Scarification in sulfuric acid consistently increased germination of Opuntia edwardsii sp. nov, O. discata Griffiths, and O. lindheimeri Engelm. ♯ OPULI seeds over that of untreated seeds. Optimum constant temperatures for germination were generally 25 to 35 C and germination was not enhanced by alternating temperatures. There was a trend for increased germination following leaching in water for 12 h which suggested the presence of chemical germination inhibitors. Seeds passed through the digestive tracts of cattle exhibited average germination percentages that were 1.5 times greater than seeds removed from ripe fruits.

Velvetleaf (Abutilon theophrasti Medic. ♯4 ABUTH) seeds were exposed to variable moisture and temperatures from 15 to 100 C. As temperature increased, the percent of permeable seeds increased. Soaking 1 h at 70 C reduced hard seeds from 99 to 15 percent. Increased available water increased the effect of temperature on reduction of hard seeds. Thus, immersion in hot water was more effective than dry or humid heat. High temperature acted within a few minutes to reduce hardseededness. Viability of hard seeds was reduced at temperatures above 70 C. Germination of permeable seeds was optimal at 24 to 30 C and declined above 35 C. Germination of permeable seeds decreased after pretreatments above 40 C. Temperature and duration of heat exposure were inversely related to germination inhibition. Recently harvested seeds (3-yr-old) remained hard under alternating 4 to 34 C but became permeable after drying at 34 C. Old seeds (15-yr) did not become permeable after drying.

Rough-rice yield of drill-seeded paddy rice (Oryza sativa L. ‘Lebonnet’ and ‘Starbonnet’) at stands of 215 to 270 plants/m2 at Stuttgart, Arkansas were reduced 18% by season-long competition of spreading dayflower (Commelina diffusa Burm. f. ♯3 COMDI) at a density of 22 plants/m2. Competition durations of 20, 40, 60, or 80 days did not reduce rice yields. None of the durations of competition affected head-rice yields (percent whole milled kernels) or germination of rice seed.

Dispersal studies of musk thistle (Carduus nutans L = C. thoermeri Weinmann) seeds by wind were performed to determine the percentage of seeds removed from the vicinity of the plant where the seeds originated. For the different wind velocities tested (up to 5.6 m/s) less than 1% of the seeds were blown further than 100 m. Most seeds were deposited within 50 m of the point of release. A Gaussian model of seed dispersal based on experimentally determined parameters and published data adequately described these results.

Field tests in 1980 and 1981 indicated that smooth pigweed (Amaranthus hybridus L.) competition affected cotton (Gossypium hirsutum L. ‘GSA 71′) water relations by reducing plant water status early in the season and by shading late in the season. Early in the growing season, cotton leaf water potential and turgor pressure reductions as great as 0.53 and 0.21 MPa, respectively, were measured at a smooth pigweed density of 2.5 plants m-2. Late in the 1981 growing season, smooth pigweed reduced the photon flux density reaching the cotton canopy by 90% at midday. The shaded conditions resulted in increased cotton water potential under low soil water conditions, but the inability to reduce solute potential lowered the turgor pressure of cotton, irrespective of soil water level. Comparisons of smooth pigweed and cotton water relations indicated that smooth pigweed had the capacity to maintain higher water potential and turgor pressure than cotton. The more favorable water status of smooth pigweed resulted in part from increased water extraction at lower depths in the soil profile and higher diffusive resistance, thereby reducing transpirational water loss.

In velvetleaf (Abutilon theophrasti Medic. ♯3 ABUTH), accelerated leaf abscission was a conspicuous response following foliar chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} treatment at 35 g ai/ha. Leaf abscission of treated plants was decreased by AVG [L-2-amino-4-(2-aminoethoxy)-trans-3-butenoic acid], an inhibitor of endogenous ethylene production. Chlorsulfuron stimulated ethylene production in the abscission zone and leaves of treated plants and also increased cellulase activity in the abscission zone. Accelerated leaf abscission of velvetleaf following chlorsulfuron application appears to result from chlorsulfuron-induced increases in endogenous ethylene production and cellulase activity.

Solar heating (SH) of wet soil by mulching it with transparent polyethylene (PE) during the hot season increased soil temperature in a typical daily course which varied with soil depth. Annual weed species responded to soil heating in the laboratory with the same pattern as under SH conditions in the field. Rhizomes of bermudagrass (Cynodon dactylon L. Pers. ♯3 CYNDA) and johnsongrass (Sorghum halepense L. Pers. ♯ SORHA) were very sensitive to heat treatment, but purple nutsedge (Cyperus rotundus L. ♯ CYPRO) tubers were able to survive temperatures as high as 80 C for 30 min. Species having big and heavy seeds or vegetative propagules were able to emerge from deep layers of soil, thus practically escaping the lethal temperature prevailing in the upper layer. Transparent and black PE mulching effectively prevented water loss from soil, as compared with perforated PE and nonmulched control. CO2 concentration in the soil atmosphere under transparent PE mulching increased rapidly during the first week and reached a maximal level which was 20-fold higher than that formed in nonmulched soil. Ethylene at 0.2 ppm was detected only in a mulched soil environment. No differences in levels of CH4 or CO were detected.