Before considering dormancy, which imposes a block to the completion of germination, it is appropriate first to consider the processes that comprise germination. Germination commences with the uptake of water by the dry seed (imbibition) and is completed when a part of the embryo, usually the radicle, extends to penetrate the structures that surround it. Water uptake by the mature dry seed is tri-phasic, where initial water up take is rapid (1 st phase) followed by a plateau phase (2 nd phase). Another increase in water uptake occurs only after germination is completed, as the radicle elongates (3 rd phase (Bewley 1997)). The absorption of water into the dry seed results in temporary structural d isturbances from the equilibrium condition, in particular the membranes. This leads to an immediate loss of solutes and low molecular weight metabolites. This is symptomatic of a transition of the membrane phospholipid components from the gel phase achieved during maturation drying to the normal, hydrated liquid-crystalline state (Crowe and Crowe 1992) . After a short period of re-hydration, the membranes are returned to their more stable configuration. This in turn stops further solute leakage from the seed.
One of the first changes upon imbibition is the resumption of respiratory activity, which can be detected within minutes. After a steep initial increase in oxygen consumption, the rate declines until the radicle penetrates the surrounding structures. Here another burst of respiratory activity occurs (Salon et al 1988, Botha et al 1992 ) . The Krebs cycle enzymes now become activated 5 . It was also noted many years ago that germinating seeds of different species frequently produce ethanol ( Morohashi et al 1972 ). This occurs because of an internal lack of oxygen, due to the restriction upon gas exchange by the structures that surround the seed and the internal composition of the seed (if it is densely packed). This deficiency in oxygen may result in increased pyruvate concentrations above what is needed for Krebs cycle and the electron transport chain. Tissues of the mature dry seed contain mitochondria, and although these organelles are poorly differentiated as a consequence of maturation drying, they contain sufficient Kreb's cycle enzymes and terminal oxidases to provide adequate amounts of ATP to support metabolism for several hours after imbibition ( Attucci et al 1991). During germination of embryos, there appear to be two distinct patterns of mitochondrial development. These patterns, which are particularly obvious in cotyledons, depend on the nature of the stored reserves. In starch-storing seeds, repair and activation of pre-existing organelles predominate, whereas oil-storing seeds typically produce new mitochondria ( Morohashi 1986). This would suggest that the regulation of nuclear and mitochondrial genomes occurs in the early stages of germination.
In order for radicle emergence and other physiological changes brought about by germination protein synthesis must now occur. All of the components necessary for the resumption of protein synthesis upon imbibition are present within the cells of mature dry embryos, although polysomes are absent. However, within minutes of re-hydration there is a decline in the number of single ribosomes as they become recruited into polysomal protein-synthesising complexes ( Dommes et al 1990 ). Initial protein synthesis is dependent on extant ribosomes, but newly synthesised ribosomes are produced and used within hours of initial polysome assembly ( Dommes et al 1990) . Messages encoding proteins that are important during seed maturation and drying, such as late embryogenesis abundant (LEA) proteins, are likely to be degraded rapidly upon imbibition (Han et al 1996) . Preformed mRNAs are also present within the dry embryo. Some of these are residual messages associated with previous developmental processes and as such can be used quickly during early germination (Comai et al 1990) . New mRNAs are transcribed as germination proceeds. Most of these will encode for proteins that are required in cellular metabolism ( Bewley et al 1990) .
The next stage in germination is the emergence of the radicle from the testa. It is generally regarded that when the radicle extends through the structure surrounding the embryo germination is over and seedling growth begins. Radicle extension may or may not be accompanied by cell division. Two phases of DNA synthesis occurs in the radicle post-imbibition. The first takes place soon after imbibition and probably involves the repair of DNA damaged during maturation drying and re-hydration as well as the synthesis of mitochondrial DNA. DNA synthesis associated with post-germinative cell division accounts for the second phase (Osborne 1994) . However it also vitally important to understand the processes of seed dormancy in order to understand germination properly.
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