Seed-Enhancement Technologies Promote Direct Seeding and Overcoming Biotic and Abiotic Barriers in Degraded Dryland Ecosystem ^†

Abstract

Restoration programs face several challenges in degraded drylands and desert environments, such as high temperatures, soil salinity, water scarcity, loose soils with low water-holding capacity, and poor fertility. Seed-enhancement technologies (SETs) are proposed to improve soil physical and chemical properties, improve seed germination and seedling recruitment, and promote plant growth. SETs improve seed, seedling, and adult plant growth through three main approaches: adding materials to seed coats (seed coating), removing barriers of seed coats (seed scarification), and physiologically altering metabolites through priming (seed priming). These three main approaches (categories) are further divided into several other subcategories. This review aims to define the general categorization of SETs, adopt the proper SETs for arid lands, and indicate the benefits of SETs to overcome the biotic and abiotic barriers in agricultural systems and the ecological restoration of degraded dryland ecosystems. In general, integrating different seed-enhancement technologies (SETs) for rehabilitating degraded lands with a mixture of seeds from various species is recommended, especially since some technologies tend to be species-specific.

1. Introduction

Seed-enhancement technologies (SETs) contribute to dryland restoration, specifically by overcoming the biotic and abiotic barriers to efficient seed employment, promoting precision agriculture practices, using cutting-edge technologies to succeed in planting seeds, and facing degradation effects in arid land while minimizing inputs, mitigating environmental impacts, and paving the path for more sustainable plant enhancement [1].

2. SET Categorization and Sub-Categorization

SETs introduce treatments that physically, biologically, and chemically manipulate and apply materials to the seed to enhance germination and seedling emergence, alleviate the effect of both abiotic and biotic stress, and accelerate early seedling growth. SETs’ capability to promote direct seeding and alleviate biotic and abiotic challenges is achieved through five different applications: (i) modify seed physical attributes that could break physical seed dormancy and adjust seed size and shape for precise seed delivery; (ii) enhance the seed’s physiological status and help to overcome stress tolerance during germination and even later in seedling and adult plant growth; (iii) alleviate seed storability concerns through the capability of storing seeds after treatment; (iv) offer seed protection against pathogens, pests, and seed predators; and (v) improve the seed microenvironment by promoting the seed’s zone via water retention, supporting the adsorption of nutrients and biomaterials, resolving physical and chemical soil inconveniences, and providing a proper microhabitat for germination and enhance seedling establishment. Different SET categories and subcategories are illustrated in Figure 1.

2.1. Modifying Seed Coat (Seed Coating): The Addition Concept

The addition of exterior materials to the seed can modify seeds’ physical attributes and facilitate their delivery process, enhance their physiological status and microenvironment, protect seeds in soils, and prolong seed storability. This can be achieved with different techniques and constituents under various subcategories that have been recognized, such as seed encapsulation, seed pelleting, encrusting techniques, etc. [1].

2.2. Modifying Seed Coat (Seed Scarification): The Removal Concept

Scarification comprises the techniques that lead to eliminating the outer part of the seed to achieve one (or more) enhancement applications to overcome degraded- and arid-land-restoration barriers. Additionally, it enhances the physical attributes of seeds that affect the delivery process and break seed dormancy. On the other hand, scarification SET shows an effect on seed storability and seed dormancy [2]. Scarification can be recognized through several approaches, including chemical, mechanical, heating, flaming, and flash flaming.

2.3. Modifying Seed Physiology: Seed Priming

Seed priming is a fast, cost-effective technology for treating seeds with mild doses of natural and synthetic materials that allow partial seed imbibition to start the germination metabolic processes without actual germination. There are several subcategories of seed priming, including (i) water (hydropriming); (ii) osmotic solution (osmopriming); (iii) inorganic and organic chemicals (chemical priming) that include halo (salt), hormonal, and biostimulants priming; (iv) beneficial microbes (biopriming); (v) nanoparticles (nano priming); (vi) solid matrices (matrix priming); and (vii) physical priming, such as using low or high temperatures (thermopriming) via radiation or a magnetic field.

3. SETs for Overcoming Challenges Affecting Seeding Success

Direct seeding is the main technique used for large-scale restoration/rehabilitation of degraded lands. This technique faces several challenges in dryland soils. SETs can enhance the success of the seedling establishment and alleviate biotic and abiotic challenges by affecting one or more of the main five applications illustrated in Figure 1;

(1) SETs can modify seeds’ physical attributes, such as seed size, shape, and flowability, improving seed delivery. For example, in pelleting technique, outer materials are added to the seed through coating [3], enhancing seeds’ shape, weight, and bulk density, enabling more control over precision seeding, and eventually improving plant establishment [4]. Additionally, with scarification, seeds become more homogeneous and easier to manipulate in the restoration process, particularly by enhancing seeds’ sowing distribution requirements (i.e., shape, weight, flowability, bulk density, etc.) [5].

(2) SETs enhance seed physiological status via breaking seed dormancy and enhancing stress tolerance during germination and even later in seedling and adult plant growth. These technologies can alter the physiological status of the seed through imbibition methods, including priming, coating with biomaterials and inoculants [5], and breaking seed dormancy through processes such as chemical and mechanical scarification [4].

(3) SETs improve seed storability, which concerns the capability of storing seeds after treatment. The advantage of priming over other SETs, such as seed encrusting and pelleting, is the ability of the former to improve seed storability [6]. On the other hand, applying scarification may indirectly reduce seed volume, representing significant savings in space for banking more native seeds. Seeds can be stored under the appropriate conditions (temperature, light, humidity) to optimize shelf life and seed storability [7].

(4) SETs can protect seeds against pathogens, pests, and seed predators through coating as well as priming SETs by imbuing the seed with protection agents, including fungicides, insecticides, and predator deterrents/repellents, to improve plant establishment [3].

(5) SETs also improve the seed microenvironment by promoting the seed’s zone via (i) increasing water availability by water-retention materials and surfactants, (ii) nutrients, (iii) microorganisms, (iv) hormones, and (v) beneficial biomolecules to seedlings, paving the way for more sustainable and effective delivery of growth factors for seeds and seedlings [5].

4. Seed-Enhancement Technologies for Dryland Peculiarities

The success of SETs, specifically in drylands, is mainly correlated to their efficiency, sustainability (long-term effect), and overall beneficial impact on enviro-socio-economic aspects. More sustainable SETs mean longer-time enhancement throughout different plant-growth stages. For example, SETs that increase soil microbial activity cause improvement in rhizosphere conditions and/or enhance seedling survival, boosting the restoration of non-fertile soils of the deserts. Moreover, SETs incorporating organic nutrient sources, water-retention materials, biomaterials, and beneficial soil microorganisms support more sustainable improvement in seed emergence and seedling survival under the harsh climatic conditions of deserts [6].

SETs effectively help plants to overcome several abiotic and edaphic stresses in drylands, particularly soil salinity, drought stress, soil alkalinity, and heat. For example, several SETs, such as hydropriming, osmo-priming, halo-priming, chemo-priming, nano-priming, radiation-priming (laser and UV), and magnet-priming, help plants overcome salinity stress during germination and even later in seedling and adult plant growth [8]. Moreover, drought stress has been mitigated using hydro-priming, chemo-priming, and nano-priming [9], while overcoming heat stress was achieved by thermo-priming.

The addition concept (coating) enhances the seed microenvironment. For instance, a coating with water retention materials increases seeds’ water-holding capacity in dryland, maximizing the benefits of precipitation in the root zone and mitigating drought stress. In addition, adding materials such as salicylic acid helps in enhancing seedling growth and survival under drought conditions [6].