Ethanol priming: An effective approach to enhance germination and seedling development by improving antioxidant system in tomato seeds

Abstract

Tomatoes reportedly have a positive response to seed priming. The present study evaluates the effects of ethanol priming on germination, seedling vigour and antioxidative responses of tomato seeds. Priming was achieved by exposing seeds of 'Roma' and 'Nagina' to 2, 4 and 6% aerated ethanol solutions for 24 h. Priming with low levels (2 and 4%) of ethanol improved seed germination, seedling vigour and enhanced antioxidative activity that results in better performance of tomato seeds. However, priming with 6% ethanol failed to improve seed germination and seedling development which relates to the decreased anti-oxidative activity in tomato seeds due to high ethanol level.

Full text

Acta Sci. Pol., Hortorum Cultus 12(4) 2013, 129-137
ETHANOL PRIMING: AN EFFECTIVE APPROACH
TO ENHANCE GERMINATION AND SEEDLING
DEVELOPMENT BY IMPROVING ANTIOXIDANT
SYSTEM IN TOMATO SEEDS
Irfan Afzal1, Fahad Munir2, Chaudhry Muhammad Ayub2,
Shahzad Maqsood Ahmad Basra1, Amjad Hameed3, Fawad Shah4
1Seed Physiology Lab, Department of Crop Physiology, University of Agriculture,
Faisalabad, Pakistan
2Institute of Horticultural Sciences, University of Agriculture, Faisalabad,
Pakistan
3Nuclear Institute for Agriculture and Biology, Jhang Road Faisalabad, Pakistan
4Commodity Inspection Division, Washington State Department of Agriculture,
Yakima, WA-98902, USA
Abstract. Tomatoes reportedly have a positive response to seed priming. The present
study evaluates the effects of ethanol priming on germination, seedling vigour and anti-
oxidative responses of tomato seeds. Priming was achieved by exposing seeds of ‘Roma’
and ‘Nagina’ to 2, 4 and 6% aerated ethanol solutions for 24 h. Priming with low levels
(2 and 4%) of ethanol improved seed germination, seedling vigour and enhanced antioxi-
dative activity that results in better performance of tomato seeds. However, priming with
6% ethanol failed to improve seed germination and seedling development which relates to
the decreased anti-oxidative activity in tomato seeds due to high ethanol level.
Key words: seed priming; dormancy, Lycopersicon esculentum, antioxidant enzymes,
seedling development
ABBREVIATIONS
FGP – final germination percentage,
T50 – time taken to 50% germination,
MGT – mean germination time,
GI – germination index,
GE – germination Energy measured after 4th days of incubation,
Corresponding author: Irfan Afzal, Seed Physiology Lab, Department of Crop Physiology, Uni-
versity of Agriculture, Faisalabad, Pakistan, e-mail: iafzal@uaf.edu.pk

130 I. Afzal, F. Munir, C.M. Ayub, S.M.A. Basra, A. Hameed, F. Shah
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Acta Sci. Pol.
MET – mean emergence time,
FEP – final emergence percentage,
CAT – Catalase,
SOD – Superoxide dismutase.
INTRODUCTION
Tomato is an excellent source of many nutrients and secondary metabolites impor-
tant for human health such as folate, potassium, vitamins C and E, flavonoids, chloro-
phyll, ȕ-carotene and lycopene [Wilcox et al. 2003]. Increasing levels of dietary lyco-
pene has been recommended by many health experts [Tonucci et al. 1995, Giovannucci
1999]. Lycopene has the ability to mitigate epithelial cancers, such as breast and pros-
tate cancers, and also coronary disease. Tomato cultivars containing higher lycopene
germinate and grow more slowly than traditional ones, and a high lycopene tomato
contained high levels of abscisic acid (ABA) which is responsible for seed dormancy
[Ramirez-Rosales et al. 2004]. Moreover, freshly harvested tomato seed often exhibit
low germination due to primary dormancy. Dormancy persists for up to one year in
tomato seeds [Liu et al. 1996] resulting in erratic and unacceptable seedling emergence.
Seed priming is a simple and low cost technique used to break dormancy and im-
prove uniformity of radicle emergence [Liu et al. 1996]. Seed priming is a controlled
hydration process that involves exposing seeds to low water potentials that restrict ger-
mination but permits pregerminative physiological and biochemical changes [Bradford
1986]. Priming enhances seed performance by increasing germination rate and uniform-
ity resulting in faster and better seedling development in various crops [Taylor et al.
1998, Powell et al. 2000, Afzal et al. 2008]. Enhancement of seed germination by prim-
ing has also been associated with stimulation of antioxidant activities [Bailly et al. 1998,
2000, Chiu et al. 2002, Afzal et al. 2011]. Priming also reduces germination time rather
than subsequent relative growth rate and is a valuable tool to improve seedling quality
in rootstock tomato seedling production [Mavi et al. 2006].
Ethanol has been reported to stimulate germination of seeds of many plant species
[Bewley and Black 1982]. Seed pretreatment of many species with ethanol or other
anaesthetic-like substances during imbibition resulted in breakdown of dormancy [Tay-
lorson and Hendricks 1979, Cohn et al. 1989, Hallett and Bewley 2002]. Due to etha-
nol’s role in breaking seed dormancy, the germination performance and activities of
antioxidant enzymes capable of scavenging free radicals and peroxides were investi-
gated in tomato seeds after exposure to various levels of ethanol as a priming agent.
This research was undertaken to provide insights in the relationship of ethanol priming
and dormancy breakdown of tomato seeds by affecting antioxidant enzymes.
MATERIALS AND METHODS
Seeds of tomato (Lycopersicon esculentum Mill.) cv. Nagina and Roma were ob-
tained from the Vegetable Research Institute, Faisalabad, Pakistan and had initial seed
moisture contents of 8.23% and 8.13% respectively on a dry weight basis. The seeds
were stored at 15°C for one year after harvest.

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Priming protocol. Seeds (2 g) were soaked in aerated solution of 2%, 4% and 6%
ethanol (100% undenatured) in 100 ml glass beakers for 24 h at 25°C. For hydroprim-
ing, seeds were soaked in distilled water under similar conditions. Hydroprimed and
non primed seeds were used as controls. During priming, fresh air was supplied con-
tinuously. After ethanol priming, seeds were washed with distilled water [Bradford
1986] and dried back after spreading in a thin layer on dry filter papersat 27°C for 48 h.
The seeds were then placed in polythene bags and stored in a refrigerator at 7°C for
further studies.
Seed germination. Four replicates of 25 seeds each were placed in 9 cm diameter
Petri dishes on Whatman No. 1 filter paper at 25°C in a growth chamber (Vindon, Eng-
land) for 12 days. Five ml of distilled water was used to moisturize each Petri dish.
Visible root protrusion was recognized as germination. Time to 50% germination (T50)
was calculated according to the formulae of Coolbear et al. [1984]. Mean germination
time (MGT) was calculated according to Ellis and Roberts [1981]. Germination index
(GI) was calculated as described in the Association of Official Seed Analysts [1983].
Energy of germination was recorded on the 4th day after planting. It is the percentage of
germinating seeds on the 4th day after planting relative to the total number of seeds
tested.
Seedling emergence. Control and treated seeds were sown in plastic trays (25 in
each) containing moist sand, replicated four times and placed in a growth chamber
(Vindon, England) maintained at 25°C under continuous fluorescent light for 2 weeks.
Emergence was recorded daily according to the seedling evaluation following the Hand-
book of Association of Official Seed Analysts [1983]. Seedlings were harvested after
two weeks of sowing and washed with deionized water. Washed seedlings were sepa-
rated into root and shoot for fresh and dry weight determination. Dry weight was deter-
mined after oven drying at 65°C for 2 days.
Estimation of antioxidant enzymes. Tomato seeds (0.5 g), both ethanol primed or
non primed were ground in 0.8 ml of 50 mM cold phosphate buffer (pH 7.8) in chilled
mortar and pestles. The homogenate was then centrifuged at 15 000 g for 20 min at 4°C.
The supernatant was removed and used for the determination of activities of enzymes.
The enzyme assay for estimation of catalase (CAT) contained 50 mM phosphate
buffer (pH 7.0), 5.9 mM H2O2, and 0.1 ml enzyme extract. The reaction was initiated by
adding the enzyme extract. The decrease in absorbance of the reaction solution at
240 nm was recorded after every 20 s with a spectrophotometer (Hitachi U-2100,
Tokyo, Japan). An absorbance change of 0.01 unit min-1 was defined as 1 unit of CAT
activity [Dixit et al. 2001]. Enzyme activities were expressed on a protein basis. Protein
concentration of the enzyme extract was measured by the dye binding assay as de-
scribed by Bradford [1976].
Superoxide dismutase (SOD) activity was assayed by measuring its ability to inhibit
the photochemical reduction of nitrobluetetrazolium (NBT) following the method of
[Dixit et al. 2001, Giannopolitis and Ries 1977] with some modification. The SOD
reaction solution (3 ml) contained 50 μM NBT, 1.3 μM riboflavin, 13 mM methionine,
75 mM EDTA, 50 mM phosphate buffer (pH 7.8) and 50 μl enzyme extract. The tubes
containing the reaction solution were irradiated under a light (15 W fluorescent lamps)
at 78 μmolām-2 s-1 for 15 min. The absorbance of the irradiated solution at 560 nm was

132 I. Afzal, F. Munir, C.M. Ayub, S.M.A. Basra, A. Hameed, F. Shah
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Acta Sci. Pol.
determined with a spectrophotometer (Hitachi U-2100, Tokyo, Japan). One unit of SOD
activity was defined as the amount of enzyme which caused 50% inhibition of photo-
chemical reduction of NBT.
Statistical analysis. All experiments were repeated twice in a completely random-
ized design; data recorded each time were pooled for statistical analysis by using soft-
ware MSTATC to determine significance of variance (P < 0.05). The least significant
difference test was used to compare the differences amongst treatment means.
RESULTS
Ethanol priming significantly (P < 0.05) affected germination and seedling devel-
opment of both tomato cultivars (tab. 1). Priming with 2 and 4% ethanol resulted in
lower T50 and MGT and higher FGP, GI, radicle and plumule lengths compared with
untreated and hydroprimed seeds (tab. 1). However, priming with 6% ethanol failed to
improve these parameters in both cultivars. In both cultivars, lowest T50 and MGT were
noted in seeds primed with 2% ethanol that was followed by 4% ethanol (tab. 1). Ma-
ximum FGP, GI, radicle and plumule length was noted in seeds primed with 2 and 4%
ethanolin both cultivars whereas GE was only improved in 2% ethanol primed and hy-
droprimed seeds (tab. 1).
Table 1. Effect of ethanol priming on the germination and seedling growth of two tomato (Ly-
copersicon esculentum Mill.) cv Roma and Nagina
Treatments T50 (days) MGT
(days) FGP (%) GI GE(%)
Radicle
length
(cm)
Plumule
length
(cm)
control 6.48 a 9.34 a 66.67 d 10.54 c 19.10 e 1.19 b 4.63 d
hydropriming 5.76 b 9.19 b 77.32 b 14.27 b 30.20 c 1.26 b 4.96 c
priming with 2% ethanol 5.19 d 8.70 d 81.33 a 15.91 a 42.94 a 2.20 a 5.82 b
priming with 4% ethanol 5.53 c 9.04 c 82.00 a 15.44 ab 37.66b 2.07 a 5.99 a
priming with 6% ethanol 5.75 b 9.21 b 71.33 c 11.87 c 22.48d 0.67 c 4.88 c
Roma
LSD at 0.05 0.07 0.12 1.63 1.47 1.82 0.24 0.19
control 8.98 a 10.88a 22.67 e 2.059d 18.99a 2.17 c 2.60 c
hydropriming 8.41 b 10.15 c 40.96 c 5.54 c 15.28 b 2.01 c 2.92 b
priming with 2% ethanol 7.39 c 9.65 d 58.00 a 7.19 a 11.90 c 3.21 b 3.69 a
priming with 4% ethanol 7.54 c 9.72 d 46.67 b 6.33 b 18.09a 3.39 a 3.77 a
priming with 6% ethanol 8.42 b 10.38 b 31.33 d 5.78 c 10.41 c 1.49 d 2.91 b
Nagina
LSD at 0.05 0.07 0.18 0.45 0.09 0.58 0.052 0.19
For each tomato cultivar, means within a column followed by the same letters are not significantly different
at P  0.05
T50 – time taken to 50% germination, FGP – final germination percentage, MGT – mean germination time,
GI – germination index, GE – germination energy measured after 4th days of incubation

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Hortorum Cultus 12(4) 2013
0
2
4
6
8
10
12
CAT (U/ m g protein)
Control Hydropriming 2% ethanol 4% ethanol 6% ethanol (a)
Fig. 1. Catalase (CAT) and superoxide dismutase (SOD) activities in seeds of 2 tomato cultivars
Roma and Nagina after priming with 2, 4 and 6% ethanol. Data are the means ±SE of at
least 4 different seed samples
All seed treatments resulted in lower MET compared with the control seeds (tab. 2).
The highest FEP was recorded in seeds primed with 2 and 4% ethanol as compared to
the control ones (non-primed and hydroprimed) and 6% ethanol primed seeds. In both
cultivars, priming with 2 and 4% ethanol resulted in better seedling development i.e.
higher root and shoot length, and seedling fresh and dry weight compared with control
(tab. 2). Overall, hydropriming and 6% ethanol priming failed to improve seedling de-
velopment of both cultivars (tab. 2).

134 I. Afzal, F. Munir, C.M. Ayub, S.M.A. Basra, A. Hameed, F. Shah
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Acta Sci. Pol.
Table 2. Effect ofethanol priming treatments on the seedling development of two tomato
(Lycopersicon esculentum Mill.) cv Roma and Nagina
Treatments
MET
(days)
FEP
(%)
Root
length
(cm)
Shoot
length
(cm)
Seedling
fresh weight
(mg)
Seedling
dry weight
(mg)
control 10.61 a 33.33 e 4.67 d 5.29 e 0.33 b 0.05 c
hydropriming 10.14 a 57.00 c 4.89 c 6.08 d 0.40 b 0.09 b
priming with 2% ethanol 9.63 c 69.33 a 6.57 a 7.41 a 0.89 a 0.14 a
priming with 4% ethanol 9.88 b 60.00 b 6.39 a 7.03 b 0.65 ab 0.14 a
priming with 6% ethanol 9.84 b 42.33 d 5.39 b 6.33 c 0.40 b 0.1 b
Roma
LSD at 0.05 0.20 0.75 0.18 0.05 0.14 0.02
control 11.61 a 47.33 d 5.23 d 4.09 c 0.29 b 0.04 c
hydropriming 11.12 b 52.20 c 6.03 c 4.25 b 0.31 b 0.07 b
priming with 2% ethanol 11.29 b 58.67 b 7.55 a 4.66 a 0.36 a 0.15 a
priming with 4% ethanol 11.36 b 60.67 a 6.97 b 4.35 b 0.35 a 0.13 a
priming with 6% ethanol 11.31 b 52.00 c 6.20 c 4.39 b 0.30 b 0.08 b
Nagina
LSD at 0.05 0.24 0.59 0.07 0.05 0.02 0.02
For each tomato cultivar, means within a column followed by the same letters are not significantly different
at P  0.05
MET – mean emergence time, FEP – final emergence percentage
There was a significant increase in SOD and CAT in both seeds primed with 2 and
4% ethanol as compared to untreated seeds in both cultivars. But SOD and CAT activi-
ties were negatively affected in seeds primed with water and 6% ethanol (fig. 1).
DISCUSSION
Ethanol priming overcame tomato seed dormancyand stimulated germination and
seedling development of both cultivars tested. Responses were similar in earlier and
synchronized germination and emergence observed with 2 and 4% ethanol compared
with non-primed, hydroprimed and 6% ethanol treated seeds as depicted by lower MET,
T50 and MGT, and higher GI, FEP and FGP. Moreover, ethanol priming further en-
hanced antioxidant defense mechanisms by improving SOD and CAT activities which
were responsible for improving germination of both tomato seeds [Siadat et al. 2012].
The negative response of 6% ethanol indicates it may be toxic during priming due to
excessive accumulation in imbibed seeds [Crawford1977].
Higher radicle and plumule lengths, root and shoot lengths as well as seedling fresh
and dry weights observed in 2% and 4% ethanol primed seeds of both cultivars might be
the result of earlier germination and emergence [Liu et al. 1996]. This earlier synchro-
nized and faster emergence might be due to the enhanced synthesis of DNA, RNA and

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Hortorum Cultus 12(4) 2013
protein [Bray et al. 1989] and activation of hydrolytic enzymes responsible for starch
breakdown [Afzal et al. 2012] during priming. Earlier, Gallardo et al. [2001] also re-
ported new proteins associated with priming. These results are also in accordance with
findings of Mavi et al. [2006] who reported priming treatments increased tomato seed-
ling (fresh and dry weight). Reduced radicle and plumule length of both cultivars repre-
sent the first indication of ethanol toxicity followed by reduced germination at higher
ethanol concentration [Taylorson and Hendricks 1980/81].
The data presented here demonstrate clearly priming with 2 and 4% significantly in-
creased SOD and CAT in both tomato cultivars. This supports the argument that SOD
rapidly dismutes superoxide to H2O2, and inhibits hydroxyl radical production, and
CAT helps in quenching H2O2 which ultimately protect seeds from its toxicity
[McDonald 1999]. The positive role of ethanol in tomato seeds might increase the
phospholipids head group spacing which in turn promotes germination by optimizing
the binding and activation of an essential peripheral membrane protein [Hallett and
Bewley 2002]. This increase in antioxidant activity is associated with enhancement of
seed germination by priming [Bailly et al. 2000, Chiu et al. 2002]. Conversely, slow
germination of seeds primed with 6% ethanol might be due to low antioxidant activity
[Bailly et al. 2002].
CONCLUSIONS
Results show that tomato germination and seedling development can be enhanced by
2% and 4% ethanol priming through maintaining higher anti-oxidative mechanisms for
eliminating excessive total peroxide. Therefore, ethanol priming with low concentration
is an effective approach to enhance stand establishment of local tomato cultivars.
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POBUDZANIE ETANOLEM: SKUTECZNA METODA WZMAGAJĄCA
KIEàKOWANIE I ROZWÓJ SIEWEK POPRZEZ ULEPSZENIE SYSTEMU
ANTYOKSYDACYJNEGO U NASION POMIDORA
Streszczenie. UwaĪa siĊ, Īe pomidory wykazują pozytywną reakcjĊ na pobudzanie na-
sion. Niniejsze badanie ocenia wpáyw pobudzania etanolem na kieákowanie, ĪywotnoĞü
siewek oraz antyoksydacyjne reakcje nasion pomidora. Pobudzanie osiągniĊto dziaáając
na nasiona odmian ‘Roma” i „Nagina” 2, 4 i 6% napowietrzonymi roztworami etanoli
przez 24 godziny. Pobudzanie za pomocą niskich poziomów (2 i 4%) etanolu poprawiaáo
kieákowanie nasion, ĪywotnoĞü siewek oraz wzmagaáo aktywnoĞü antyoksydacyjną, która
daje lepsze wyniki w odniesieniu do nasion pomidora. Jednak pobudzanie 6% etanolem
nie polepszyáo kieákowania nasion ani rozwoju siewek, co ma związek z obniĪoną aktyw-
noĞcią antyoksydacyjną u nasion pomidora ze wzglĊdu na wysoki poziom etanolu.
Sáowa kluczowe: pobudzanie nasion, spoczynek, Lycopersicon esculentum, enzymy an-
tyoksydacyjne, rozwój siewek
Accepted for print: 1.03.2013