IRANIAN JOURNAL OF WEED SCIENCE
Year:2005 | Volume:1 | Issue:2|Papers Count:6

Unsatisfactory germination of annual canary grass (Phalaris minor) is contributed to seed don11ancy. Factors affecting the breaking of seed don11ancy were investigated to evaluate their effects on the germination of seed samples collected from three different provinces of Iran. Results indicated that the percentage of germination and radicle growth rate of seed samples varied significantly. Treating seeds with sulphuric acid, pre-chilling seeds prior to incubation, and placing seeds on filter paper moistened with gibberellic acid at 100 ppm during incubation, all significantly enhanced the germination percentage of the seed samples. Applying a combination of these factors resulted in 90 to 98% seed germination and significant radicle elongation. Consecutive freezing and thawing of annual canary grass seed usually had no effect on enhancing seed germination in this weed.

One of the major problems with effective weed control is the presence of dormancy state in weed seeds. No seeds of toothed dock (Rumex dentatus) or common lambsquarters (Chenopodium album) germinated at 10, 15, 20, 25, 30 or 35˚C, whereas redroot pigweed (Amaranthus retroflexus) seeds germinated at most temperatures. Stratification, scarification and light effectively stimulated germination of toothed dock and common lambesquarters seeds. These results suggest that toothed dock and common lambesquarters are positively photoblastic. Stratification effectively stimulated seed germination of redroot pigweed, whereas scarification of seed by sulphuric acid, even for 5 min, killed the embryo. Also, light only increased seed germination at low temperature. Probably this is the reason why redroot pigweed seeds under low temperature require a light stimulation by red light because active phytochrome for germination is produced enough in seed coat.

To study the competition potential of six winter wheat genotypes against the weed Goldbachia laevigata, two field experiments were conducted at the Weed Research Station of the Plant Pest & Disease Research Institute, Karaj, Iran, during two growing seasons from 2000 to 2002. The six wheat genotypes used in this study were 6618, M-75-15, M-75-13, Alamout, Qafqaz and M-75-5. The experiments were arranged in a randomized complete block design with a factorial arrangement of treatments and with four replicates. 6618 and M-75-5 showed the highest and lowest competitive potential with the weed, respectively. The genotypes with higher competitive potential were characterized as having better canopy height, total of fertile tillers, increased leaf area index, an increase in cumulative total dry matter, increased crop growth rate and relative growth rate. These criteria can be used in plant breeding programs to screen for more competitive wheat genotypes against G. laevigata.

The effects of shading by redroot pigweed (Amaranthus retroflexlls) on corn (Zea mays) were determined in growth cabinets and growth rooms. Corn was grown in a growth cabinet to different growth stages (4-,7- and 10 leaves) and then moved to a second growth cabinet where it was shaded by redroot pigweed plants at the 15 leaf growth stage. Treatments were arranged in a completely randornized design with four replications. The results indicated that the rate of corn leaf appearance was reduced and tassel emergence was delayed when the corn was shaded by the weed at 4- and 7-leaf stages. Shoot and root dry matter accumulations were influenced by shading. Partitioning of dry matter was changed and corn root / shoot ratio was reduced when corn plants were shaded. Leaf area per plant was reduced significantly only when light competition started at the 4-leaf stage.

Alternaria alternata (isolate 423) has been identified as a promising mycoherbicide for the biocontrol of redroot pigweed (Amaranthus retroflexus). Transmission electron micrographs of A. alternata conidia showed the presence of pigments enmeshed within the cell wall. Light microscopy showed that conidia grown on media containing tricyclazole at 50 or 100 µg mL-1, which inhibits melanin biosynthesis, lacked pigmentation. After long term storage (5 - 12 months) at 25°C and 15% relative humidity (RH), conidia on semi-permeable discs were placed on top water agar at 25°C for 5 h. Germination of non-pigmented conidia was significantly higher than that of pigmented ones. When seedlings of redroot pigweed were transferred to a plant propagator at 25°C with 92±2% RH, 2 - 6 h after being inoculated with conidial suspension (3× 106 conidia mL-1), the disease symptoms were significantly more severe than those transferred to the propagator immediately or 24 h after treatment.

The efficacy of2,4-D, 2,4-D+MCPA, bromoxynil, tribenuron methyl in the control of 17 annual broad-leaf weed species was evaluated in five herbicide trials, each lasting two years during 2002 and 2003, in five Iranian provinces: Tehran, Khuzestan, Semnan, Kermanshah and Khorasan. The herbicides were applied when the target weeds were at the 2-4 leaf stage. Results indicated that most target species were significantly controlled at least by one of these herbicides. However, Rumex crispus, R. dentatus, Fumaria officinalis and Polygonum aviculare were relatively tolerant to all the herbicides. Among the weed species, three of them including Sinapis arvensis, Rapistrum rugosum, Betae maritima, were controlled completely by all herbicides. In controlling Centaura depressa, Chenopodium album, Cephalaria syriaca, Carthamus oxyacantha and Sisymbrium officinalis, 2,4-D and 2,4-D+MCPA were significantly more effective than tribenuron methyl or bromoxynil. Whereas, in the control of Goldbachia laevigata, bromoxynil was more effective than the other herbicides. Stellaria media was more sensitive to tribenuron methyl than to the other herbicides.