Seed Scarification Methods and their Use in Forage Legumes
Forage legumes are important crops because of their ability to improve soil health by fixing atmospheric nitrogen (N), reduce fertilizer cost and produce high quality forages. However, one of the major problems associated with the legumes is hard seed content which results in poor stand establishment because of reduced germination and un-uniformed seedling emergence. It is important to reduce the hard seed content and improve germination for successful stand establishment of these crops. Seed scarification (a technique to physically damage the seed coat to reduce hard seed while keeping the seed viable) is used to soften hard seeds. Researchers have been using different scarification methods since early 20th century and have reported variable results. Heat, freeze-thaw, mechanical and acid scarification are among the most popular methods. This study reviews available literatures and summarizes information on scarification methods with special emphasis on seeds of Medicago, Trifolium and Astragalus species. Information obtained in this search improves data bank of seed scarification methods and helps researchers to improve the methods.
Received: September 16, 2011;
Accepted: December 24, 2011;
Published: February 24, 2012
Legumes are a member of the Fabaceae family which contains 700 genera and about
19,000 species (Graham and Vance, 2003). They are distinctive
with unique floral structures, pod-like fruits and nodule formation systems
in the root. Many species from this family have been economically important
to humans as far back 3,000 year ago when wild Fabaceae family was domesticated
in America and Asia (Kaplan and Lynch, 1999). By the
early 21st century, legume production has occupied approximately 15% of the
earths surface (Graham and Vance, 2003). The significant
increase of legume land use is mainly due to production of high quality forage
(Evers, 2011), ability to improve soil quality by fixing
atmospheric N (Faria et al., 1989) and the extension
of spring growth (Balasko and Nelson, 2003). A good example
of an attractive forage legume is Medicago species (e.g., Medicago
sativa L.), the most widely and extensively used representative forage legume
in the world (Bouton, 1996). Other important forage legumes
include Trifolium and Astragalus species (Carleton
et al., 1971; Acharya et al., 1993;
Patane and Bradford, 1993; Martin
and De La Cuadra, 2004). In many instances, N fixation by Trifolium
species is significant. For example, red clover (T. pretense L.) and
white clover (T. repens L.) are able to fix up to 373 and 545 kg N ha-1
year-1, respectively (Carlsson and Huss-Danell,
2003). Recently, Astragalus species such as cicer milkvetch (CMV:
A. cicer L.) has attracted attention of many scientists and producers.
This is a perennial legume, highly tolerant to overgrazing because of its vigorous
root systems (Tilley et al., 2008) and does not
produce bloat problem in ruminant animals (Twidwell and Kephart,
2002; Acharya et al., 2006).
Despite the great importance and characteristics, establishment of forage legumes
is difficult. One of the major constraints in successful stand establishment
of forage legumes is hard seed. High hard seed content in a seed lot can cause
delayed or decreased seedling emergence. As a result, stands become thin, sporadic
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 a seed lot of forage legumes is important before planting.
Seed scarification, a physical damage to break the hard seed coat without lowering
the quality of seeds, has been studied for more than a century (Dixon,
1901; Harrington, 1916; Stewart,
1926; Rincker, 1954; Rolston,
1978; Stanwood, 1980; Rutar
et al., 2001; Zeng et al., 2005; Dittus
and Muir, 2010).
Seed scarification methods have been developed and modified over time to make
these more practical and effective. Important methods of seed scarification
include heat, freeze-thaw, mechanical and acid scarification. Heat scarification
is the method that uses high temperatures to break or crack seed coat (Staker,
1925; Tomer and Maguire, 1989). Freeze-thaw scarification
is a method that breaks the seed coat by exposing seeds to temperature alternations
between low and high (Stout, 1990; Rutar
et al., 2001). Mechanical scarification is a technique to physically
create scars on seed surface to increase water imbibition of the seed (Uzun
and Aydin, 2004; Rostami and Shasavar, 2009; Olisa
et al., 2010; Jayasuriya et al., 2012).
Acid scarification is a chemical method to melt seed coat and soften hard seed
(Can et al., 2009). The objective of the present
study is to review and summarize some important seed scarification methods and
their use in forage legume species, especially in Medicago, Trifolium
and Astragalus species, aiming to improve productivity and profitability
of these valuable forage legumes. The information generated in the review is
useful not only for researchers and producers but also for seed companies.
Heat scarification has been one of the most popular methods because it is simple
and easy to use. Two main heating devices used in heat scarification include
oven (Harrington, 1916; Staker,
1925; Stewart, 1926; Lute, 1927;
Rincker, 1954; Rutar et al.,
2001) and hot water bath (Uzun and Aydin, 2004;
Patane and Gresta, 2006; Can et
al., 2009). Efficacy of heat scarification varies significantly depending
on heating devices, treatment times and temperatures.
Heat scarification using the dry heat in oven seems to be effective on hard
seed reduction and germination improvement when appropriate treatment time and
temperature are used (Harrington, 1916; Staker,
1925; Stewart, 1926; Lute, 1927;
Rincker, 1954; Tomer and Maguire,
1989; Rutar et al., 2001). Various temperatures
(below 40°C to over 100°C) and treatment times (one min to 21 h) were
used in different studies and variable results were reported. Some treatments
were more effective than the others. For example, heat scarification had little
influence on hard seed reduction when treatment temperatures were between 40
and 50°C (Harrington, 1916; Lute,
1927; Rutar et al., 2001). Neither reduction
of hard seed (range 14-29%) nor improvement of germination (range 87-96%) was
observed in seeds of three cultivars (Osjecka 10, Osjecka
88 and Slavonka) of alfalfa (M. sativa L.) when they
were heated at 40°C for 4 h (Rutar et al., 2001).
Heat scarification had no influence on seeds of red clover and sweet clover
(Melilotus officinalis L. Pall.) when they were subjected to treatment
temperature at 50°C for 21 h (Harrington, 1916).
Lute (1927) also reported that temperature below 50°C
had no influence on alfalfa seeds.
||Effect of heat scarification on alfalfa seeds with various
temperatures and four treatment timings. Arrows indicate the death point
of alfalfa seeds in each treatment time. There is a missing value for the
4 h treatment at 93°C (Staker, 1925)
Higher temperatures may be more effective on hard seed reduction compared
to the lower temperatures; however, negative impact was observed when temperatures
were higher than 80°C (Rincker, 1954; Rutar
et al., 2001). Greater amount of dead seeds (38%) were observed when
alfalfa seeds were heated at 80°C for one h, while lesser dead seeds (1%)
were found at 40°C when heated for 5 h (Rutar et
al., 2001). Alfalfa seeds were completely damaged when heated at 104°C
for 1 h (Rincker, 1954). Staker
(1925) conducted a study to investigate mortality of alfalfa seeds during
heat scarification. In this study, alfalfa seeds were heated over four different
times (half, one, two and four h) at various temperature regimes starting from
60 to 105°C. Germination declined at temperatures greater than 80°C
and the number of dead seeds increased when temperatures were 90, 91 and 93°C
with treatment times of four, one and two and half h, respectively (Fig.
Nevertheless, heat scarification with high temperatures was shown to be effective
and successful in improving germination in many studies. For example, Rincker
(1954) scarified alfalfa seeds using heat scarification at 104°C with
treatment time of one, two, three and four min. Germination was improved greatly
from 47% in untreated control to 84% after four min of heat scarification. Stewart
(1926) showed that heat scarification at 80°C was effective when heated
for 2 h in dry heat as it increased germination of alfalfa seeds from 69 in
untreated control to 99%. He also reported that weed seed could be filtered
from alfalfa seed lots by applying heat scarification. Weed seeds contaminated
in alfalfa seeds were killed when heated at 85°C for 4 h while germination
of alfalfa seeds was increased. Such weed species were dodder (Cuscuta
sp.), atriplex tumbleweed (Atriplex hastate L.) and Russian thistle (Salsoli
tragus L.). Varying effectiveness of heat scarification is reported when
treatment temperatures are higher than 80°C. As compared to the higher temperatures,
there were no observations of abnormal seedlings or dead seeds when heat scarification
was conducted with temperatures higher than 60°C and below 80°C (Staker,
1925; Tomer and Maguire, 1989). Hard seed of alfalfa
was successfully reduced from 21% in control to 3% when heated at 60°C for
2 h in an oven, on the other hand, germination was improved from 72 in untreated
control to 91% (Staker, 1925; Tomer
and Maguire, 1989). The effectiveness of heat scarification with the use
of oven was tested recently for winter annual medic (M. rigidula)
and CMV (Islam and Kimura, 2010). The seeds of Laramie
medic and three cultivars of CMV (Monarch, Oxley and
Lutana) were heated at 60°C in an oven. Treatment timings used
were one, two, three, four and five h. The heat scarification neither reduced
hard seed nor improved germination of medic and two cultivars (Oxley and Lutana)
||Comparison of palisade layer of (a) M. orbicularis (511
x) and (b) A. hamosus (596 x); adapted from Patane
and Gresta (2006). M. orbicularis has thicker seed coat than
However, hard seed reduction (61% in untreated control to 11% after 1 h in
the heat scarification) and germination improvement (28% in untreated control
to 76% after 1 h in the heat scarification) was observed in Monarch CMV. The
observations by Tomer and Maguire (1989) and Islam
and Kimura (2010) suggested that the effectiveness of heat scarification
varies depending on genus and species and even species with the same genus (M.
sativa vs. M. rigidula) and cultivars within the same species (Oxley
and Lutana vs. Monarch). This variation may be caused by the difference in seed
coat structures. For example, Patane and Gresta (2006)
reported that the palisade layer, responsible for water resistance of seeds,
of M. orbicularis seeds was approximately twice as thick as
that of A. hamosus seeds (Fig. 2); as a result,
seeds of M. orbicularis were less affected by hot water and sulfuric
acid scarification than A. hamosus (milkvetch) seeds. Oxley and
Lutana may have similar seed coat structures. In other words, these two cultivars
may have similar thickness and composition of seed coat providing similar hard
seed contents. These structural differences may be due to the genetics of the
species and cultivars or environment in which the seeds were produced or a combination
Unlike the heat scarification using oven, heat scarification with a hot water
bath has not been shown to be effective on hard seed reduction of many Medicago
species. Patane and Gresta (2006) soaked button medic
(M. orbicularis L. Bartal) and milkvetch seeds in a hot water bath with
five different temperatures, starting from 60 to 100°C at an interval of
10°C. These treatments had no effect on button medic seeds but had great
effect on milkvetch. Hard seed of milkvetch was reduced from 93 in untreated
control to 8% when treated at 80°C for 10 min. Can et
al. (2009) also reported that hot water bath treatment had no influence
on hard seed of button medic even when seeds were heated at 90°C. It has
also been reported that hot water bath treatment had little effect on other
Medicago species such as M. hispida, M. arabica, M.
falcata (Uzun and Aydin, 2004), M. rotate,
M. turbinate, M. scutellata (Can et al.,
2009) and M. marina L. (Scippa et al.,
2011). In contrast, germination of M. arborea was increased
from 45 in untreated control to 71% when the seeds were soaked in a hot water
bath at 100°C for four min (Travlos and Economou, 2006).
The positive effect of hot water bath treatment was seen in the legume species
common in arid environment (Travlos and Economou, 2006;
Travlos et al., 2007).
Mechanism behind the reduction of hard seededness by a freeze-thaw scarification
is to make tiny scars on hard seed coat and make seed coat brittle to enhance
germination (Busse, 1930; Pritchard
et al., 1988; Stout, 1990; Hall
et al., 1993). A force that produces scars on seed surface through
this technique is depending on the size, shape and water content of seeds and
treatment intensity and durations (Candolle, 1922; Steinbauer,
1926; Busse, 1930). Methods for cooling in freeze-thaw
scarification include freezer (Midgley, 1926; Shibata
and Hatakeyama, 1995); carbon dioxide (CO2) snow, dry ice, liquid
air (Busse, 1930; ultra-low freezer (Stout,
1990); acetone (Rutar et al., 2001); liquid
N (Brant et al., 1971; Stanwood,
1980; Pritchard et al., 1988; Acharya
et al., 1993; Patane and Gresta, 2006).
Researchers have reported that temperatures at -80°C or lower may be more
effective than temperatures greater than -80°C (Midgley,
1926; Busse, 1930; Stout, 1990).
When alfalfa seeds were subjected to freeze-thaw scarification with cooling
temperatures at -5°C or at -15°C for 36 h and with warming at room temperatures
for six d (one cycle), germination was not improved (Midgley,
1926). In another study, milkvetch seeds were stored in freezer at -22°C
for 2, 4, 7, 30, 60, 90 and 180 d, however the highest germination was below
78% after this freeze-thaw scarification (Shibata and Hatakeyama,
1995). Busse (1930) compared effectiveness of three
freezing temperatures, -20°C (by dry ice), -80°C (by CO2
snow) and -190°C (by liquid air). Freeze-thaw cycle was completed when the
cooled seeds were placed at room temperature (slow warming) or placed on a warm
metal plate (rapid warming). Germination was increased from 53, 62 and 60% in
untreated control to 86, 78 and 98% when treated with dry ice, CO2
snow and liquid air with rapid warming, respectively. Repeated freeze-thaw cycles
were necessary to improve germination of alfalfa seeds when dry ice was used.
Repeating freeze-thaw cycle seems to be more effective in making a fragile
seed coat (Busse, 1930; Stout, 1990;
Rutar et al., 2001). For example, Stout
(1990) conducted a study in two cultivars of alfalfa (Peace
and Anik) with freeze-thaw scarification at -80°C for cooling
(by ultra-low bio freezer) and 20°C for warming (room temperature) alternations.
Each cooling and warming was continued for two h. Hard seed was decreased as
the treatment cycle was increased through the third cycle for Peace seeds (from
24 to 5%) and through the fifth cycle for Anik seeds (from 60 to 14%; Fig.
3). Germination was the greatest at the third cycle in Peace seeds (95%)
and at the fifth cycle in Anik seeds (81%). Rutar et
al. (2001) used the same treatment temperatures (-80°C for cooling
and 20°C for thawing) and time (2 h) as the study of Stout
(1990) in freeze-thaw scarification on alfalfa seeds, however the cooling
device was dry ice and acetone instead of an ultra-low bio freezer. Dry ice
and acetone cooled down seeds as a result of evaporation and these devices might
not cool down all seeds equally as the freezer did. There was no influence of
dry ice and acetone on hard seed or germination of alfalfa seeds.
Freeze-thaw scarification using liquid N (-196°C) showed varying results
depending on the species. Seeds of Oxley CMV were soaked in liquid N for cooling
(10 min) and then the seeds were soaked in steam bath at 100°C for warming
(10 min) (Acharya et al., 1993). Germination
of Oxley CMV seeds was increased from 0 in control to 86% by the freeze-thaw
scarification when it was repeated for 15 times. In contrast, no influence was
observed on germination or hard seed of milkvetch when seeds were scarified
by freeze-thaw scarification with liquid N for cooling (five min) and room temperature
for warming (Patane and Gresta, 2006).
||Hard seed and germination (%) of two cultivars of alfalfa
exposed to freeze-thaw scarification with the temperature alternations between
-80°C (2 h) and 20°C (2 h) (Stout, 1990)
Freeze-thaw scarification with the use of liquid N had no influence on hard
seed in 27 species, including Grim alfalfa, Chesapeake
red clover, Eski sainfoin (Onobrychis viciifolia Scop.),
A1564 soybean (Glycine max L. Merr.) and Purple
common vetch (Vicia sativa L.) when soaked in liquid N for up to 180
days, while seeds of sesame (Sesamum indicum L.) and flax (Linum usitatissimum
L.) were damaged by liquid N (Stanwood, 1980). Other
studies showed increased abnormal seedlings by freeze-thaw scarification using
liquid N. Seeds of rabbitfoot clover (T. arvense L.) were soaked in liquid
N for cooling (5 min) and then placed at room temperature (slow warming) or
hot water bath (40°C; rapid warming) for subsequent warming for 1 h (Pritchard
et al., 1988). These treatments increased seedling abnormality but
did not increase germination of the seeds as freeze-thaw cycle increased. Seedling
abnormality of alfalfa (PI238151, PI258765, PI259522 and PI286378) was also
increased when seeds were soaked in liquid N for 24 h (Wiesner
et al., 1994). Freeze-thaw scarification using liquid N may have
potential to reduce hard seed or improve germination (Acharya
et al., 1993); however, in many instances, it had no influence on
hard seed or germination (Stanwood, 1980; Pritchard
et al., 1988; Wiesner et al., 1994;
Patane and Gresta, 2006).
For mechanical scarification, many researchers have used mechanical scarifiers
(Carleton et al., 1971; Townsend
and Mcginnies, 1972; Miklas et al., 1987;
Singh et al., 1991; Patane
and Gresta, 2006; Dittus and Muir, 2010) or sandpapers
to rub seeds manually (Baes et al., 2002; Uzun
and Aydin, 2004; Patane and Gresta, 2006; Can
et al., 2009).
A small vane-type scarifier has five rotation settings and scarification level
is controlled by throwing seeds into the scarifier at different air pressures
(Carleton et al., 1971). This scarifier increased
germination of Lutana CMV from 39% in control to 60% when the seeds were thrown
into the scarifier four times at 50 rpm, whereas rotation frequency higher than
50 (55 and 60 rpm) increased damaged seeds up to 36%. Another scarifier called
Model No. 2 Foresberg Huller (Fig. 4) was made for larger
seeds and it increased germination of Lutana CMV seeds from 20 in control to
78% when seeds were thrown into the scarifier four times at 1400 rpm (Carleton
et al., 1971).
The other scarifiers introduced in the study of Carleton
et al. (1971) were Model S scarifier and huller (Crippen Manufacturing
Co., MI, USA) and Wes Gro process brand polisher (Northrup King Seed Co., Ontario,
These scarifiers had four rotation frequency settings, 700, 900, 1300 and
1600 rpm and have capability to scarify a large volume of seeds; however, both
scarifiers had little influence on germination of Lutana CMV seeds as the highest
germination obtained was only 53% (Carleton et al.,
1971). On the other hand, a scarifier made and used by Townsend
and Mcginnies (1972) successfully reduced hard seed of CMV seeds. This scarifier
was a small drum type, in which abrasive paper was attached inside the drum.
This was connected to air pressure that blew seeds into the drum by different
level of air pressures. Among the three air pressures of 40, 60, 80 pound per
square inch (psi) used in the study, 60 and 80 psi reduced hard seed from 40
in control to 2% when seeds were placed into the scarifier for two times (Townsend
and Mcginnies, 1972). There were no differences in hard seed when seeds
were scarified with 50 or 60 grits sandpapers attached inside the drum. Damaged
seeds were increased with increased treatment times at 80 psi pressure.
The same scarifier was used in the study of Patane and
Gresta (2006) to scarify seeds of button medic and milkvetch. Seeds were
placed into the scarifier for 10 times. Little influence of this scarification
was observed on hard seed reduction and improvement of germination. On the other
hand, when seeds were rubbed manually with sandpapers (100 grits), initial hard
seeds (average of the two species 94%) and germination (11%) of the two species
became 1 and 92%, respectively. Mechanical scarification using sandpapers was
also effective on other Medicago (M. hispida, M. arabica,
M. scutellata, M. lupulina and M. falcata) and Trifolium
species (T. resupinatum, T. subterraneum, T. alexandrinum,
T. meneghinianum and T. striatum). Germination of these Medicago
(initial 10-18%) and Trifolium species (initial 10-40%) were increased
to 90-97% and 85-95% when rubbed with sandpapers, respectively (Uzun
and Aydin, 2004). Effectiveness of sandpaper scarification was also reported
on M. rigidula, M. rotata, M. orbicularis, M. scutellata
and T. spumosum (Can et al., 2009). Seeds
were manually shaken in a bottle in which sandpaper was installed inside. Germination
of these Medicago species (initial 3-15%) and T. spumosum (initial
5%) was increased to 73-100% and 80%, respectively. Sandpaper scarification,
however, did not affect seeds of M. scutellata, M. turbinate,
T. striatum (Uzun and Aydin, 2004); T.
cherleri, T. spadiceum, T. lappaceum, T.
scabrum, T. angusrifolium, T. strictum and
T. badium (Can et al., 2009). It
appears that mechanical scarification is effective on Medicago (Uzun
and Aydin, 2004; Patane and Gresta, 2006; Can
et al., 2009), Trifolium (Uzun and Aydin,
2004; Can et al., 2009) and Astragalus
(Patane and Gresta, 2006); however the effectiveness
of mechanical scarification may vary depending on the genus and species (Uzun
and Aydin, 2004; Can et al., 2009).
Acid scarification is considered as one of the most effective scarification
methods used for seed scarification. Sulfuric acid is the most popular and effective
chemical to reduce hard seed of legume seeds (Pandrangi
et al., 2003; Martin and De La Cuadra, 2004;
Uzun and Aydin, 2004; Patane and
Gresta, 2006; Can et al., 2009). The effectiveness
of acid scarification depends on concentration of acid, duration of scarification
and species and cultivars used (Martin and De La Cuadra,
2004; Alderete-Chavez et al., 2011).
Many researchers used acid scarification in Medicago and Trifolium
species. For example, seeds of six Medicago (M. rigidula,
M. polymorpha, M. rotata, M. orbicularis, M. turbinata
and M. scutellata) and eight Trifolium species (T.
spumosum, T. cherleri, T. spadiceum, T.
lappaceum, T. scabrum, T. angustifolium,
T. strictum and T. badium) were soaked in sulfuric
acid (95-97%) for five min (Can et al., 2009).
Germination of T. lappaceum was increased from 0 in untreated
control to 90%; however, no or little changes were observed in rest of the Medicago
and Trifolium species (Table 1). Other studies also
showed that acid scarification for five min has no effect on seeds of M.
orbicularis (Crawford, 1976; Russi
et al., 1992a, b) and M. scutellata
(Uzun and Aydin, 2004). Acid scarification was also
not effective in alfalfa seeds. Pandrangi et al.
(2003) scarified seeds of alfalfa with various concentrations of sulfuric
acid (0.1, 0.2, 0.5, 1.0, or 2.0 N) under several treatment timings (2.5, 5,
10, 15, 20, 30, 45 and 60 min).
|| Effect of acid scarification on Medicago and Trifolium
species (Can et al., 2009)
All concentrations had little influence on germination of alfalfa seeds (range
85-95%) and reduction of germination (range 74-76%) occurred when seeds were
soaked longer than 45 min. Acid scarification was not also effective on seeds
of milkvetch and button medic when three concentrations of sulfuric acid (30,
50 and 70%) were used for 15, 25, 35 and 60 min (Patane
and Gresta, 2006).
In contrast, (Martin and De La Cuadra, 2004) reported
that the hard seed of M. polymorpha could be successfully reduced
by acid treating the seeds for 15 min. In this study, there was a significant
reduction in hard seed (from 84 to 7%) and increase in germination (from 10
to 89%) in M. polymorpha (Fig. 5). Balouchi
and Sanavy (2006) also showed the positive influence of sulfuric acid (96%)
scarification on seeds of M. polymorpha and M. rigidula.
Germination of M. polymorpha and M. rigidula was
increased from 12 and 13% in untreated control to 96% after treating with sulfuric
acid for 10 min. Germination of five cultivars of alfalfa seeds was shown to
be increased in another study from 45 in control to 85% when seeds were soaked
in concentrated sulfuric acid for 30 min (Tomer and Maguire,
1989). Germination of Chinese milkvetch (A. sinicus L.) seeds was
also improved from 8 in untreated control to 93% when soaked them in concentrated
sulfuric acid for 20 min (Kim et al., 2008).
The positive effect of acid scarification was reported for other forage legumes,
such as Lupinus angustifolius (Burns, 1959),
Centrosema pubescens (Pe et al., 1975),
Vigna mungo (Tomer and Kumari, 1991), V. umbellate
(Tomer and Singh, 1993), Ornithopus compressus
and O. pinnatus (Fu et al., 1996), Trigonella
corniculata (Pandita et al., 1999) and Tamarindus
indica (El-Siddig et al., 2001), L.
leptophyllus (Alderete-Chavez et al., 2010a),
Crotalaria retusa (Alderete-Chavez et al.,
Variable successes have been reported by many researchers for different species
and even cultivars within the same species. For example, acid scarification
had little influence on many Medicago (Crawford,
1976; Russi et al., 1992a, b;
Pandrangi et al., 2003; Martin
and De La Cuadra, 2004; Uzun and Aydin, 2004; Can
et al., 2009), Trifolium (Can et al.,
2009); Astragalus species (Patane and Gresta,
2006), whereas it was effective in seeds of M. polymorpha (Martin
and De La Cuadra, 2004; Balouchi and Sanavy, 2006),
M. rigidula (Balouchi and Sanavy, 2006),
T. lappaceum (Can et al., 2009)
and several cultivars of alfalfa when seeds were treated with 95-97% sulfuric
acid for 5, 15 and 30 min, respectively (Tomer and Maguire,
1989). These variable results by acid scarification may be due to the composition
change in seed coat.
||Alfalfa seeds under microscope. Untreated alfalfa seed (a)
shows waxy coating and no damage, (b) while acid treated (sulfuric acid)
seed shows no waxy coating. Acid treatment has removed and damaged seed
coat (Pandrangi et al., 2003)
Acid scarification might have removed the thick palisade layer of seed coat
(Fig. 2). The removal of the hard seed coat by acid scarification
was shown in alfalfa seeds (Pandrangi et al., 2003;
Fig. 6). The untreated alfalfa seed had waxy coating while
acid scarified seeds had none. Effectiveness of the acid scarification may vary
within the cultivars of same species, depending on their seed coat structures
(Fairey and Lefkovitch, 1991; Taia,
2004; Patane and Gresta, 2006).
Forage legumes are highly valuable crops because of their ability to fix atmospheric
N and reduce fertilizer input cost and produce high quality forage. Hard seed
in many of the forage legumes is one of the major constraints, however, for
successful stand establishment. The hard seeds lower germination rate creating
less competitive stands against weeds over resources (e.g., water, light and
nutrients) in the establishment year (Dittus and Muir, 2010).
Scarification methods such as heat, freeze-thaw, mechanical and acid scarification
are useful tools to soften hard seeds, improve germination and enhance seedling
establishment. However, effectiveness of the methods varies depending on the
duration of imposed treatments and species or cultivars to be used (Taia,
2004). Over treatment or longer time scarification may impose negative impacts
on or injury to the seeds. This review suggests that there is no single method
of scarification that can be recommended for all legume species in general.
As effectiveness of scarification methods varies among species and even cultivars
within the same species, detailed scarification studies on legume seeds, especially
on newly released cultivars, are necessary.
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A review of research progress on cicer milkvetch (Astragalus cicer
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