Study Some Ecological Characteristics on Germination and Seedling Growth of Milky Thistle (Silybum marianum (L.) Gaertn)

Introduction: Milky thistle (Silybum marianum) is an annual or biennial plant of the Asteraceae family. Possibly native near the coast of southeast England, it has been widely introduced outside its natural range, for example into North America, Iran, Australia and New Zealand where it is considered an invasive weed. Seed is an important stage of plant life history. Most invasive plants primarily rely on seedling recruitment for population establishment and persistence. The rapid spread of many invasive plants is frequently correlated with special seed traits. Seed trait variations exist not only among species but also within species. Seed traits variations within a species are essential for the seedling establishment at different habitats. Environmental factors, such as temperature, soil solution osmotic potential, solution pH, light quality, management practices and seed location in the soil seedbank, affect weed seed germination and emergence. Fluctuations in temperature can influence seed germination differently than those under constant temperatures; however, such information is not available on Milky thistle. A light requirement for germination is the principal means by which germination can be restricted to an area close to the soil surface, and species requiring light for germination are potentially more likely to be prevalent in no-till and pasture systems. Soil pH affects the development and competitiveness of crops and weeds by affecting the availability of essential minerals, nutrients, the solubility of toxic elements, and soil microflora. Seed burial depth (buried by tillage or other means) also affects germination and seedling emergence of several weed species. Better knowledge of the factors that influence seed germination and seedling emergence of Milky thistle could contribute to the development of control measures and help determine its potential for invasion into new areas. The objectives of this study, therefore, were to determine the effect of various environmental factors on seed germination and seedling emergence of Milky thistle. Material and Methods: Germination of Milky thistle was determined by placing 25 seed in a 9-cm-diam Petri dish containing two layers of Whatman No. 1 filter paper and moistened with 5 ml of distilled water or a treatment solution. Temperature: Germination of freshly harvested seed was determined in growth chambers under fluctuating day/night temperatures (15/5, 20/10, 15/25, 15/30 and 20/35˚ C). Salinity: In this experiment, seeds were exposed to seven levels of increasing salinity using NaCl solutions including 0, 10, 20, 40, 80, 160 and 320 mM. Petri dishes were incubated as described in the general protocol under the light⁄dark regime. pH: To examine the effects of pH on seed germination, buffered solutions of pH 5 to 9 were prepared. Seed burial depth: The effect of different burial depths on seedling emergence was investigated in a growth chamber. Seeds were buried at six different depths (0, 0.5, 1, 2, 4, and 8 cm) in 15-cm-diam plastic pots. Statistical analysis: Data were subjected to two-way analysis of variance (ANOVA) and the difference between treatment means was separated using FLSD test. A significance level of 5% was applied by SAS 9.2. Results and Discussion: The results showed that effect of different temperature regimes on germination percentage and rate, plumule length and seedling dry weight were significant (P