*3.2.2 Secondary dormancy*

Secondary dormancy is generally induced by unfavorable environmental conditions following shedding of mature seeds from the parent plant which were not

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

dormant at shedding [2]. Secondary dormancy is induced when changing environmental conditions cause undesirable germination conditions, such as unfavorable temperature, extended light or darkness, water stress, or lack of oxygen. This type of dormancy does not only decrease with time, but it can also be re-induced in nondormant seeds when conditions for germination like light are lacking [26]. When exposed to inappropriate conditions like critical temperature, anoxia, limited light etc. secondary dormancy might occur after imbibed after-ripening seeds have lost primary dormancy. In the soil seed bank, secondary dormancy enables cycling, through which different depths of dormancy are progressively gained or lost, until the environment is favorable for germination, and then seedling establishment [6].

#### **3.3 Classification according to Baskin and Baskin**

Baskin and Baskin [64] based on an initial dormancy classification scheme proposed by Nikolaeva [11] to put forward a more comprehensive hierarchical classification system made up of five classes of seed dormancy which are in turn sub divided into levels and types.

#### *3.3.1 Physiological dormancy (PD) or class A*

Physiological dormancy is present among species distributed over the entire phylogenetic tree of gymnosperms, basal angiosperms, monocots and eudicots [64, 70]. PD is caused by low growth potential of embryo, which cannot overcome mechanical constraint of seed (or fruit) coat. It is the most abundant form of dormancy common in seeds of angiosperms and all major angiosperm clades. Class A has three levels: deep, intermediate and non-deep.

#### *3.3.1.1 PD deep*

Embryos excised from seeds with deep PD either do not grow or produce abnormal seedlings. GA treatment does not break the level of PD dormancy. Three to four months of cold (subtype a) or warm (subtype b) stratification are needed before germination can occur. Examples of plant family of sub type are *Aceraceae*, and b is *Ericaceae* [71].

#### *3.3.1.2 PD intermediate*

Embryos excised from such seeds produce normal seedlings. GA treatment promotes germination in some but not in all species. Seeds require 2–3 months of cold stratification. Dry storage after-ripening can shorten the cold stratification period. Example of a plant with PD intermediate is Acer Pseudoplatanus (*Aceraceae*) [70].

#### *3.3.1.3 PD non-deep*

Embryos excised from such seeds produce normal seedlings. GA treatment can break this dormancy but depending on the dormancy can also be overcome by scarification, after-ripening in dry storage, and cold (0–10°C) or warm (>15°C) stratification. The sensitivity of seeds to light and GA increases as non-deep PD is released. Members of Asteraceae and poaceae have non-deep PD [72].
