A review on hardseedness and breaking dormancy in tropical forage legumes

Muhammad Rusdy

Laboratory of Forage Science and Grassland Management Faculty of Animal Science, Hasanuddin University, Indonesia
muhrusdy79@yahoo.co.id

Abstract

Introduction of forage legumes into grassland is very beneficial to increase productivity and nutritive value of tropical grasses. However, their successful establishment is hampered by their high hard seed that impaired germination.This paper reviews some work concerning the hardseedness in tropical forage legumes and methods of their breaking dormancy. The hard seed in forage legumes was mostly found in herbaceous forage legumes. The type of seed dormancy was mostly physical dormancy with acid scarification was the most efficient treatment to break seed dormancy, followed by sandpapering and hot water. Some forage legume seeds showed physiological dormancy that can be broken by gibberellic acid and pre-chilling treatments.

Keywords: establishment, external dormancy, internal dormancy, leguminous seeds, scarification

Introduction

The major constraints to increasing livestock production in tropical country like Indonesia is the scarcity and fluctuating of yield and quality of year round forage supply. During the rainy season which lasts about 4 - 6 month, there is adequate forage of good quality for ruminant animals, but during the dry season, the available forage generally is low and does not meet the feed requirement of ruminants, both in terms of quantity and quality. Consequently, during the dry season, the animals raised in the natural grassland have to be supplemented with crop residues, browse plants, concentrates or forage legumes. As crop residues availability is seasonal and the high prices of concentrates, the use forage legume as feed supplement to animals fed grass base diets could be the best option to obtain sustainable animal production in the tropics.

Forage legumes offer important opportunities for sustainable grassland-based animal production because they can increase forage yield and quality, and raising the efficiency of conversion of forage to animal products. As forage, legumes have many advantages compared to grasses. Legumes have higher protein and mineral levels and can maintain this nutrient levels during growth than do grasses, which decline rapidly in quality with advancing maturity. Legumes also have both higher digestibility and intake than grasses and their nutritive value tends to remain higher as plants mature (Tothil 1985). Species such as Leucaena leucocephala has been grown with grasses as fodder banks to provide a high quantity and quality feeds for grazing ruminants (Norton 1998).

N fixed by forage legumes can act as substitutes for high prices of inorganic N-fertilizer. N is the primary nutrient limiting plant production in most natural ecosystems (Williams 2017). Through their ability to fix atmospheric N, legumes convert unusable nitrogen from the atmosphere into ammonia. This biologically fixed N can become available to companion grass thorough plant decomposition and recycling of N from animal excreta which ultimately increases the productivity and quality of grass.

Forage legumes can be planted during crop rotation to enrich soil and protect it from the damaging effects of wind and rain. With the growing consciousness that the soil must possess a vegetal cover to prevent erosion, legumes are being used more for erosion control. They are grown as cover crops in strips alternating with clean-tilled crops, in grass-legume mixtures, ley farming or in alley cropping system.

Despite the great importance, establishment of most forage legumes is difficult. One of the major constraint in successful stand establishment of forage legumes is its high degree of hard seed, which can cause delayed or decreased their germination, seedling emergence and growth. As a result, stands become sparse, sporadic, less uniform and less competitive with weeds or undesirable species. Such legume stands reduce not only N fixation but also lower yield and quality. Therefore, reduction of hard seed content in seed lot of forage legumes before planting is important. In the present paper, many work on hardness in seeds of forage legumes and efficacy of scarification methods on breaking dormancy of seeds of tropical forage legume species are presented and reviewed.

Hardseedness in forage legume species

Both herbaceous forage legumes (Table 1) and forage shrub/tree legumes (Table 2) showed hard seeds, as evidenced from induction of seed germination by some breaking dormancy treatments. Further, from the both tables it can be shown that herbaceous legumes showed more hardseedness than shrub/tree legumes.

The high hardseedness in legumes is in agreement with Rolston (1978) that most Fabaceae (subfamilies Caesalpiniodeae, Mimosoideae and Papilionoideae) have impermeable seed coat that impose seed dormancy. This impermeability of the seed coat is caused by the presence of one or more palisade layers of lignified Malphigian cells (macrosclereids) tightly packed together and impregnated with water-repellant chemicals. Seeds of such legume species will not germinate quickly when subjected to favorable conditions because their hard seed coat prevents water and gases entering the seeds. Under natural conditions, this impermeability gradually decreases, so that a certain percentage of seeds germinates in each period (Morais et al 2014).

Not all seeds of forage legume species are hard seeded. Seeds of some legumes, mainly forage tree legumes like Calliadra calothyrsus, Gliricidia sepium, Sesbania grandiflora (Gutteridge and Stur 1998) (Table 2) are not hard and do not need to betreated before sowing, although seeds of some species like Sesbania sesban, S. rostrata and S. virgate, seed scarification is recommended to ensure uniform seed germination (Veasey and Freitas 2002). Seeds of species without hard seed coat requires specialized storage conditions if seed viability is to be maintained. Ideally seed of these species should be stored in sealed containers at a moisture content of less than 10% and at a temperature of 4°C or less. For example, Calliandra seeds retained viability when stored in a refrigerator (4°C) for 2.5 years but viability was reduced by 15% when seed was stored at room temperature for one year (NAS 1983).

Efficiency of seed scarifications on germination of forage legumes

In forage tree legume species, the most efficient breaking dormancy method was immersion in concentrated sulfuric acid (Table 2) while in herbaceous forage legumes, the most effective treatments were immersion in concentrated sulfuric acid, sandpapering, hot water and cold water (Table 1). In general, chemical scarification with concentrated sulfuric acid, mechanical and hot water scarifications resulted in significantly higher total germination percentages than the other treatments involved. Because acid, mechanical and hot water treatments associated with disruption of seed coat, it can be inferred that most seed dormancy in forage tropical legumes is physical dormancy.

In the present review, efficiency of sulfuric acid scarifications were varied with legume species and duration of scarification. The harder of the seed coat, the longer of scarification time may be needed. Insufficient soaking of seeds leaves the seed coats of species glossy; coats of correctly treated seeds are dull, but not deeply pitted, conversely oversoaking may pit the seed and even expose the endosperm to acid.

The high efficacy of acid scarification on breaking seed dormancy is also reported in temperate forage legumes, like Medicago and Trifolium species (Can et al 2009). Sulfuric acid is thought to disrupt the seed coat and expose the lumens of the macrosclereids cell, permitting imbibition of water which triggers the release of simple sugar that could be readily used for protein synthesis, thereby encouraging germination (Jackson 1994).

Following acid scarification, mechanical scarification by hand with sandpaper was quite effective in increasing germination of forage legume seeds (Table 1 and Table 2). This response supports evidence that the seed coat of the plant is the main inhibitor of germination. Lignified palisade cell layer in the seeds could be damaged after sandpapering and germination occurs with water penetration (Yildiztugay et al 2012).

Although mechanical scarification was an effective treatment for promoting germination but this method was labor intensive and many seeds were broken in the scarification process, especially to small seeded legumes. This make the mechanical scarification with sandpaper is inefficient for large quantities of seeds.

Hot water scarification has been reported as an economical method for breaking seed dormancy, especially for hard-coated seeds. Its efficiency varied with legume species, temperature and soaking duration (Table 1 and Table 2). It is suggested that the harder seeds need higher temperature and longer soaking duration, however embryo may get destroyed on contact with high temperatures of water for a prolonged period. Hot-water treatment has been reported to enhance germination by affecting various factors, viz. seed coat permeability for water and gaseous exchange and release of inhibitors (Sharma et al 2008). It is believed that hot water, which is a form of thermal scarifications, breaks physical dormancy in seeds by causing randomly located cracks in seed coats without altering the anatomy of the micropyle. The cracks then allow water to enter, which initiates germination (Baskin and Baskin 1999).

Table 1 shows that only germination of Macroptilium atropurpureum that enhanced by cold water treatment; cold water scarification even was more efficient than acid scarification (Table 1). It is suggested that Macroptilium atropurpureum seeds not as hard as most tropical legumes that hardly to be permeated by water. It seems that seed dormancy in this plant is not very strong.

Table 1 also shows that besides enhanced by sulfuric acid and sandpapering, germination in Pueraria phaseloides was also enhanced by application of gibberellin, which indicates occurrence of seed physiological dormancy This indicates that in this plant, seed dormancy is caused by both physical dormancy and physiological dormancy. Gibberellic acid is known can activate synthesis of protein and other metabolites that are required by the embryo for germination (Golmohammadzadeh et al 2015). Physiological dormancy prevents embryo growth and seed germination until chemical changes occur. These chemicals include inhibitors that often retard embryo growth to the point where it is not strong enough to break through the seed coat or other tissues.

Presence of both physical dormancy and physiological dormancy is also found in Leucaena leucocephala seeds (Omran 2013) (Table 2) as evidenced by increasing germination using sulfuric acid or sand paper scarification and keeping the seeds under pre-chilling conditions. Physiological dormancy can be broken by pre-chilling because of occurrence of imbibition of water and protein hydration during the pre-chilling stage (Flannigan and Woodward, 1993).

Conclusion

• Most seed dormancy in tropical forage legumes is found in herbaceous legumes.
• Most seed dormancy in tropical forage legumes is physical dormancy.
• The most efficient treatment to break seed physical seed dormancy is sulfuric acid scarification, followed by mechanical and hot water scarification.

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Received 28 May 2017; Accepted 22 November 2017; Published 1 December 2017

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