How Seed Traits Can Affect the Performance of Cover Crops and What Can We Do to Enhance It?

1. Introduction

Weeds are a major constraint to crop production and should be managed through direct (physical, chemical, and biological) or indirect (cultural) methods [1] to secure crop productivity and high quality of the harvestable commodity [2]. Chemical weed control remains the most common method of weed control due to high efficacy/cost ratio. However, overreliance on herbicides in combination with monoculture has led to the evolution of weed herbicide resistance [3] and loss of weed diversity [4] reflected by the emergence of a few dominant weed species responsible for high yield losses [5]. In particular, the development of herbicide resistance holds a critical role in hindering further herbicide usage for weed control [6]. Intensive use of herbicide is now questioned for their effects on the environment and human health [7]. Additionally, lack of new chemistry against weeds, as stated by Duke and Powles (2012), intensifies the need of alternative weed management approaches [8] However, intensive use of primary tillage, false seedbed during the fallow period, coupled with in-crop mechanical weeding are questioned for their impact on soil health, economic profitability and environmental impacts [9]. Therefore, ecological-based options for weed management appear as promising for both organic and conventional farming [10].

Use of cover crops, living mulches, and competitive cultivars can be used for weed management [11,12], cropping system diversity [13] and soil improvements [14,15]. Cover crops in particular, exert their effects on weeds through competition, allelopathy, physical barrier [16,17] or indirectly by providing habitat for seed predators [18,19,20]. Cover crop species with high biomass accumulation, early-season emergence and rapid growth are more efficient to outcompete weeds, even if the field demonstration of their allelopathic properties remain scare [21,22], cover crops excrete allelopathic substances that suppress weeds further [23]. However, identifying the mechanisms by which cover crops exert their negative effects on weeds in the field remain challenging [24] but is crucial to improve their efficacy. Cover crops have proven to be an effective ecological-based weed management tool in various agricultural systems [25,26,27,28]. Cover cropping is one of the main pillars of weed management in no-till and conservation agriculture systems [29]. However, since tillage and herbicide represent major drivers on weed communities [30] most studies have failed to highlight a carrying over effect of cover crops in tillage-based systems [31]. Furthermore, usage of cover should be optimized in tillage-based systems to be part of integrated weed management strategies in order to reduce the reliance on herbicide use [32].

In order to increase the use of cover crops, knowledge is required to identify the factors hampering farmers’ adoption. It is well documented that the level of weed control provided by cover crops greatly depends on rapid growth and coverage of soil surface, which in turn are influenced by the relative weed:cover crop growth at early stage of main crop establishment, such as seed germination and seedling emergence [33]. This is of particular importance, especially when cover crops exposed to stressful abiotic conditions caused by climate change and must establish high biomass and coverage faster than weeds. In addition, climate change might interfere with the allelopathic potential of cover crops because the persistence of allelochemicals in the soil is not consistent and lasts less than the desired period for effective inhibition of weed seed germination [34,35].

This review aims to critically address factors that affect the germination of cover crops in response to environmental and agronomic factors, seed traits of main cover crop species, and methods to increase cover crop seed germination. Such knowledge is required to improve the establishment and management of cover crops that will eventually boost their integration into cropping systems.

2. Factor affecting cover crops seed germination

There are several agronomic and environmental factors that impact the seed germination of cover crops. Knowing the germination response of cover crop seeds to different condition would be beneficial in effective utilization of cover crop against weeds while providing desirable agroecosystem functions.

2.1. Abiotic factors

2.1.1. Temperature and soil moisture

Temperature and soil water as two important abiotic factors impact germination and early growth of seedlings [36]. The lowest water potential at which a seed can germinate known as base water potential is partially correlated with the species indicator values [37]. Such data on base water potential could be helpful to predict seed germination under various soil moisture. However, cardinal temperatures are the best criteria to determine the optimum habitant for a specific cover crop. Seed traits, including age, nutrient status of the seeds, and quality of seeds can affect their response to temperature and soil water [38].

Some benefits of being aware of temperature and water requirements for seed germination of cover crop seeds, include usage of cover crop mixtures with identical sowing time, hence reducing the risk of failure of homogeneous seed germination. In addition, cover crop early growth under adverse climate conditions can be secured. Furthermore, both simulating the emergence of cover crops under various environmental conditions and predicting the exact date of cover crop emergence are feasible by having such data [39,40].

Tribouillois et al. [41] determined the germination response of a variety of cover crop species to a wide range of temperature and water potential showing suitable temperature was highest for cover crop from Brassicaceae followed by Poaceae, Asteraceae, and Fabaceae. Most of tested species germinated well under warm condition of summer while some species from Fabaceae showed sensitivity to higher temperature regimes.

Generally, cover crop species studied in the research conducted by Tribouillois et al [41] showed two contrasting types of final germination percentage (Figure 1). In one type, germination of all Fabaceae, all C3 Poaceae, some Brassicaceae (Brassica napus, Sinapis alba and Eruca sativa), Phacelia tanacetifolia and Helianthus annuus was steady in temperature range 24–35°C and then decreased near 40°C. In opposition, in the second group, including Brassicaceae, the two C4 Poaceae, Guizotia abyssinica and Polygonum fagopyrum germination was negligible at extreme ends of tested temperature range (Figure 1).

Tribouillois et al. [41] reported that by decreasing water potential germination of some cover crop species , such as Lupinus angustifllius, Vicia faba, Trigonella foenum-graecum, and Pisum sativum decreased but the slope of reduction varied greatly among species (Figure 1). The least base water potential was recorded for species of Poaceae (-1.6 MPa) (especially C3 species) followed by Brassicaceae (-1.4 MPa ) and Fabaceae (-0.6 MPa) (Figure 1)

In comparison to species from Poaceae and Brassicaceae, seed germination of Poaceae were sensitive to water stress (Figure 1). Most Fabaceae species were sensitive to low water availability, which indicates that they are better suited to rainy climates.

Regardless of botanical family, tested cover crop species were grouped based of favorable temperature and water potential which is very informative for choosing a cover crop for a given climate condition and growing season. Tribouillois et al. [41] argued that the value of base water potential of large-seeded plants is higher than that of small-seeded plants, as they require more water for being imbibed.

2.2. Seed dormancy

It is well established that due to poor terminating of cover crop growth and seed dormancy characteristics cover crop are of great potential becoming weedy in subsequent crops.

Seed dormancy is the failure of an intact viable seed to complete germination under favorable conditions (Bewley, 1997). Seed dormancy is a complex trait that both its development and breaking is regulated by a combination of environmental and genetic factors [42,43,44].

Domestication and breeding of major crop species have resulted in removal of most dormancy mechanisms in their seeds inherited from their wild ancestors [45]. However, cover crop seeds frequently have several dormancy mechanisms as they have not undergo vigorous domestications process (genetic and morphological changes within the plant that makes it suitable for cultivation) [42,45,46]. Seed dormancy through different ways can limit the agricultural use of many cover crop species, especially for Fabaceae family. Hence, having information on the genetic and environmental factor affecting seed dormancy is required to prevent them becoming weedy in subsequent crops.

Previous research have demonstrated that combined effects of cover crop genotype and climate conditions during seed development on the maternal plant and then storing condition during postharvest determine the mechanism and level of seed dormancy [46,47,48]. It is well established that climate conditions surrounding the parental plants has highest contribution to germination ability of their resultant seeds [49]. In addition, as seed rain during growing season or after incomplete termination can contribute to the weediness of cover crops in farmlands [50,51].

Several seed dormancy-breaking methods have suggested for alleviation and overcoming dormancy, which varies depending of dormancy type (Table 1). In spite of effectiveness of such proposed stimulatory methods for releasing cover crop seeds from dormancy these practices would add more cost to usage [52].

3. Seed traits of main cover crop species

Worldwide, commonly cover crop species cultivated in different agricultural ecosystem are from genus Vicia, Trifolium, Secale, Lolium, Hordeum, Sorghum, Raphanus, and Sinapis. These species could be classified into three botanical groups, viz. Poaceae, Fabaceae, and Brassicaceae [34], which their seed germination requirements varies greatly among groups [53,54]. This section provides useful information, particularly from a seed germination perspective, to select of crop species for specific production situation.

3.1. Fabaceae

Cover crops of Fabaceae botanical family are popular because of their ability to convert atmospheric nitrogen into plant available forms. Rapid establishment, high capability of biomass accumulation, improving soil organic matter, enhance soil structure and reduce soil erosion risk, and suppress weeds are some properties of legume cover crops [55,56].

Despite several agronomic benefits and desirable agroecosystem functions of cropping systems that incorporate Fabaceae plants as cover crops, seed dormancy imposes limitations on usage, Vicia spp. and Trifolium spp. as main cover crop species (Table 1). From a weed management point of view, these species are noxious and there is limited option for their control in main crops [57]. Seeds of legumes demonstrate both physiological and physical seed dormancy and thereby can persist in the soil seed bank [58].

3.1.1. Vicia spp.

Vicia villosa is considered as the only species from Vicia genus that can survive moderate to harsh winter conditions [59] Produced seeds by V. villosa, similar to other species of this genus, are dimorphic comprising of both soft and hard seed coats. Hard seeds of V. villosa persist more than two years and higher rate of dormancy-breaking during first 6 months. Furthermore, it is estimated that >45% of vetch seeds recently shed from maternal plant are able to germinate [58] .

Similar to other member of legume, combinational dormancy (physiological and physical) occurs in seeds of V. villosa. In many hard seeds, after removal of physiological dormancy, seeds are capable of germinating over a wide range of environmental conditions (Table 1). Releasing seeds of V. villosa from dormancy would accelerate by the after-ripening environment in the summer. Hence, summer, dormancy of V. villosa seeds would be alleviated and emergence of seedlings would take place in autumn. Afterwards, best-established seedling survive in harsh winter [60].

According to this information, it could be concluded that mitigating seed dormancy of hairy vetch and providing the enough water for its successful germination are two main factors determining the acceptance of this crop as a fall-planted legume cover crop. Dormant seeds add to the soil seedbank via two different ways, contaminated seeds aimed cultivation of cover crop and from unsuccessful determined cover plants.

It is reported that priming is not effective in releasing dormancy in hard (Viable seeds that do not imbibe water and thus fail to germinate in an apparently favorable) and physiologically dormant seed with while negatively affect seedling growth, particularly under water deficit (Rolston 1978).

Hard seeds and regrowth after it termination with mechanical means usually result in weediness of V. villosa in subsequent crops. As under reduced tillage condition the common method of cover crop vegetative growth termination mowing or roller crimping, regrowth of V. villosa is a challenge in the conservation systems. In order to reduce the risk of regrowth, conducting mowing during full flowering (50 to 100%) and early pod setting of plants is suggested [50]. Furthermore, adoption of early-flowering cultivars is more suited as they can be terminated more earlier than cultivation of warm season crops in spring. Hence, in addition to lower percentage of dormant seeds and overwinter survival; early flowering is the major specific traits

In addition to seed dormancy, different genotypes of Vicia villosa exhibit pod dehiscence [46] resulting in shattering of seed prior harvest operation yield and adding dormant seeds to the soil seedbank. Percentage of indehiscent and dehiscent pods are related, in part, environmental condition surrounding growing plants [61]

Despite negative effects of evolution dormancy in seeds of V. villosa and its pod dehiscence, these traits that would make cost effective the utility of cover crops in agroecosystems [62]. This mainly due to improvement of self-regeneration no-till cropping systems[58].

Faba bean (Vicia faba L., broad bean, horse bean), another important member of legumes is cultivated as a winter annual it tolerate chiling temperatures in opposition to field pea, cold do not terminate V. faba growth, furthermore, this legume fixes more N₂ than other cool-season legumes, like winter pea (Pisum sativum L.) and lupin (Lupinus albus L.) [63]. Peas are sensitive to cold and, limiting their cultivation during winter in temperate regions. The main advantage of seeds of the specie belong to genus are not hard and tolerate bad soil [64].

3.1.2. Trifolium spp.

Several species of the genus Trifolium are commonly adopted for cover cropping, due to their rapid growth and allelopathic activity (containing phenols and isoflavonoids) on weeds [65,66].

Seeds of different clover species can germinate in low temperatures and grow well in shady, cool, and moist conditions, which is common under closed canopy of cash crops. Hence, clovers are best option to be used for interseeding proposes [67]. Nevertheless, small seed size, low seedling vigor, development of seed dormancy, and poor establishment are some weaknesses of clovers are hindering their extensive application as cover crop [68].

Similar to most members of Fabaceae family, seeds of clover exhibit a variable ratio of hard seeds. Proportion of hard produced by plant varies depending on soil and environmental factors, such as temperature, relative humidity, soil texture and fertility, photoperiod [69]. Accordingly, varieties of the same species show variation in seed hardiness percentage. hence, clover species may persist in soil seed bank, making problem in the next crop [70]. Research suggests that growth characteristics of Trifolium spp abilities vary greatly among species, suiting each species for intended uses.

Collectively, factor contribute to weediness of legume cover crops and V. vilossa in particular are development of combinational dormancy mechanism in seeds, capability of regrowth after mechanical termination, and pod dehiscence. When compared to other member of Fabaceae, V. villosa is less domesticated. To minimize the weediness threat of V. villosa in subsequent cash crops, breeding to reduce pod dehiscence, proper cultivar selection, avoiding any environmental stress to growing plants, choosing suitable termination time and method could be useful recommendations.

3.2. Poaceae

There are numerous annual and perennial grass species that can be used as cover crops. Globally, commonly used cereals as cover crop are Secale cereale L., Avena sativa L., Lolium perenne , and Sorghum bicolor (L.) Moench. [71]. Grasses present special traits suitable for weed suppression proposes within crops, mainly superficial root systems, allowing them to control weeds without competition for water with the main crop [72]. The best results from cultivating grasses achieved when they are stablished in optimum time, which in turn is dependent on seed germination process [73].

Winter annual grasses germinate during fall coinciding with cool and moist condition. This cycle is regulated by presence of non-deep physiological dormancy commonly overcome by high temperature of summer high temperatures during dry-after-ripening [74]. Furthermore, the optimum temperature for germinating seeds of these plants is about 16°C, preventing germination of non-dormant and freshly shed seeds in summer (Table 1).

Jiménez-Alfaro et al. [73] by collecting seeds from six winter annual grass species growing in Spanish olive gardens evaluated their seed germination in response to various temperature regimes to determine their suitability to be used as ground cover in Mediterranean agroecosystems.

Their results showed that contrary to previously published works, dormancy showed low effect on preventing summer germination. However, this low level of dormancy is helpful in inhibiting seed germination immediately after dispersal and under hot and dry conditions during summer.

In general, low temperature and adequate moisture, which are common characteristic of fall season of temperate regions, provide suitable condition for seed germination of these winter annual grasses [73].

Secale cereale is the most common winter grass cover crops in which the amount of nitrogen scavenged, the main benefit of adoption of this species as cover crop, is greatly dependent on to biomass production, growing season length, burial depth of seed in the soil [75].

Secale cereale is one of the most recently domesticated cereals so it poses a great danger to spread as important weed [76]. This species is very challenging in cereals crop like wheat and barley as there is no chemical option for its control. In addition, seeds of its wild relatives exhibit varying level of dormancy, which enable S. cereale to maintain its presence in the following crops in the rotation [77].. Hence, careful management of this cover crop, in particular termination operation, for preventing its weediness is crucial,

3.3. Brassicaceae

Cover crops belonging to Brassicaceae family (mustards or Cruciferae) contain various allelochemicals, mainly glucosinolates. Derivate of this compound, including organic cyanides, oxazolidinethione, and isothiocyanates can suppress weeds [78]. By incorporation of residues of mustards into soils, its allelochemicals acts as biofumigant against germination and growth of weeds [79].

To maximize efficacy of Brassicaceae species in enhancing agroecosystem productivity and hindering its weediness in subsequent cash crop, optimum timing of termination is necessary. Under poor termination, the high growth rate and pod-shattering characteristics of some Brassicaceae cover crop make surviving plants problematic weeds. Additionally, seeds added to the soil seed bank remain dormant for many years and become a challenge in the next crops [80] (Table 1).

Seeds of mustard are very small hindering them from emerging from deep layers and coarse-texture soil layers [81]. Therefore, preparing a soft and fine seedbed is essential for successful establishment. This is a very important issue should be considered about Brassicacea as the main mechanism they suppress weed is through rapid soil coverage [82].

From this review, it could be argued that suitable establishment time and optimum density of cover crops are the most important challenge to get desired ecosystem services and highest degree of weed suppression from all three main cover crop groups namely Fabaceae, Poaceae, and Brassicaceae and others regardless of their seed and seedling emergence traits. Climate variables, oil properties, management practice, and species characteristics altogether contribute influence these challenges [83].

Tribouillois et al. [83] investigated emergence dynamics of cover crop species mainly from three botanical families (Fabaceae, Poaceae, Brassicaceae) under different field conditions to estimate emergence duration and time in response to different sowing conditions by a static model. Results indicated a drastically high variation in emergence duration and percentage depending on situations of each cover crop species. Furthermore, they concluded that emergence of cover crop is strongly related to water availability.

In addition, they showed that crucifer cover crop species, such as Brassica rapa and Sinapis alba by having short emergence duration are capable to be cultivated in late summer. This is because their germination and emergence process take place within a few days enabling them to benefit from rare rainfall or moisture of seedbed. In opposition, sowing of legumes with delayed emergence, probably because of slower seed imbibition, are sensitive to water deficit. The rapid emergence of Brassicaceae may explain their ability to suppress weed effectively.

4. Solutions for enhancing cover crop seed germinability

4.1. Agronomic practices

4.1.1. Sowing time and planting geometry

As living mulch, cover crops undersown between rows of cash crop between crop planting lines resulting, inhibition of seed germination of photoblastic seeds and suppression of seedling growth [84]. Undersown cover crops for weed suppression are used for low, taprooted competitive crops like sugar beet, cotton, and canola, that sown in wide row spaces (Baumann et al., 2001).

Main types of cover crops sowing methods are as drilling and broadcasting (aerial spreading or interseeding) of seeds. Drilling seed by burying the seeds into the soil would result in a better cover crop establishment when compared with broadcasting method [85]. In small-seeded species, needed seeding rates is higher for broadcasting seed as cover crop establish poorly [86].

Broadcasting cover crop seeds into living cash crops (like corn and sugar beet) growing, particularly at crop maturity, can allow for better cover crop establishment as seeds benefits from warm and moist conditions created by leaves [87]. Broadcasting cover crops into cash crops at crop maturity has several advantages mainly more biomass production, although seeding rate is higher (at least 25 to 50% ) than that of drilling sowing method [88,89]. Furthermore, interseeding would result in poor establishment of cover crops as seeds left on the soil surface are exposed to biotic and abiotic stresses, such as, water deficit, low-light conditions and seed predators [75,90]. Mirsky et al. [91] suggested soil depth range of 3 to 5 cm to obtain highest seed germination percentage.

On the other hand, in no-till conditions, broadcasting cover crop seeds into crop residue reaming of harvesting either winter or summer crops provide a protective means for seed germination and seedling emergence of cover crop against adverse factors, wind speed, soil evaporation, chilling temperature [92,93]. A linear relationship between cover crop stand counts and seeding rate has been reported with exception with species from Poaceae. In cover crops of Poaceae limited available water will further restrict thier seeding rate in broadcast interseeded method [94].

Rapid emergence of cover crops sown in tillage system is would result in more weed management as cover crop emerge more rapidly, which is due to its better access to soil moisture [95,96]. Poor soil-seed contact in no-till usually limit seed germination as locating seeds on the straw deprive seeds from water for germination. On the contrary, deep tilling by burying weed seed worsen the weed problem [84]. Hence, providing suitable seed contact to soil by optimizing seeding depth (2-3 cm) is crucial for successful germination and seedling growth of cover crops.

4.2. Seed pretreatment

4.2.1. Seed priming

Seed priming is the process of accelerating water absorption by seeds and onset the metabolism phases of germination before radical protrusion and then drying and stabling to original moisture level [97]. Seed priming by initiating physiological and biochemical contents of treated seeds; enhance aspects of seed germination and seedling emergence of a wide range of crop species (Table 2).

Seed priming improves seed germination and seedling establishment of cover crops at early growing season. In addition, it causes rapid growth of cover crop through increasing of water uptake, nutrients, securing higher as well as more uniform cover crop stand [98,99]. Seed germination and seedling emergence response to seed priming vary among species (Table 2). Cover crop species with small seed size and hard seed coating [100,101] are more likely to benefit more. In addition, both priming media and duration impact seed germination and seedling emergence [102].

In semi-arid areas, lack of moisture in early autumn inhibits seed germination and seedling growth of cover crops. Hence, accelerating germination of cover crops by priming not only make tolerate their seedling to water stress but also enhance their competitiveness against weeds. For example, Yusefi-Tanha et al [103] reported that priming of hairy vetch seeds with potassium nitrate and distilled water prompted guaiacol peroxidase and catalase activity in seedling and subsequently enhanced the ability of the seedling to resist against oxygen free radicals resulting from the peroxidation of different compounds. Furthermore, they demonstrated that the performance of different priming in enhancing germination of hairy vetch varied depending on ambient temperature.

Table 2. Seed mass and seed germination response of a wide range of cover crop species to seed treatment (priming and coating).

Cover crop species	1000-Seed weight (mg) §	Seed treatment
		Priming	Coating
Guizotia abyssinica	3.3	+ [127]	unknown
Helianthus annuus	48.0	+ [128]	+ [129]
Brassica carinata	5.0	Unknown	unknown
Brassica juncea	3.0	+ [104]	unknown
Brassica napus	2.7	+ [130,131]	+ [132]
Brassica rapa	3.7	+ [133,134]	+ [135]
Camelina sativa	1.3	+ [136,137]	unknown
Eruca sativa	1.3	+ [138]	unknown
Raphanus sativus	13.0	+ [139]	unknown
Sinapis alba	8.0	Unknown	unknown
Lathyrus sativus	176.0	+ [140]	unknown
Lens nigricans	21.5	Unknown	unknown
Lupinus angustifolius	179.4	Unknown	unknown
Medicago lupulina	1.5	Unknown	+ [141]
Melilotus officinalis	2.5	Unknown	unknown
Onobrychis viciifolia	23.0	+ [142]	+ [141]
Pisum sativum	168.8	+ [143,144]	+ [145]
Trifolium alexandrinum	3.0	+ [99]	unknown
Trifolium incarnatum	4.7	Unknown	unknown
Trifoliumhybridum	0.83	Unknown
Trifolium resupinatum	1.48	Unknown	unknown
Trifoliumpratense	2.04	+ [146]	+ [113]
Trifoliumsubterraneum	6.28	+ [147]	unknown
Trifoliumrepense	075	+ [148]	+ [141]
Trigonella foenum graecum	16.0	+ [149]	unknown
Vicia faba	359.6	+ [150]	unknown
Vicia sativa	53.8	+ [151]	unknown
Vicia villosa	26.7	+ [103,108]	unknown
Phacelia tanacetifolia	1.8	+ [152]	unknown
Avena sativa	39.4	+ [153]	+ [154]
Lolium hybridum	3.4	Unknown	unknown
Lolium multiflorum	2.7	+ [155]	+ [156]
Secale cereale	32.3	+ [157]	unknown
Secale multicaule	18.8	Unknown	unknown
Setaria italica	2.2	+ [158]	unknown
Sorghum sudanense	13.8	+ [159]	unknown
Fagopyrum esculentum	25.0	Unknown	unknown

§ 1000-Seed weight [160,161].

Table 2. Seed mass and seed germination response of a wide range of cover crop species to seed treatment (priming and coating).

Cover crop species	1000-Seed weight (mg) §	Seed treatment
		Priming	Coating
Guizotia abyssinica	3.3	+ [127]	unknown
Helianthus annuus	48.0	+ [128]	+ [129]
Brassica carinata	5.0	Unknown	unknown
Brassica juncea	3.0	+ [104]	unknown
Brassica napus	2.7	+ [130,131]	+ [132]
Brassica rapa	3.7	+ [133,134]	+ [135]
Camelina sativa	1.3	+ [136,137]	unknown
Eruca sativa	1.3	+ [138]	unknown
Raphanus sativus	13.0	+ [139]	unknown
Sinapis alba	8.0	Unknown	unknown
Lathyrus sativus	176.0	+ [140]	unknown
Lens nigricans	21.5	Unknown	unknown
Lupinus angustifolius	179.4	Unknown	unknown
Medicago lupulina	1.5	Unknown	+ [141]
Melilotus officinalis	2.5	Unknown	unknown
Onobrychis viciifolia	23.0	+ [142]	+ [141]
Pisum sativum	168.8	+ [143,144]	+ [145]
Trifolium alexandrinum	3.0	+ [99]	unknown
Trifolium incarnatum	4.7	Unknown	unknown
Trifoliumhybridum	0.83	Unknown
Trifolium resupinatum	1.48	Unknown	unknown
Trifoliumpratense	2.04	+ [146]	+ [113]
Trifoliumsubterraneum	6.28	+ [147]	unknown
Trifoliumrepense	075	+ [148]	+ [141]
Trigonella foenum graecum	16.0	+ [149]	unknown
Vicia faba	359.6	+ [150]	unknown
Vicia sativa	53.8	+ [151]	unknown
Vicia villosa	26.7	+ [103,108]	unknown
Phacelia tanacetifolia	1.8	+ [152]	unknown
Avena sativa	39.4	+ [153]	+ [154]
Lolium hybridum	3.4	Unknown	unknown
Lolium multiflorum	2.7	+ [155]	+ [156]
Secale cereale	32.3	+ [157]	unknown
Secale multicaule	18.8	Unknown	unknown
Setaria italica	2.2	+ [158]	unknown
Sorghum sudanense	13.8	+ [159]	unknown
Fagopyrum esculentum	25.0	Unknown	unknown

§ 1000-Seed weight [160,161].

Under low temperature conditions, hydropriming (soaking seeds in water) of hairy vetch had higher positive impact on seed germination in comparison with either halopriming or hydropriming. In contrast, under higher temperature (15 C) the efficacy of priming was not significantly different from non-primed condition, showing the advantage of priming only under adverse conditions. Yusefi-Tanha et al [103] concluded that both halopriming and hydropriming were more efficient improving seedling establishment and early growth of hairy vetch at lower temperature by enhancing physiological parameters and germination process.

In another study, effect of seed priming duration on germination of some cover crop species of seed size and germination traits, including cereal rye (Secale cereale L.), perennial ryegrass (Lolium perenne L.), hairy vetch (Vicia villosa Roth), and oriental mustard (Brassica juncea L.), [104]. They determined the effectiveness of priming for seedling emergence of perennial ryegrass and hairy vetch under compacted for evaluating the seedling vigor.

Similar to above-mentioned study, Snapp et al [104] demonstrated that seed priming accelerates germination for hairy vetch, mustard, and perennial ryegrass. Perennial ryegrass, with the smallest seed size among evaluated species was the only species, of which seed germination was improved substantially with priming under non-stress condition. They showed that seedling emergence of hairy vetch and perennial ryegrass in the compacted soil was improved by seed priming (Table 2).

Emergence of hairy vetch and perennial ryegrass from compacted soil was improved by 39% and 42% when compared with unprimed seed, respectively[104]). This is a valuable result as typically cover crops cultivate in fields with bad soil conditions.

Hydro-priming and osmo-priming (soaking seed in chemicals that reduce osmotic potential of seed) are regularly applied to improve seed performance in various cultivated crops [105].

Increased seed germination by priming seeds with potassium nitrate (KNO₃) can be attributed one or more of mechanism softening impermeable seed coat, releasing of ethylene within embryonic tissues, and washing out of seed germination-inhibitor compounds from seeds [106,107]. For example, [108] pointed out that Hydro-priming is suitable for older seeds of pod vetch [Vicia villosa ssp. dasycarpa (Ten.)] while found osmo-priming (with KNO₃) a better pre-treatment for freshly harvested seeds.

4.2.2. Seed coating

covering seeds with external materials to improve their handling, protection , and, to a considerably lesser extent, germination enhancement, seedling vigor, and stand establishment is called seed coating [109].

Seed coating with biostimulants consisting of microbial inoculants, beneficial bacteria and fungi, nitrogen containing compounds, biopolymers, and plant extracts is more environmentally friendly and effective compared to less sustainable conventional pesticides and fertilizers [110,111,112]. Amongst other seed coating techniques seed pelleting, film coating, and seed encrusting are the most commonly used. Seed germination and seedling vigor of coated seeds are not only influenced by chemical properties of applied compounds but also to a higher extend physical properties and thickness of the coating. Hence, an optimum coating thickness also should be determined for a given cover crop species in order to seed coating be effective.

Qiu et al. [113] investigated the seed germination and seedling growth of red clover (Trifolium pratense L.) and perennial ryegrass (Lolium perenne L.) seed response to coating with different combinations of soy flour, diatomaceous earth, micronized vermicompost, and concentrated vermicompost extract. Results indicated that germination percentage, uniformity, speed, as well as seedling growth of coated seed of red clover were higher when compared with non-treated seeds control.

In opposition to red clover, seed coating with various biostimulants reduced seed germination for perennial ryegrass while growth seedling produced by coated seed were significantly enhanced. The results of this study emphasis the importance of species-specific response to coating treatment when adopting seed coating for improving the germination and subsequent establishment of desired cover crops.

5. Conclusion

The delivery of most ecosystem services is related to cover crop biomass productivity and result from successful establishment and early growth, which in turn are affected greatly by cover crop seed traits. Here we showed for the first time that seed traits of cover crops are major drivers of cover crop weed supressivness. Furthermore, information on the response of cover crop seed germination to biotic and abiotic factors as well as methods for improving germination and seedling emergence is crucial. Farmers facing climate change are looking for species/varieties with appropriate seed traits which coupled with innovative farming strategies could allow them to obtain a fair return on investment. Information presented in this review on the seed traits and treatments of cover crops would be helpful for a diversity of stakeholders (e.g., farmers, extension services, researchers, seed companies) to use cover crops more effectively.