**1. Introduction**

Good-quality seed has a significant potential of increasing on-farm productivity and enhanc‐ ing food security [1]. Seed quality is the foundation for profitable production and marketing [2, 3]. High-quality seeds are genetically and physically pure, vigorous and free from insect pests and pathogens [4]. High-quality seeds with enhanced vigour contribute nearly 30% of the total production. Plant uniformity is an expression of high seed quality achieved by high vigour of seeds [5]. Seed quality is influenced by several factors during seed development, such as maturation, harvesting, drying, cleaning, grading, packing and storage. Farmers and growers are constantly looking for high-quality seeds to ensure uniform field establishment and increased production [6].

Availability, quality and cost of seeds influence the global production and ultimately food security [7]. The informal seed systems (farmers organized and managed without legal documentation) constitute for 75–90% of their food crop cultivation [8]. In the developing world, informal seed systems remain the prevailing source of seed for smallholder farmers. Improper storage environment, sensitivity of germinating seeds and young seedlings to dehydration stress lead to loss of desiccation tolerance with seed hydration [9–11] and predicted climate change (erratic rainfall patterns and unpredictable temperature extremes) may further exacerbate seed quality. Low-vigour seeds can be improved using a variety of seed technologies that will thrive under small holder cultivation conditions and also improve the supply of good-quality seed in the local seed industry.

Efficient seed germination and early seedling establishment are important for commercial agriculture because they represent the most susceptible stages of the life cycle of crop plants [12]. Rapid and uniform seedling emergence leads to successful establishment as it produces a deep root system before the upper layers of soil dry out, harden, or reach supra-optimal temperatures [13]. Germination begins with water uptake by seed and ends with the emer‐ gence of the embryonic axis, usually the radicle [14]. A wide range of techniques are now used to help sowing seeds and to improve or protect seedling establishment and growth under the changing environments and seedbed constraints. These techniques constitute the postharvest processing necessary to prepare seed for sowing and optional treatments that are generally described in the industry and scientific literature as 'seed enhancements' or 'seed treatments'. Many scientists have suggested techniques for improving crop germination performance in the field keeping in view the responses of seed to temperature and water availability in the soil. These techniques may be differentiated into physiological (seed priming, coating and pelleting), physical (magnetic, radiation and plasma) and biological (seed enhancements) aspects [15–20]. In 2015, the projected value by global chemical seed treatment industry was up to \$ 5.4 billion. Bayer Crop Sciences and Syngenta have 75% share in seed treatment market. In this chapter, we will focus on physiological, biological and physical enhancements of seeds.

Several reports are available, for instance, Heydecker and Coolbear [15] had reported on seed treatments to break dormancy, improve germination and impart stress tolerance and subse‐ quently Taylor et al. [17] continued this work. Halmer [18, 21] focused on practical aspects of seed treatment technologies and categorized it into conditioning, protection and physiological enhancements. Bray [22] and McDonald [23] further continued work on exploring the mechanisms of physiological enhancements especially seed priming. Regarding physical enhancement, only one key review [24] addressed the effect of magnetic field on growth and yield of crops without primarily focusing on seed germination. Up to now, 1253 research articles have been published on seed priming and out of that almost 100 articles are being published every year since 2010. While there is no consistency in publications on magnetic field treatments, i.e., on this topic roughly 3–4 publications per year and a total of 164 articles have been published up to now [25]. With the recent advances in molecular biology of seeds, here we presented conceptual insights into physiological, biochemical, morphological and biophysical markers that can be used for further improvement of seed quality of crop plants. In addition, in-depth mechanisms of seed germination promotion by physiological, biological and physical seed treatments have also been discussed.
