Chloroplasts isolated from spinach (Spinacia oleracea L.) leaves were used to study mechanisms of toxicity from 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron). Light was needed to initiate diuron injury. The addition of ascorbate plus 2,6-dichlorophenylindolephenol (DPIP) as an electron donor system completely protected the chloroplasts from diuron-induced toxicity. The protective effect from the electron donor system occurred only in functional chloroplasts. Diuron caused rapid and extensive chlorophyll degradation at chlorophyll: diuron ratios of 200:1 and lower. At higher ratios the effect was much less measurable. The electron donor system gave complete protection in the presence of methylamine HCl, a known inhibitor of photophosphorylation, indicating that a deficiency of ATP was not the primary cause of diuron toxicity. Time-course studies showed that carotenoid pigments began to degrade before initiation of chlorophyll degradation. These results are interpreted as supporting a hypothesis that diuron induces phytotoxicity by catalyzing lethal photosensitized oxidations in the cell. This may occur as a result of (a) a greater concentration of oxidized chlorophyll caused by an interruption of electron flow and (b) an inhibition of NADPH formation which is necessary to maintain a functional carotenoid protective mechanism.

Trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 10−5 M applied to etiolated corn (Zea mays L. ‘Michigan 500′) seedlings 6 or 12 hr before the isolation of chromatin from the roots markedly reduced ribonucleic acid (RNA) synthesis supported by the chromatin. The addition of Escherichia coli RNA polymerase failed to overcome the inhibition. Trifluralin increased the melting temperature of the chromatin. The presence of trifluralin during the isolation and reaction procedure inhibited RNA synthesis indicating possible trifluralin binding to the chromatin with subsequent reduction of template availability for transcription. Trifluralin did not inhibit chromatin activity in soybean [Glycine max (L.) Merr. ‘Hark’] seedlings.

Applications of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) at 10−4, 10−5, and 10−6 M to 4-day-old etiolated soybean [Glycine max (L.) Merr. ‘Mark’] seedlings 6 hr before the isolation of chromatin enhanced the chromatin directed ribonucleic acid (RNA) synthesis by 16, 65, and 40%, respectively. The magnitude of this inductive effect increased with an increasing interval of time from 6 to 24 hr between application and chromatin isolation. The basis for the effect appeared to be an atrazine-mediated increase in chromatin template availability. The presence of atrazine during extraction and the assay was sufficient to evoke the response. This effect on chromatin activity was not observed for 2-hydroxy-4-(ethylamino)-6-isopropylamino)-s-triazine (hydroxyatrazine) or for atrazine on corn (Zea mays L. ‘Michigan 400′).

Common milkweed (Asclepias syriaca L.) seed did not survive storage in water for 1, 2, and 12 months at 40, 30, and 5 C, respectively; but under dry storage seed viability was destroyed only at 40 C for 5 months. Stratifying common milkweed seed in water at 5 C was more effective in breaking dormancy than at 22 C. Mechanical scarification also caused a significant increase in germination. Exogenous 6-furfurylamino purine (kinetin) and gibberellic acid-3 (GA) increased germination while indole-3-acetic acid (IAA) had no effect. The optimum germination temperature scheme for common milkweed was an alternating 20 to 30 C, the maximum between 35 and 40 C, and the minimum between 15 and 20 C. Common milkweed seedlings were more susceptible to moisture stress than kochia [Kochia scoparia (L.) Schrad.] but less susceptible than hemp dogbane (Apocynum cannabinum L.) and sunflower (Helianthus annuus L.). In a greenhouse study common milkweed was less drought tolerant than eight other weed species. Common milkweed seedlings tolerated wider pH ranges than honeyvine milkweed [Ampelamus albidus (Nutt.) Britt.], kochia, and sunflower. Common milkweed seedlings were less tolerant to salt solutions than four other species. Common milkweed emerged from 6-cm depths in Sharpsburg silty clay loam (sicl) at 20 or 30 C but only from the 3-cm depth at an alternating 20 to 30 C. Results from root box studies showed that common and honeyvine milkweed produced less shoot and root weight than kochia and sunflower and had a lower growth rate index of roots than sunflower. Common milkweed seedlings resprouted 21 days after planting when clipped at ground level. As days from planting to clipping increased, sprouting activity increased.

The competitive effects of sicklepod (Cassia obtusifolia L.) on soybeans (Glycine max L. ‘Bragg’) were studied on Malbis sandy loam soil and Chesterfield sandy loam soil during 1968 to 1970. Weed density was a linear function of soybean yield for the densities studied (0 to 15 weeds/m2). The regression coefficient of weed density (weeds per square meter) on bean yield (kilograms per hectare) was −92 and −101 for the two soils, respectively. When the data were combined across both soils and years, the regression coefficient was −95. Densities of 7.7 weeds/m2 reduced soybean yield 19 to 32% and 34 to 35% on the two soils, respectively. The weight of weeds at harvest was also inversely correlated to soybean yield. Sicklepod was more competitive when grown 15 to 30 cm from the drill than in the drill. Soybean yields were reduced in some instances when sicklepod was allowed to compete for as little as 4 weeks. If weeds were removed at 2 to 4 weeks after soybean emergence, no further weed removal was needed for maximum yield of soybeans.

Shade cages admitting three quantities of sunlight were placed in a dense field infestation of Canada thistle (Cirsium arvense (L.) Scop.), and the thistles were sprayed with (2,4-dichlorophenoxy)acetic acid (2,4-D) and with 2,4-D plus glucose in 1968 and again in 1969. Necrosis was greatest under full sunlight and when glucose was added. Regrowth the years after spraying increased with decreasing daylight intensities, but it was not significantly influenced by the addition of glucose. Percentage of necrosis in the year of treatment was not correlated with the percentage of regrowth the following year.

A ‘penncross' bentgrass (Agrostis palustris Huds.) golf-type green was treated in the spring with seven herbicides at three rates each for three consecutive annual applications to assess their phytotoxicity. Herbicides which caused slight foliage injury the first year were tri-calcium arsenate, dimethyl tetrachloroterephthalate (DCPA), and polychlorodicyclopentadiene isomers (bandane); moderate injury occurred with 2,6-di-tert-butyl-p-tolyl methylcarbamate (terbutol) and N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine (benefin). No injury was observed with O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide (bensulide) and 1-(2-methylcyclohexyl)-3-phenylurea (siduron). For the second and third year bandane was reapplied at half and tri-calcium arsenate at one fourth the original rate. Loss of bentgrass sod the second year was slight with terbutol and bandane and moderate with benefin and DCPA. No sod loss occurred with tri-calcium arsenate, siduron, and bensulide. Sod loss the third year was slight with tri-calcium arsenate, moderate with siduron and DCPA, and severe with bensulide, benefin, bandane, and terbutol. A persisting sod loss the fourth season, an observation year, was nil with tri-calcium arsenate; slight with siduron, benefin, and DCPA; slight to moderate with bensulide; moderate with bandane; and severe with terbutol.

Growth of tops and rhizomes of established johnsongrass (Sorghum halepense (L.) Pers.) stopped almost completely in the cool winter season. Rhizome growth started in spring later than topgrowth. Rhizomes constituted more than 90% of the total subterranean weight. In a sward of pure johnsongrass, 60%, 30%, and 10% of the total subterranean weight were found in the 0 to 15-cm, 15 to 30-cm, and 30 to 45-cm soil layer, respectively. Water-soluble sugar content in rhizomes was high in early winter and in early summer and low in early spring and in fall. Less water-soluble sugars were found in shallow than in deeper rhizomes. In winter the sugars accumulated mainly in the deepest rhizomes. No clear relation was detected between the sprouting percentage of rhizome buds and growth, sugar content, or seasonal climatic factors. No case of complete sprouting inhibition was recorded during the year.