نشریه دانش علفهای هرز
سال پنجم شماره 2 (پاییز و زمستان 1388)

After dry storage, germination of newly harvested intact and naked seeds of wild barley (Hordeum spontaneum Koch) were determined at 20. C. Intact seeds did not germinate after 8 weeks whereas, naked seeds germinated and no significant differences between dry storage periods were observed for germination of these seeds. Cold stratification periods had no effect on germination percentages of non-dormant seeds of wild barley. The minimum, optimum, and maximum temperatures for germination of wild barley seeds were 5, 20, and 30. C, respectively. Results showed that wild barley glumellae had either physical or chemical effects on seeds germination because, all naked seeds germinated but when intact seeds were rinsed in ethanol 70% and distilled water, the germination percentage were 0 and 54%, respectively, which was lower than that of naked and intact seeds.

Nitrification inhibition (NI) by climax ecosystems has been suggested for decades, and this inhibitory effect seems to be a feature of wild genotypes rather than commercial cultivars. Many plants particularly grasses were suggested to have NI activity, and recently Brachiaria humidicola (BH) was shown to have promising control on nitrification rates through root exudates. In this study effects of different treatments such as N form (NH4+ vs NO3-) and N concentrations (1, 2 and 4 mM N), plant age, light intensity and different collecting mediums for root exudates on the NI activity of root washings were investigated. This was done using a series of nutrient solution experiments. The results showed that BH root exudates collected in distilled water, independent of light intensity, plant age, N-forms, N-concentrations and root exudates collection periods, had no significant inhibition on nitrification. However, root exudates collected in a 1 mM NH4Cl medium had significant inhibition on nitrification process in a soil bioassay. This inhibition was more highlighted when plants were grown in presence of ammonium rather than nitrate. In comparison to drying with rotary evaporator, freezed dried root exudates indicated significant NI in root exudates of plants which were grown in NH4+ under low light, while this effect was not seen under higher light intensity or nitrate nutrition. Measuring electric conductivity of solutions from root washing also showed higher conductivity when ammonium presented in root medium, particularly in root exudates collecting medium over extended time (24 instead 6 hours

Mouse barley (Hordeum murinum), from the poaceae family, is an abundant annual grass weed in wheat fields of South Khorasan province. Determining the ecological factors of this grass weed will contribute to development of its control programs. Effects of environmental factors on germination and emergence of mouse barley was investigated in the laboratory. Although mouse barley germination was greater than 85% at salinity level of 160 mM, further increase of salinity caused a remarkable decrease in its germination, following a 3% decrease in germination at salinity level of 320mM. Increasing osmotic potential from 0 to -0.8 MPa, resulted in an 80% decrease in its germination. Mouse barley germination was not affected by the pH and in a pH range of 4 to 10 remained approximately 90%. Emergence depth test showed that the maximum emergence of mouse barley occurred for seeds placed on the soil surface (86%) and no seedlings emerged from the seeds buried at 10cm depth. High germination ability of mouse barley under diverse environmental conditions may greatly contribute to it as a problematic weed species in the wheat fields of the region.

In order to determine the role of plant leaf area in radiation distribution within the canopy and a better understanding of how crops and weeds intercept light a study of the complexity of plants is necessary. The effect of intercropping on leaf area distribution and dry matter accumulation in corn, soybean and weeds canopy was studied in a field experiment at a research field of Tehran University (Karaj campus), during 2007 growing season. Treatments were arranged in a factorial experiment based on randomized complete blocks with three replications. The treatments were five different mixing ratios of corn (Zea mays L.) and soybean (Glycine max L.) including 100/0, 75/25, 50/50, 25/75 and 0/100 (corn/soybean). Crops were planted at four levels of weed infestations, including weed free, infested to redroot pigweed (Amaranthus retroflexus L., AMRET), infested to jimsonweed (Datura stramonium L., DASTR) and mixed stands of both weeds species (DASTR+AMRET). Results showed that in weed free corn pure stand, 30.36% of the maximum leaf area was distributed in 90-120 cm layer, but when corn was grown with jimsonweed or infested with both weed species (DASTR+AMRET), the maximum leaf area were established in the upper layer. Soybean weed free monoculture produced 34.66% of its total biomass in the layer of 30-60 cm, but contaminated soybean with DASTR+AMRET, allocated 32.97% of its biomass in the 60-90 cm layer. In this treatment DASTR had also its maximum biomass (49.54%) in the 120-150 cm layer. Soybean canopy in monoculture couldn’t compete with weeds and was suppressed, but intercropped soybean with the corn especially in 50%: 50% mixing ratio, suppressed the weeds successfully. Therefore we can concluded that complementarily effect of corn/soybean intercropping created better condition for optimum utilization of solar radiation to successfully suppress weeds and maintain crop production.

A weed survey of turfgrass fields was conducted at Bajgah during 2008- 2009. The turfgrass fields had been covered with Sport turf. Quantitative measurements viz frequency (F), field uniformity (FU), mean field density (MFD), mean occurrence field density (MOFD) and relative abundance (RA) were recorded. Fourteen species of weeds of 9 families were recorded. Most of them were included in Asteraceae, Fabaceae, Poaceae, and Plantaginaceae. The most important broad leaved and narrow-leaved weeds were Taraxacum officinale L. and Cynodon dactylon [L.] Pers., respectively. Results indicated that the highest frequency (F) (100%), field uniformity (FU) (89.28%), mean field density (MFD) (58.43 m-2) and mean occurrence field density (MOFD) (58.43 m-2) belongs to T. officinale L. of the year 2008. Results were almost the same at year 2009. Taraxacum officinale L. showed the highest relative abundance in both years.

Adjutant's ability in enhancing the performance of herbicides is a major priority in adjuvant research. To identify an appropriate adjuvant for pinoxaden and clodinafop propargyl herbicides against littleseed canarygrass (Phalaris minor Retz.), common wild oat (Avena fatua L.) and ryegrass (Lolium temulentum L.), three separate experiments were conducted under greenhouse conditions. In all experiments treatments consisted of five doses of pinoxaden and two doses of each of the three commercial formulations of clodinafop propargyl (Topik, Behpik & Karent), with and without the adjuvants Adigor, Citogate, Citohef and Volk. Performance of all herbicides increased with enhancing their concentrations against the tested plants except for clodinafop propargyl in case of wild oat. The addition of Volk (followed by Adigor) had the highest effect on pinoxaden efficacy against ryegrass and littleseed canarygrass, supporting the idea that either Volk or Adigor solubilizes the cuticular waxes thus facilitating their uptake. Adding Volk and Adigor had the highest and lowest influence on pinoxaden performance against wild oat, respectively. Totally, the adjuvant receptivity for pinoxaden was higher than for clodinafop propargyl. Between the two surfactants, Citogate was more effective than Citohef in enhancing the efficacy of pinoxaden against ryegrass and littleseed canarygrass, while, Citohef was more effective in increasing the efficacy of pinoxaden against wild oat.

In order to study the effects of row spacing and weed control duration on yield, yield components and oil content of canola, a factorial experiment was conducted using randomized complete block design with 3 replications. The factors comprised row spacing at 2 levels (25 and 35 centimeter) and weed control 4, 8 leaf stages and formation of flower buds. Weeds were permitted to grow by the crop after the above mentioned growth stages. Two check treatments as control, including weedy and weed free were also selected. Evaluated traits were number of pods.plant-1, number of grains.pod-1, 1000-grain weight, grain yield, oil content and oil yield. Results showed that different row spacing had significant effect on all traits except oil content. Increase in row spacing was significantly accompanied by increase in pod number.plant-1 and 1000-grain weight. Other traits were significantly decreased with increasing row spacing. In addition, increase in weed control duration resulted in significant increase in all traits except oil content. The interaction of row spacing × weed control duration was also significant for all traits except 1000-grain weight and oil content. Based on the results, the highest values of grain and oil yield were related to 25 centimeter row spacing on total weed free check and the lowest amounts of these traits were obtained from 35 centimeter row spacing on weedy check treatment.

Velvetleaf (Abutilon theophrasti Medic) is one of the most important warm season weeds in soybean fields. To the aim of this study is to determine the critical period of velvetleaf control in two cultivars of soybean, Williams as indeterminate and Persian as determinate,with different growth patterns.. Theexperiment was conducted at the University of Agriculture and Natural Resources of Gorgan, Iran in 2007 using a randomized complete block design along with four replicates. The experiments consisted of two sets of treatments; one set was to keep the plot weed-free until the growth stages of 2- leaf (V2), 4- leaf (V4), 6- leaf (V6) stages, beginning of flowering (R1) and beginning of pod set (R3). The second set was interference treatments that velvetleaf was allowed to grow within the plot throughout the above-mentioned growth stages. Weedy and weed-free checks were also included in the study. The effect of control treatments were significant on final height, branch number per plant, leaf area index, dry matter, yield, and pod number per plant, while seed number in pod and 100-seed weights of soybean were not significantly affected. The effects of interference treatments were significant on all traits except 100-seed weights. Using the Gompertz and Logistic equations it was found that the critical period of controlling velvetleaf in both cultivars under study, considering 5% allowed decrease in yield, was between 260 to 943 CGDD or 14-51 DAE, which is approximately from 2-leaf stage to beginning of flowering. With a 10% allowed decrease, is the number would be between 320 to 752 CGDD or 18-41 DAE, which is approximately 3 to 6-leaf growth stage for both cultivars.