تأثیر دما و تنش خشکی بر جوانه¬زنی بذر تاج خروس خزنده (Amaranthus viridis L.) و خوابیده (Amaranthus blitoides S. Watson)

Abstract

Introduction: Slender amaranth (Amaranthus viridis L.) and prostrate pigweed (Amaranthus blitoides S. Watson) are two common weeds in vegetables and summer crop fields of Iran. The two Amaranthus species have all the attributes required by ecologically successful annual weeds: rapid growth, early reproduction and continuous seed production. Knowledge of the germination requirements of these weeds will helps determine the proper conditions for germination and emergence and allow better management of them. Water and temperature are determining factors for seed germination of weed. Both factors can, separately or jointly, affect the germination percentage and germination rate. Water stress is one of the main constraints on plant growth and the most common environmental stresses around the world. Water stress affects the different aspects of plant growth and causes reduction and delay in seed germination. Seed germination of all plant species requires a minimum of water to be absorbed and swelled and that is why osmotic potential should not be less than a certain amount. Materials and Methods: Seeds were harvested from vegetable fields of Karaj. For breaking dormancy, seeds were treated with concentrated sulfuric acid for two minutes. Two experiments were conducted at Islamic Azad University, Science and Research Branch, Ecology lab, in 2016. First experiment was based on completely randomized design with 4 replications .The seeds were treated with different temperatures (5, 10, 15, 20, 25, 30, 35, 40 and 45oC). Germination percentage and germination rate were measured and seed were considered to have germinated with the emergence of the radical. Intersected lines model is used to determine the cardinal temperature. Second experiment was conducted to determine the effects of simulated dry conditions (use PEG) and temperature on seed germination of slender amaranth and prostrate pigweed. Exposure to polyethylene glycol (PEG-6000) solutions has been effectively used to mimic drought stress with limited metabolic interferences as those associated to the use of low molecular weight. Over a 21d period, germination was studied in 5, 10, 15, 20, 25, 30, 35, 40 and 45°C constant temperatures and water potentials of 0, -0.2, -.04, -.06, -0.8 and -1.0 MPa. The number of 25 seeds were placed into petri dishes, then 10 ml of prepared drought solution was added to each petri dishe. Petri dishes, then, were placed in germinator in circadian alternation of 12 h light and 12 h darkness, under 9 temperatures and a relative humidity of 65 %. Data were statistically analyzed using analyses of variance in the SAS 9:1 software. Probabilities of significance were used to indicate significance among treatments and interaction effects. FLSD (p<0.05) was used to make comparisons among treatments. Results and Discussion: In the first experiment, analysis of variance showed that temperature had a significant effect on seed germination percentage and germination rate. Base, optimum and maximum temperatures were 14.37, 34.21, 44.50oC for slender amaranth; and 14.24, 31.65-19.67, 44.20 oC, for prostrate pigweed respectively. At second experiment the of temperature and water potential on seed germination had a significant effect. At all temperatures, germination percentage decreased with increasing water potential. Germination percentage of prostrate pigweed was reduced by 50% in -0.52 MPa at 25oC, but germination percentage of slender amaranth was reduced by 50% in -0.62 MPa at 25oC. Results showed that both species began to germinate at 5°C, and germination was stopped at 40 and 45oC, respectively. Germination percentages were higher at all treatment temperatures but cardinal temperatures were similar in two species. Values of X50 were higher at all temperatures in slender amaranth. Slender amaranth could germinate by 9.24% in -1 MPa but prostrate pigweed could not germinate in this water potential, which represents higher toleration of drought by this species. Conclusions: The ability to predict germination percentage expected at any temperature and water potential has applications for weed management. To control weeds, understanding germination and emergence of weed seeds helps predict their potential spread in new areas. Inter-specific differences in seed germination was found in this experiment. The results of this study showed that slender amaranth and prostrate pigweed are drought tolerant weeds and can germinate in temperature range of 5 to 40 °C, indicating their high adaptability for germination in subtropical and tropical areas but slender amaranth was more tolerate than prostrate pigweed and had more potential invasion in to arid areas.