Oxyfluorfen [2 - chloro -1 - (3 - ethoxy - 4 - nitrophenoxy) -4 - (trifluoromethyl)benzene] and RH-8817 {ethyl 5 - [2-chloro - 4 -{trifluoromethyl) phenoxy] - 2 - nitrobenzoate} require light for activation. Removal of carotenoids by treating corn (Zea mays L.) seedlings with norflurazon [4 - chloro - 5 - (methylamino) - 2 - (α, α, α - trifluoro -m - tolyl) - 3 (2H) - pyridazinone] rendered them partially tolerant of oxyfluorfen and RH-8817 even when grown in the light. The activation of substituted diphenylether herbicides may occur as a result of the absorption of light energy by carotenoids.

Comparative growth, development, and resource allocation of triazine - susceptible and triazine - resistant biotypes of common groundsel (Senecio vulgaris L.), grown under two light regimes, were studied over a 7 - week period. Dry-matter production, height, number of leaves, and leaf area of the susceptible biotype were greater than those of the resistant biotype at all harvests under both light regimes. Root/shoot ratios were lower in the resistant than in the susceptible biotype under high light. Lower values for these parameters in resistant plants are due to lowered photosynthetic capacity, which limits growth and lowers relative root production. Net assimilation rate (NAR) was lower in resistant than susceptible plants when grown under high light, but mean leaf area ratio over a harvest interval (L) of resistant plants was higher than that of susceptible plants under high light. Relative growth rate (RGR) was higher for susceptible plants before day 42 and higher for resistant plants after day 42 under high light. A lower plastochron index (PI) of resistant plants up to 5 weeks old, relative to susceptible plants of the same age, suggests that, when grown under high light, resistant plants are developmentally at least 2 days behind susceptible plants. Shading lowered dry-weight production, height, number of leaves, leaf area, NAR, RGR, PI and root/shoot ratios of both biotypes. Values for these growth parameters for susceptible plants were similar to or larger than for resistant plants when grown under low light. In both biotypes, resource allocation patterns and L shifted towards increased shoot and leaf production in response to shading.

Yields of drill-seeded paddy rice (Oryza sativa L. ‘Lebonnet’) at optimum stands of 215 to 270 plants/m2 at Stuttgart, Arkansas, were reduced 9, 18, 20, and 36% by bearded sprangletop [Leptochloa fascicularis (Lam.) Gray] densities of 11, 22, 54, and 108 plants/m2, respectively. There was a linear decrease in rice grain yield of 21 kg/ha for each bearded sprangletop plant per square meter. Weed densities of 54 and 108 plants/m2 reduced head-rice yields (whole milled kernels) and a density of 108 plants/m2 reduced germination of rice seed. The number of bearded sprangletop panicles produced per weed plant decreased as the weed density increased.

Field and greenhouse research was conducted during 1980 and 1981 to evaluate the effects of carrier volume, surfactant concentration, and treatment date on glyphosate [N - (phosphonomethyl)glycine] toxicity to annual-grass weeds and volunteer small grains. Glyphosate phytotoxicity increased as carrier volume was decreased from 190 to 24 L/ha. The presence of a surfactant in the spray solution did not influence grass control when glyphosate was applied in a carrier volume of 24 L/ha. When glyphosate was applied in 48 or 95 L/ha, the presence of surfactant resulted in better grass control than glyphosate without surfactant. When applied in 190 L/ha, glyphosate with 0.5% (v/v) surfactant gave better grass control than glyphosate alone or commercially formulated glyphosate. When glyphosate was applied to plants under water stress, little control was achieved regardless of plant growth stage. Glyphosate application to grass after head initiation also resulted in reduced control. Maximum weed control with glyphosate was attained when applications were made to seedlings growing actively because of adequate soil moisture and favorable temperatures.

Spotted spurge (Euphorbia maculata L.) and prostrate spurge (E. supina Raf.), both in subgenus Chamesyce, were examined by light and electron microscopy using a caffeine - fixation technique to sequester the phenolic pools intercellularly. Both species have typical dicotyledon-type Kranz anatomy. Sequestered phenolic pools were located in vacuoles in epidermal and mesophyll cells. Only in spotted spurge, however, were additional phenolic pools formed in bundle - sheath cells. This study was undertaken because allelopathy has been demonstrated in prostrate spurge and because phenolic compounds have been implicated in allelopathy. These results would indicate that spotted spurge should also be allelopathic.

Combinations of herbicides applied preplant incorporated and preemergence plus at layby were evaluated in corn (Zea mays L.) at three Nebraska locations in 1977 through 1980 for their effectiveness in providing season-long weed control. Pendimethalin [N-(1 -ethylpropyl) - 3,4-dimethyl - 2,6 - dinitrobenzenamine], pendimethalin + atrazine [2- chloro - 4 - (ethylamino) -6 - isopropylamino) -s- triazine ], metolachlor [2 - chloro - N - (2 - ethyl - 6 - methylphenyl) - N - (2 - methoxy -1 - methylethyl) acetamide], alachlor [2 - chloro -2′,6’ - diethyl - N- (methoxymethyl)acetanilide], cyanazine {2 -[[4 - chloro- 6 - (ethylamino) - s - triazin-2 - yl] amino] -2 -methylpropionitrile}, and cyanazine + atrazine were shown to be selective to corn and effective in controlling late - germinating weeds. Irrigated - corn yield was not increased by layby herbicide applications on the weed species and weed densities present in these experiments.