**2.3 Multiple control points in dormancy induction and breaking**

A block to germination may occur at more than one point in the sequence of steps leading to visible growth [55]. Various dormancy mechanisms occur at different levels in the course of seed development and principally in germination, indicating the concurrent nature of dormancy and germination. For example a dormant seed without a hard seed coat will imbibe water but remain blocked at some stage of metabolic activation or growth until other blocks are removed [56]. For example an impermeable seed coat inhibits water absorption, blocking imbibition, and, in many cases, the seed coat may reduce oxygen availability to the embryo [57]. If a seed successfully completes imbibition, it will normally be metabolically active [1]. However, germination-related activities may further be blocked during activation phase in dormant seeds. Even if this stage is completed successfully, a mechanical restriction of root growth may still impose dormancy on the seed, at the last step in germination. This shows that dormancy induction can occur in more than one control point depending on the plant species. The complex interactions between stimulating,

*Seed Dormancy: Induction, Maintenance and Seed Technology Approaches to Break Dormancy DOI: http://dx.doi.org/10.5772/intechopen.106153*

inhibiting and limiting factors demonstrate that a series of requisites must be met for the seed to germinate. A strong line of evidence in support of multiple control points in dormancy induction and breaking is provided by [58] who characterized germination of *Sisymbrium* and *Arabidopsis* seeds under different wave lengths of the visible spectrum and growth stimulants.

Secondary dormancy may be induced by metabolic variations, like ABA synthesis due to drought or cold ambient conditions or when the level of primary dormancy decrease, during after-ripening, because of decreases in ABA level or increases in GA or cytokinin levels [59]. At the molecular level, a well-known genetic barrier to germination is DELAY OF GERMINATION1 (DOG1) [60]. Though the binding of (DOG1) to Protein Phosphatase 2C ABSCISIC ACID (PP2C ABA), Hypersensitive Germination (AHG1) and heme are independent processes, they are however both essential for the *in vivo* functioning of DOG1's*.* AHG1 and DOG1 are both involved in the regulatory system for dormancy induction, maintenance and germination. DOG1 has a significant influence on the level of expression of ABA INSENSITIVE5 (ABI5) [61, 62].

### **3. Classification of dormancy**

Generally, different classes of dormancy require different dormancy breaking methods [4]. By misinterpreting the class of seed dormancy, researchers may fail to apply the correct methods to overcome dormancy in a given species. For this reason, method of overcoming dormancy must be directed towards the specific kind of dormancy [63]. Owing to the fact that the notion of dormancy still need to be elucidated, several classifications exist, but the most common are those proposed by Nicolaeva [11], Hilhorst [2] and that of Baskin and Baskin [64].

#### **3.1 Classification based on barrier factors**

One of the earliest classifications of dormancy is that proposed by [11] who distinguished three main types of dormancy based on barrier factors within the seed.

#### *3.1.1 Exogenous dormancy*

Dormancy is due to some features of the seed located outside the embryo. Exogenous dormancy could be due to the following; impermeability of seed coat to water owing to seed coat structure, which is hard enough to restrict the entry of moisture into the seeds. Secondly seed coat may be impermeability to gases, hence insufficient intake of oxygen necessary for catabolism of reserves. Thirdly, extremely hard seed/fruit structure such as seed coat, endosperm as in Acacia species can impose mechanical resistances on further development of the embryo. Finally, the testa may produce some biochemical substances that serve as inhibitors, blocking germination of embryo.

#### *3.1.2 Endogenous dormancy*

The cause of this dormancy is present within the embryo and can be as a result of; incomplete embryo development after ripening period, in such cases, germination does not occur until the embryos develop to their normal size. Common in family

Palmaceae, Amgnoliaceae; inhibitors are present within the embryo. In such cases germination can commence only when these inhibitors are leached out of the embryo, this is the case of Xanthium, Fraximus.

#### *3.1.3 Combined dormancy*

This type is provoked by a combination of two or more exogenous and endogenous factors which act in complementary fashion.
