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Environmental factors during seed development on the mother plant, or after seed dispersal in the soil, can affect subsequent seed germinability (Gutterman, 2000a). These factors can include moisture, nutrients, light and temperature, and they have different effects on different species (Fenner, 1991; Aamlid, 1992). The effects of temperature on plant growth (Fitter & Hay, 2002; Blackshaw & Entz, 1995) and seed maturation (Fenner, 1992) have been studied. Plants respond to the stress caused by high and low temperatures by changing their metabolic activity (Fitter & Hay, 2002). High temperatures can accelerate metabolic activity, shorten developmental periods (Bewley & Black, 1994) and cause plants to produce small leaves and extensive root systems to offset water loss from the leaves or to maintain water intake relative to leaf area (Gliessman, 1998).
Higher temperatures during development decrease individual seed mass in many species (Shimzu et al., 1979; Ong, 1983; Wulff, 1986; Roach & Wulff, 1987), whereas lower temperatures increase seed mass in others (Lacey et al., 1997). Even small differences in temperature during seed development may affect both seed mass, especially pericarp tissue (Gray et al., 1988), and the germination response of the progeny to temperature (Wulff, 1995).
Higher temperatures during seed development increase seed germinability in many species (Grant-Lipp & Ballard, 1963; Peters, 1982; Alexander & Wulff, 1985; Drew & Brocklehurst, 1990). However, in some cases an inverse relationship has been found, where higher temperatures caused increased seed dormancy (Karssen, 1970; Argel & Humphreys, 1983; Keigley & Mullen, 1986; Govinthasamy, 1994; Hume, 1994).
After dispersal, seeds of many species do not germinate until they experience a period of after-ripening (Baskin & Baskin, 1998). Environmental factors such as moisture, temperature and oxygen can affect after-ripening (Bewley & Black, 1994). During after-ripening, the narrow range of conditions under which seeds can germinate gradually becomes wider. Also, during after-ripening, physical and chemical changes that alter the tensile strength of seed coats often occur in seeds and increase their permeability to water and gases. In some cases, changes can occur in the embryo or the embryo coverings (Kozlowski & Pallardy, 1997).
An inverse relationship between seed moisture content and maximum after-ripening (seen as germination) has been found in several species (Foley, 1994; Mohamed et al., 1998). Seeds of some species lose their impermeability to water after a certain time, even under benign conditions of storage. On the surfaces of many seeds there are cracks that become deeper over time and lead to increased germinability (Werker, 1997).
Whether on the mother plant or in the soil, seeds from one population or one plant or even one flower head do not experience the same environmental conditions during ripening (Gutterman, 1985). For this reason, even seeds from the same genotype can have different degrees of dormancy. Even though the effects of temperature and after-ripening have been reported for many species, little is known about the mechanisms of these effects on germination patterns.
To explore the underlying causes of after-ripening and temperature during seed development, Scotch thistle (Onopordum acanthium) was chosen, because this species responds to changes in temperatures during development or after dispersal and exhibits intermittent germination. This species has both physical and physiological cypsela (seed) dormancy (Qaderi, 2002) and becomes more germinable after dry storage, regardless of conditions and duration (M. Qaderi, A. Presti & P. Cavers, unpublished data). Because every cypsela has its own microenvironment, either on the mother plant or in the soil, its germinability may be different from that of others. Variability in germinability among cypselas leads to intermittency of seedling emergence, which has not been documented extensively and thus needs further study.
Recently, Qaderi & Cavers (2000) have shown that plants grown in a glasshouse produced more readily germinable cypselas than those from the same population grown under field conditions during the same period. From this, we hypothesized that contrasting temperatures during plant growth and cypsela development of O. acanthium, which affect the developmental processes and morphological characteristics of plants and cypselas, result in differences in the germinability of fresh and after-ripened cypselas. Experiments were designed to determine the effects of high and low temperatures on plant developmental processes, including flowering time and period, cypsela developmental stage (e.g. embryogenesis, maturation and desiccation), plant vegetative characteristics (including stem height and diameter, leaf size and above- and below-ground biomass) and reproductive yield (including capitulum size, cypsela number and weight) of O. acanthium. The relationship between germination responses and physical and structural characteristics of cypselas was investigated for cypselas matured under different conditions. This is one of the first studies in which the effects of after-ripening and temperature during maturation on germination patterns and the structural characteristics of seeds have been investigated in the same batch of seeds (cypselas).