Germination Ecology of Johnsongrass Seeds (Sorghum halepense (L.) PERS.)

Introduction Johnsongrass (Sorghum halepense (L.) PERS) is one of the most controversial and problematic weed. It is damaging at more than 30 different crops in 53 different countries. S. halepense (L.) is a perennial weed reproducing by seed and rhizome. Since it produces many seeds and rhizomes, it is difficult to control it. A weed germination plays an important role in attaining a prosper establishment in a typical agri-ecosystem; and this trend is adjusted with some environmental factors such as light, temperature, salinity, pH and soil moisture. If you consider the pattern of germination and emergence of weed species, you will able to provide comprehensive information to develop weed management strategies in the future. Thus, the purpose of current research has been to evaluate the breaking methods of the seed dormancy, effect of constant and alternative temperature, light, salinity and drought stress and burial depth on germination and seedling emergence of Johnsongrass.
Materials and Methods Seeds of Johnsongrass (S. halepense L.) were collected in June 2013 from plants located at the research farmlands of the Agriculture research centre of Fars province in Zarghan town, Iran. Experimental treatments of Breaking Dormancy consist of six level of scarification with 95-98% acid sulfuric (4, 8, 15, 30, 45 and 60 minutes), in the other one, there were the soaked seeds in the water for 24, 48, 72 and 96 hours, and in the next group the seeds were heated in a 95- 98 boiling water for two and five minutes, and again in the next group, for 15, 30, 45 and 60 days, the seeds were chilled in 3 C, and in the last group, the seeds stored in 3 and 12 months after harvest comparing to control treatment. A number of 25 seeds were transferred to incubators to identify a suitable temperature and light regime for subsequent experiments of germination and determine under alternative day/night temperatures (15/5, 20/10, 30/15 and 35/20 C) and constant temperature day/night (5, 10, 15, 20, 25, 30, 35, 40 and 45 C) in both light/dark and dark regimes. The salt and osmotic potentials were applied in -1, -3, -5, -7, -9, -11 levels and control treatment in order to survey the stress of salt, drought on seed germination and to compare both stress on it. NaCl was applied in the Vant Hoff method to influence salt stress and also PEG 6000 was applied in the Michel methods to affect osmotic potential. In the burial depth experiment, a number of 30 seeds, for 30 days, were laid in the depth of 0, 0.5, 1, 2, 3, 4, 6, 8 cm in plastic vases with 15 cm diameters. All experiments were conducted twice in the form of randomized complete-block design with four replications. Each replication was arranged on a different shelf of growth chamber and considered as a single block in the laboratory experiments. The data of the replicated experiments were pooled for analysis, as the time interaction and treatment was absolutely meaningless. A functional three-parameter logistic model Was fitted to the germination values (%) obtained at different concentrations of NaCl or osmotic potential. Also, a sigmoidal decay curve was fitted to show the seedling emergence (%) values at different burial depths.
Results and Discussion Seed germination was influenced significantly on different treatments of breaking dormancy (Fig 1). Sulfuric acid treatments showed significant difference rather than control treatment. In 30 and 45 min sulfuric acid scarification, germination was recorded more than 95 %. Based on the achieved results it is very probable that the hard seed prevents germination and this issue causes the weed remains stable in seed bank. Germination percentage was not affected by light regime and temperature interaction (both in constant and alternative) (Fig 2a and 2b), the influence of different temperatures on the feature was significant (Fig 2a). The same germination has been seen in all temperatures (both light and light/ dark regime) in all temperature, The more temperature increased, the seed germination increased too. In both regime, at 25/15, 30/20 and 35/25 C (day/night), germination was recorded more than 95 % (Fig 2a). The maximum and the minimum germination percentage of Johnsongrass in light/ dark regime, were accorded in alternative temperature in 30/25 C (99.5%) and 15/5 C (35.5 %) respectively. In constant and alternative temperatures, the germination was similar. In the constant range of 25-45 C temperatures, the feature was occurred more than 90 % (Fig 3a). The maximum and the minimum germination percentage, in light/ dark regime, were recorded in 25 C (99%) and 5 C (0%) respectively (Fig 3a). The experiment on light revealed Johnsongrass seeds was not influential to light, therefore it wasnt photoblastic species and the germination percentage will not exceed through light. The germination percentage could not be decreased by light as a major factor. Accordingly it is reported that Johnsongrass seed was not photoblastic and the light regime had the slightest effect on the number of seed germination. As the matter of fact the temperature has greater importance comparing with light. The deeper the seedling planted, the fewer emergences were accorded. The emergence at the surface of the soil (zero depth) was 83.33% and in depth of 3 cm it was 100%. The minimum emergence for 18.35%, devoted to depth of 8 cm (Fig 5). With an increase in five cm burial-depth, the seedling emergence decreased for 81.65 %. According to the fitted sigmoid model, the depth which caused a decrease for the maximum 50% seedling emergence calculated 5.60 cm. The results showed that weed seedling emergence was limited in depth of 6 cm and over. The decrease in seedling emergence may be linked to seed energy with increasing buried depths. Agronomic practices causing a decrease in seed germination and an increase seed death is able to help the weed management. Our study model proved that 5.60 cm was the depth in which 50% the maximum emergence decreased (Fig 5) and therefore, tillage more than 6 cm could help reducing germination and emergence of this species.
Conclusions Based on the above results, we come to conclusion that Johnsongrass seed is able to tolerate a variety of environmental circumstances and as soon as providing a suitable condition such as temperature, in particular, it is capable to improve beyond 90% of germination. Salinity and drought stress play no important role in germination. Although, the more increase in depth, the less seedling emergence happens. Consequently one of the best methods of managing this weed is to till the field over 6 cm.