Seed dormancy is generally regarded as the inability of an intact and viable seed to complete germination under optimal conditions (Bewley 1997) . Dormancy is controlled by several environmental factors, such as light, temperature and the duration of seed storage (after ripening). Studies of genetics and physiology have shown the important roles of the plant hormones abscisic acid and gibberellin in the regulation of dormancy and germination. Studies of gibberellin (GA)-deficient, abscisic acid ( ABA )-deficient, and signaling mutants in Arabidopsis and tomato have identified the crucial role of ABA in seed dormancy, as well as the requirement for GA for germination (Debeaujon et al 2000, Hilhorst 1995). The observation that inhibitors of ABA biosynthesis, such as norfluorazon, promote germination indicates that the maintenance of dormancy in imbibed seeds is an active process involving de novo ABA synthesis (Debeaujon et al 2000). It was recently found that the ethylene insensitive2 ( ein2 ) and ethylene response ( etr ) mutants of Arabidopsis are also hypersensitive to ABA (Beaudoin et al 2000, Ghassemian et al 2000). These findings, in combination with the non-dormant phenotype of the ein2 abi3-4 double mutant, indicate that ethylene may suppress seed dormancy by inhibiting ABA action (Beaudoin et al 2000).
In addition to these, mutants have been selected that control dormancy through the seed coat or other maternal factors. A number of seed-coat or testa mutants have a maternally inherited reduced seed dormancy (Debeaujon et al 2000). This indicates the importance of the testa structure as a constraint to radicle emergence. In Arabidopsis , dormancy is apparently imposed by the seed coat because removal of the testa allows the germination of GA-deficient mutants. Evidently, lack of germination may also be due to a reduced growth potential of the embryo. A knockout mutant of the Dof AFFECTING GERMINATION1 ( DAG1 ) gene, which encodes a Dof (DNA-binding with one finger) transcription factor, caused reduced dormancy (Papi et al 2000). In contrast to those of other reduced dormancy mutants, this phenotypic effect is determined by the maternal genotype ( Léon-Kloosterziel et al 1996, Molina-Cano et al 1999). In essence, Dormancy and germination are complex traits that are controlled by a large number of genes, which are affected by both developmental and environmental factors. Seed dormancy and germination depend on seed structures, especially those surrounding the embryo, and on factors affecting the growth potential of the embryo. The latter may include compounds that are imported from the mother plant and also factors that are produced by the embryo itself, including several plant hormones.
Arabidopsis thaliana is a small flowering plant that serves as the major model system in plant molecular genetics ( Meinke et al 1998) . Arabidopsis has many advantages for this type of experiment, in particular its quick germination time and the sequenced genome of the plant (The Arabidopsis genome initiative 2000) . The sequenced regions cover 115.4 megabases of the 125-megabase genome and extend into centromeric regions. The genome contains 25,498 genes encoding proteins from 11,000 families, similar to the functional diversity of Drosophila and Caenorhabditis elegans– the other sequenced multicellular eukaryotes (The Arabidopsis genome initiative 2000). The plant is also quite small therefore making it suitable for use in the greenhouse, thus allowing the seedling to grow in optimal conditions throughout its life cycle. Most seeds will of course only germinate between certain temperatures. Too low and the seed takes up water but cannot germinate and therefore rots, too high and growth within the seed is prevented. Fortunately most seeds are tolerant of a wide range of temperatures but it is wise to try to maintain a steady, not fluctuating temperature of 22°C. This will allow us to show that it is indeed stratification, and stratification only that made particular ecotypes grow at different rates. Whereas had they grown in the wild one would expect temperature, humidity, day-night cycles to be constantly changing and as such credible data could not be attained.
Abstract,Introduction,Germination,Dormancy,Temperature,Light,Ecotypes
Ecotype-Storage,Soil preperation,Seed Sowing,Germination-Conditions,Measurement of Results
Cond1,Cond1rep,Cond2,Cond3,Cond4,Cond5,Cond6,Cond6rep,Cond7,Cond8