**2. A brief history of seed priming**

**1. Introduction**

occurs (**Figure 1**) [2].

Efficient seed germination is important for agriculture. Successful establishment of early seedling indeed requires a rapid and uniform emergence and root growth. Germination of orthodox seeds commonly implies three distinct phases (**Figure 1**) consisting in (1) Phase I: seed hydration process related to passive imbibition of dry tissues associated with water movement first occurring in the apoplastic spaces; (2) Phase II: activation phase associated with the re-establishment of metabolic activities and repairing processes at the cell level; and (3) Phase III: initiation of growing processes associated to cell elongation and leading to radicle protrusion. Phases I and III both involve an increase in the water content while hydration remains stable during Phase II. It is commonly considered that before the end of Phase II, germination remains a reversible process: the seeds may be dried again and remain alive during storage and able to subsequently re-initiate germination under favorable conditions.

2 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

**Figure 1.** Seed hydration curves and germinating phases in unprimed and primed seeds.

Water-based seed priming is defined as a pre-sowing treatment that partially hydrates seeds without allowing emergence [1]. Various treatments may indeed be applied during the reversible phase of germination (point 3). They widely differ according to the osmotic potential of the priming solution, the duration, the external temperature, and the presence of specific chemical compounds. The efficient treatments trigger metabolic processes activated during the phase II of germination, which are then temporally stopped before a loss of desiccation Man established contact with seed physiology since the beginning of agriculture and quickly realized that many seeds do not germinate easily and uniformly. Ancient civilization was fascinated by the capacity of an apparently « dead seed » to resurrect and to produce a viable young and healthy seedling after germination. The Greek Theophrastus (ca. 372–287 BC) already focused on seed physiology and suggested that germination process may be tempo‐ rarily interrupted [8]. Pre-hydration of legume seeds before sowing was performed by Roman farmers in order to increase the germination rate and synchronize germination as reported by the Roman naturalist Gaius Plinius Secundus. Several centuries later, these techniques were still used for a wide range of species according to the French agronomist Olivier de Serres (1539–1619) [8]. In 1664, Evelyn [9] mentioned that temperature prior sowing may have an impact on further germination while one century later, Ingenhousz [10] analyzed the impact of light on seedling emergence.

During the nineteenth century, numerous botanists started to describe morphological proc‐ esses associated with seed germination [11, 12]. Sachs [13] experimented the impact of various compounds (including tyrosine and asparagine) before and during germination. The discov‐ ery of plant hormones in the 1920s underlined the crucial role of these compounds in seed desiccation tolerance, reserve mobilization, as well as cell division and cell elongation occur‐ ring during germination. The possibility to influence final germination as a consequence of pre-sowing treatment has led to a wide range of empirical methods for numerous cultivated plant species during the year 1970s [14].
