talk was first discussed, it was suggested that I might deal with developments in weed science outside the USA, but one had only to pause for a moment to realise that this was a hopelessly vast topic which could not usefully be dealt with in one presentation. I had, therefore, to choose between discussing the advances occurring in the more developed territories such as Europe and Japan and the advances or lack of advance in the less developed countries. It was natural enough for me to narrow my discussion to the developing countries, not only because of my personal experience in this sphere, but also because I believe there is a great deal to consider in this context which is of relevance to the theme for today, namely “a positive concern for society”.

Seedlings of switchgrass (Panicum virgatum L. ‘Blackwell’), sideoats grama (Bouteloua curtipendula [Michx.] Torr. ‘Premier’), and native vine mesquite (Panicum obtusum H. B. K.) tolerated 0.28 kg/ha of 3,6-dichloro-o-anisic acid (dicamba) applied preemergence. After emergence, sideoats grama tolerated 0.56 kg/ha dicamba. Preemergence or postemergence applications of 1.12 and 2.24 kg/ha severely retarded shoot production of all three species. Soil residues in the surface 2.5 cm reduced root production of grass seedlings, whereas placement of dicamba 7.6 or 15.2 cm deep was not detrimental. All grass species germinated and grew for 21 days in soil containing up to 63 ppb of dicamba without reduction in shoot production. Overall order of susceptibility to dicamba was ranked as vine mesquite > switchgrass > sideoats grama.

Seeds of six weed species were buried in the field at depths of 2.5, 7.6, and 15.2 cm. Seed germination after various light and temperature treatments was determined before burial and after recoveries in darkness at 3-month intervals over 12 and 20-month periods. In one experiment the five species studied were ungerminable before burial. After 6 months burial, most species responded to light but became ungerminable again after this time. These results suggest cycles of dormancy where seeds pass from total dormancy at maturation to a phytochrome-controlled dormancy after burial and then return to total dormancy. During the phytochrome-controlled dormancy period, some species were highly light sensitive. In another experiment the response of yellow rocket (Barbarea vulgaris R. Br.) seeds to thirteen light and temperature treatments was determined before and after burial. Without light treatment, germination of the recovered seeds at various temperatures was low. Germination was usually high after brief exposures to red light, particularly in seeds recovered from 15.2 cm. Very large increases in germination were induced by light exposures of 3 × 10−9 Einsteins cm−2 incident in the 600 to 680-nm region (1 sec). The evidence suggests changes in total phytochrome (Ptot) are responsible for a seasonal change in light sensitivity and a previously undescribed type of dormancy.

Removal of nitrogen and phosphorus by waterhyacinth [Eichhornia crassipes (Mart.) Solms] in static and flowing water was investigated. Milligrams of phosphorus absorbed per plant per day in static water averaged 1.1, 2.1, 3.1, and 1.6 in 10, 25, and 50% Hoagland's solution and in sewage effluent, respectively, while in flowing water the values were 1.7, 2.5, and 3.3 for 10, 25, and 50% Hoagland's solution. Milligrams of nitrogen absorbed per plant per day from these same solutions averaged 5.3, 11.4, 19.8, and 6.6 from static water and 9.9, 18.4, and 20.8 from flowing water. Transpiration per plant per day averaged approximately 175 mi in static water and 225 ml in flowing water. One hectare of waterhyacinth plants under optimum conditions could absorb the average daily nitrogen and phosphorus waste production of over 800 people.

The effects of repeated annual applications of nine herbicides on a planting of asparagus (Asparagus officinalis L. ‘Martha Washington’) were studied over a 7-year period. Weed competition during the cutting season seriously reduced yields. Large populations of common lambsquarters (Chenopodium album L.) developed where 3-amino-2,5-dichlorobenzoic acid (chloramben) and N,N-dimethyl-2,2-diphenylacetamide (diphenamid) were used. Large increases in redroot pigweed (Amaranthus retroflexus L.) occurred where dimethyl tetrachloroterephthalate (DCPA) was used. The fiber content of asparagus treated with 3-(p-chlorophenyl)-1,1-dimethylurea (monuron) and 3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea (linuron) increased during slow growing conditions. Spear emergence in the spring was delayed when 2,6-dichlorobenzonitrile (dichlobenil) was applied near bud break. There was no indication that continuous use of any of the herbicides studied resulted in a buildup in the soil or plants causing a deleterious effect upon the yield of asparagus.

Annual applications for 4 years of 2,6-dichlorobenzonitrile (dichlobenil), N,N-dimethyl-2,2-diphenylacetamide (diphenamid), 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 3-amino-2,5-dichlorobenzoic acid (chloramben), and 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat) to a red raspberry (Rubus sp. ‘Newburgh’) planting had no direct deleterious effect upon the raspberry plants. Dichlobenil, simazine, and paraquat gave excellent weed control. Weed competition in control plots seriously reduced yields. Shoot development was delayed when dichlobenil was applied near the time of normal bud break.

Dimethyl tetrachloroterephthalate (DCPA) caused localized swelling of the hypocotyl of tomato (Lycopersicon esculentum Mill. ‘VF-99′). DCPA caused phloem and xylem parenchyma cells to become multinucleate and to develop into masses undifferentiated tissue within the stele of the tomato hypocotyl. Xylem procambium differentiated earlier than that of controls and matured into shortened and often disconnected elements. Xylem formed prior to treatment was often disrupted due to pressure from the dividing xylem parenchyma. Xylem parenchyma tended to break down resulting in the formation of air cavities within the stele. Cells in the stele of treated hypocotyls tended to stain more intensely with safranin than did the cells in the steles of untreated tomato hypocotyls indicating a difference in cell wall composition. Treated tissue had a higher concentration of total oxalates than did untreated tissue.

Combinations of 4-amino-3,5,6-trichloropicolinic acid (picloram) and 5-bromo-3-sec-butyl-6-methyluracil (bromacil) pellets applied to the soil gave antagonistic responses on oats (Avena sativa L. ‘Clintland 64′) with either 5.6 plus 11.2 kg/ha or 11.2 plus 11.2 kg/ha of picloram plus bromacil. In nutrient culture a significant antagonistic response occurred over a 15-day period with a picloram to bromacil ratio of 30 to 1. Of the ratios evaluated, picloram appeared to interfere with the phytotoxic effect of bromacil. The uptake of 14C-bromacil alone by oats from nutrient solution was nearly double that of 14C-bromacil when combined with picloram. Thin layer chromatographic analyses revealed that 47% of the 14C activity from bromacil in the oat was intact bromacil and that 70% of the labeled activity from picloram was unaltered picloram. The loss of nutrient solution by evapotranspiration was less from containers with picloram and bromacil than from those with bromacil alone. Picloramtreated roots were shorter than untreated or bromacil-treated roots.

Reserve polysaccharides were stored at high levels in rhizomes of mugwort (Artemisia vulgaris L.). Polysaccharide level in the rhizome dropped within 24 hr after treatment with 2,3,6-trichlorophenylacetic acid (fenac). Fructosans were hydrolyzed to yield increased levels of simple sugar. Sucrose concentration was slightly lower in treated rhizomes. Glucose was low in untreated rhizomes, and fenac treatment resulted in slight increases in glucose levels. Rhizomes became enlarged, the epidermis was disrupted, and “root primordia” were observed in rows parallel to the main rhizome axis after 1 to 5 days in 10 ppmw of fenac solution. The new primordial growth occurred through activation of the interfascicular cambium.

An oil soluble formulation of the dodecyl (51% w/w) and the tetradecyl (13% w/w) amine salts of (2,4-dichlorophenoxy)acetic acid (2,4-D) was diluted with diesel oil and applied in homogeneous sprays of 100, 200, and 400-μ. droplets to seedlings of sunflower (Helianthus annuus L. ‘Peredovik’). At each droplet size all combinations of five dosage levels and five spray volumes were compared. For the same applied dosage the amount of stem curvature increased as droplet-size was decreased. When compared with 100-μ droplets, approximately three times and six times as much active ingredient was needed to produce the same effect with 200-μ and 400-μ droplets, respectively. For a fixed droplet-size, the effective dosage could be increased equally well by increasing the concentration or by increasing the number of droplets per unit area.

Corn (Zea mays L.) was grown continuously without tillage (no-tillage) and with conventional tillage for 7 years to evaluate several herbicides for use in both crop culture systems. The only consistently satisfactory herbicide combinations for the no-tillage corn were 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) and 1,1′-dimethyl-4,4′bipyridinium ion (paraquat) or 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), simazine, and paraquat. Annual weed population shifted rapidly with different herbicide systems; fall panicum (Panicum dichotomiflorum Michx.) was the major annual weed where triazines were used as the residual herbicide. After several years of corn grown with no-tillage, hempdogbane (Apocynum cannabinum L.) became a significant problem in some plots. Corn yields with no-tillage were equal to yields in tilled areas provided weed control was satisfactory.

Thirteen selections of kochia (Kochia scoparia (L.) Schrad.), self-pollinated for four generations, varied in response to (2,4-dichlorophenoxy)acetic acid (2,4-D) and 3,6-dichloro-o-anisic acid (dicamba). The most susceptible selection was injured more by 0.35 kg/ha of the dimethylamine salt of 2,4-D than the most tolerant by 0.70 kg/ha based on visual injury rating, growth in plant diameter relative to untreated controls, and seed production. The 0.28 kg/ha rate of dicamba injured the most susceptible selection as much as 0.54 kg/ha injured the most tolerant based on visual injury rating and change in plant diameter. Plant size and maturity influenced the response of kochia to herbicides, but the differences in response among selections could not be explained by variations in size or maturity. All kochia selections were tolerant to 4-amino-3,5,6-trichloropicolinic acid (picloram) at 4.28 kg/ha.

Floral induction in kochia (Kochia scoparia (L.) Schrad.) was influenced more by light period and temperature than by soil fertility or soil moisture. Floral induction occurred when the light period per 24-hr cycle was shorter than a critical length. However, the critical duration of light per 24-hr cycle at which floral induction occurred varied from about 13 to 15 hr among kochia selections. The time from emergence to flowering varied from 57 to 100 days. The kochia selection from New Mexico required a shorter light period and a longer time after emergence for floral induction than did selections from North Dakota, South Dakota, and Minnesota. Kochia selections from the latter three states varied greatly in flowering date.

Isopropyl m-chlorocarbanilate (chlorpropham) at 6.7 kg/ha applied in mid April controlled dodder (Cuscuta campestris Yunck. and Cuscuta indecora Choisy) that emerged in May but did not consistently control dodder that emerged in June. Mid March applications sometimes controlled dodder that emerged in May but did not control that which emerged in June. When p-chlorophenyl N-methylcarbamate (PCMC), an inhibitor of microbial enzyme activity, at 1.7 kg/ha was applied with chlorpropham at 6.7 kg/ha in mid March or mid February, dodder that emerged in May and June was controlled consistently. A reduced rate (4.5 plus 1.1 kg/ha) of the combined materials applied in mid March also consistently controlled dodder that emerged in May and June.

The effect of timing of preemergence application of N,N-dimethyl-2,2-diphenylacetamide (diphenamid) on phytotoxicity to pepper (Capsicum annuum L.), efficacy of weed control, and damping-off incidence was studied in greenhouse and field experiments. Delay of application from day of sowing to close to emergence reduced phytotoxicity to pepper while only partially reducing herbicidal action. Addition of a mixture of the contact nonresidual herbicides 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium ion (diquat) and 1,1′-dimethyl-4-4′-bipyridinium ion (paraquat) to a late preemergence application of diphenamid resulted in control of weed seedlings which emerged after sowing of pepper and significantly increased the yield. The increase of Rhizoctonia damping-off incidence due to diphenamid was also reduced by delayed application.

Investigations with exogenous test solutions showed that abscisic acid (ABA) inhibited germination of nondormant seeds and that gibberellins (GA) promoted the germination of yellow foxtail (Setaria lutescens (Weigel) Hubb.) The effect of the promoters was much more readily demonstrated with excised caryopses than with intact seeds. Cytokinins reversed ABA inhibition of germination; GA did not, but the two promoters in combination were more effective than the cytokinin alone. The reversal occurred only at one end of the embryonic axis, the shoot end. An endogenous inhibitor was extracted from dormant and nondormant yellow foxtail seeds and compared with ABA by chromatography, electrophoresis, and bioassays. The extracted inhibitor was very similar to ABA in six paper-chromatography solvent systems, three paper electrophoresis runs at different pH values, and three bioassays. Paper electrophoresis was an effective means of separating the two plant hormones, ABA and gibberellic acid (GA3).

Tobacco (Nicotiana tabacum L. ‘Samsun N. N.’) leaves were treated with (2,4-dichlorophenoxy)acetic acid (2,4-D) at a concentration of 500 mg/L by the syringe infiltration technique and then sampled at regular intervals. Electron microscopic examination of the treated tissue revealed that the response of the mesophyll cells to 2,4-D was related to the age of the leaf at the time of treatment. Expanding leaves from the upper portion of the plant exhibited a high degree of tolerance to 2,4-D. Mature, fully-expanded leaves from the lower part of the plant were highly sensitive to the herbicide, with the mesophyll cells undergoing rapid structural breakdown. This breakdown involved a rupturing and disintegration of the tonoplast, plasmalemma, and membranes of chloroplasts and mitochondria.

A 2-year field study was conducted to determine the effects of fertilization on elodea (Elodea canadensis Michx.), eurasian watermilfoil (Myriophyllum spicatum L.), and heartleaf pondweed (Potamogeton pulcher Tuckerm.). Plants, water, and sediment were sampled and inorganic mineral contents determined. Water concentrations of NO3, NH4, P, and K increased sharply following fertilization and generally reached maximum values within 7 days after treatment. Concentrations of these nutrients decreased rapidly after reaching maximum values. In the first year, plant growth was not increased due to fertilization while in the second year only heartleaf pondweed produced significantly greater yields. Elodea and eurasian watermilfoil grew better in the control environment, presumably due to less competition from algae, whose growth was increased by the addition of fertilizer. At the end of the study the top 1.3 cm of bottom sediment had increased in cation exchange capacity (C. E. C.), organic matter, total N, and available P for all treatments while the underlying sediment remained relatively unchanged. Available P was greater in the surface of the fertilized sediment than in the surface of the control sediment.