Changes oil content and fatty acid composition of peanut by
irrigation
Sevim AKÇURA1, İsmail TAŞ2, Kağan
KÖKTEN3, Mahmut KAPLAN4, Aydın Şükrü
BENGÜ5
1Çanakkale Onsekiz Mart University, Institute of
Natural and Applied Sciences, Field Crops Department, Çanakkale, Turkey
Corresponding author e-mail:
sevimakcura@yahoo.com
Orcid No: 0000-0002-7699-748X
2Çanakkale Onsekiz Mart University, Agricultural
Faculty, Agricultural Structures and Irrigation Department, Çanakkale,
Turkey
Orcid No: 0000-0003-0872-2529
3Bingöl University, Agricultural Faculty, Field Crops
Department, Bingöl, Turkey
Orcid No: 0000-0001-5403-5629
4Erciyes University, Agricultural Faculty, Field Crops
Department, Kayseri, Turkey
Orcid No: 0000-0002-6717-4115
5Bingöl University, Vocational College of Health
Services, Medical Services and Techniques Department, Bingöl, Turkey
Orcid No: 0000-0002-7635-4855
Abstract: Oil content and fatty acid composition are the most
significant quality criteria of peanuts (Arachis hypogaea L.),
and such a parameter is greatly influenced by irrigation. This study was
conducted to investigate the effects of irrigation intervals and
irrigation levels on oil content and fatty acid composition of peanuts.
Experiments were conducted in the 2017 and 2018 growing seasons.
Halisbey, NC-7, and Sultan peanut cultivars were used as the plant
material. Irrigation intervals were set as two and four days, and
irrigation levels were set as 50% (I50), 75%
(I75), 100% (I100) and 125%
(I125) of Class-A pan evaporations. Oil content,
unsaturated and saturated fatty acids of peanut cultivars were
determined. Experimental data were initially subjected to variance
analysis, then regression analyses were conducted for irrigation
intervals, and irrigation levels of the cultivars and biplot analysis
was performed to assess the cultivar x irrigation interval x irrigation
level interactions. For oil content, treatments were identified as the
most appropriate irrigation for a two-day irrigation interval of all
cultivars; I100 for four-day irrigation interval of
Halisbey and Sultan cultivars and I75 for four-day
irrigation interval of NC-7 cultivar. Oleic, linoleic, and palmitic
acids were the major fatty acids of peanuts. Cultivars exhibited
different variations in these fatty acids based on irrigation intervals
and irrigation levels. In all cultivar, the irrigation intervals and
irrigation levels with a high oleic acid content yielded low linoleic
and palmitic acid contents. According to present biplot and regression
analyses, the greatest oleic acid contents were obtained from two and
four-day irrigation intervals of I50 treatments in
Halisbey and NC-7 cultivars and from two and four-day irrigation
intervals of I75 treatments of the second year in Sultan
cultivar. Present findings revealed that for quality peanut production,
both irrigation intervals and irrigation levels should be taken into
consideration.
Key Words: Irrigation, peanut, Arachis hypogaea L.,
fatty acid composition, oil content
Introduction
Irrigation is an essential component of plant production. Proper
irrigation programs play a key role not only in improving both the yield
and the quality. Such programs play a significant role also in the
preservation and efficient use of water resources. Increasing water
demands are not sufficiently met with deficit water resources, and
existing droughts generate crisis not only in agricultural productions
but also in several other sectors. Such cases will probably get worse in
the future. Therefore, deficit water resources should be properly and
efficiently be used, especially in agriculture and the other sectors.
The proper and efficient application of irrigation water at the right
time and the right quantity to root zone of the plants without
generating any environmental problems will improve yield and quality.
Plants exhibit quite different responses against water deficits.
Therefore, the plant responds to water deficits that should be
determined for sustainable agricultural production. Such responses
should especially be determined in peanut-like plants mostly used as
industrial raw material.
Peanut is an annual leguminous plant either used for oil production in
the industry or consumed as an appetizer. Peanut is an important oil
crop cultivated worldwide in different continents, and world annual
production is about 43.9 million tons (Sezen et al., 2019). Turkey, with
available climate conditions, is among the leading peanut producer
countries of Europe (Florkowski, 2012). Peanut is cultivated as the main
and second crop in some regions of Turkey (Çukurova and Southeast
Anatolia region) but cultivated as only the main crop in Aegean,
Mediterranean, and Marmora regions. Peanut is a legume crop able to
fixate 150 kg/ha nitrogen into soils through symbiotic rhizobium
bacteria in a single growing season (Arioglu, 2014). With such an
attribute, the peanut is a significant crop ration and green fertilizer
plant (Langat et al., 2006). Harvest residues could also be used as
roughage, thus generate a significant feed source for animals (Kokten et
al., 2014).
Peanut seeds are rich in proteins, essential oil acids, minerals, and
bioactive compounds (Akram et al., 2018). Oil is the major component of
peanut seeds (36 – 54%), and seeds also contain high quantities of
proteins (16 – 36%) and carbohydrates (10 – 20%) (Davis et al.,
2016). Peanut seeds also contain coenzyme Q10, arginine, and polysterols
(Akhtar et al., 2014). However, the majority of world peanuts are
produced for cooking oil just because of light and soft taste and some
advantages of peanut oil over the other vegetable oils (Toomer, 2018).
Following the oil extraction, the remaining peanut meal is quite rich in
protein (50%), thus constitutes an important source of nutrients (Zhao
et al., 2012). Since peanut is an oil crop, characteristics of peanut
oil should be known (Abbas et al., 2018). Peanut oil contains major
saturated fatty acids of palmitic (C16: 0), unsaturated fatty acids of
oleic (C18: 1) and linoleic acids (C18: 2) (Toomer, 2018). Besides these
fatty acids, peanut oil also contains slight quantities of stearic acid
(C18: 0), arachidonic (C20: 0), eicosenoic (C29: 1), behenic (C22: 0)
and lignoceric (C24: 0) acids (Casini et al., 2003; Carrin and Carelli,
2010).
The taste and quality of peanuts and peanut products largely depend on
the chemical composition of peanut oil. With a high oleic acid content,
peanut oil has a well oxidative and frying stability. Peanut oil does
not provide solidity when exposed to air, but become solid between 0 -
3°C (O’Brien, 2004). Lipid and fatty acid composition of peanut
oil-primarily depend on the cultivar, seed maturity, environmental
conditions, agronomic practices, and geographical location (Young,
1996). Low temperatures at the seed ripening stage influence the
unsaturated fatty acid composition of peanuts (Casini et al., 2003).
Oleic acid content increases, and linoleic and palmitic acid contents
decrease toward the harvest maturity (Hinds, 1995). Andersen and Gorbet
(2002) indicated that the fatty acid profile of peanuts did not effect
varied with the sowing times and the years. It was indicated in another
study that the fatty acid profile of peanut may vary with the soil
texture (Hinds, 1995). However, it was indicated in studies conducted in
the USA that genetics significantly influenced the fatty acid
composition of peanuts, and greater genetic-dependent variations were
observed in major components (palmitic, oleic, and linoleic acid) than
the other minor fatty acids (Zhu et al., 2004).
NC-7, Halisbey, and Sultan are the most commonly cultivated peanut
cultivars in Turkey, and they belong to the Virginia group (Arıoğlu et
al., 2016). Oil contents of peanut cultivars grown in different
environments of Turkey were reported as between 35.3 - 45.4% (Özcan,
2010). In a previous experiment conducted in a single location, the oil
content was reported as 48.94% for Halisbey cultivar, 47.55% for NC-7
cultivar, and 48.62% for Sultan cultivar (Arıoğlu et al., 2016). Yol et
al. (2017) reported the oil content of peanut genetic materials as
between 31.7 - 57.0%, oleic acid contents as between 35.3 - 60.9% and
linoleic acid contents as between 16.1 - 43.6%. Greater adaptation to
certain environmental conditions may contribute to high seed yield and
quality of peanuts (Yol and Uzun, 2018). Growing techniques greatly
influence oil content and fatty acid composition. Isleib et al. (2008)
indicated that the chemical composition of peanut seeds significantly
influenced by environmental factors, genetic factors, and interactions
of these factors. Researchers also investigated the effects of drought,
water deficit and air temperature on fatty acid composition of peanuts
at different locations (Young et al., 1974; Dwivedi et al., 1996; Yao et
al., 2003; Reddy et al., 2003; Patel et al., 2008; Shinde and Laware,
2010). A limited number of researchers investigated the effects of
irrigation water levels on quality parameters of peanuts (Zhu et al.,
2004; Amir et al., 2005; Boydak et al., 2010; Aydinsakir et al., 2018).
However, a comprehensive study investigating the effects of cultivars,
irrigation intervals, and irrigation levels on oil content and fatty
acid composition of peanuts hasn’t been conducted yet.
This study was conducted to assess the variations in oil contents and
fatty acid composition of Halisbey, NC-7, and Sultan peanut cultivars
grown under sandy soil conditions, two different irrigation intervals,
and different irrigation levels for two growing seasons. Effects of
irrigation interval x irrigation level interactions on oil content and
fatty acid compositions were also investigated, and proper irrigation
practices for quality peanut production under sandy soil conditions were
tried to be identified.
Material and Method
This study was conducted for two years between May and October of 2017
and 2018 under Balıkesir Burhaniye conditions over the experimental
fields of Farmer’s Training and Extension Center of Balıkesir Greater
City Municipality. Irrigation-related soil parameters of the
experimental site are provided in Table 1. Soil texture is mostly sandy,
and sandy-loam and sand contents varied between 58.7 – 95.0%, silt
contents varied between 2.4 - 30.1%, and clay contents varied between
2.6 - 21.4%. Field capacity (FC) values varied between 10.87 - 17.78%,
and permanent wilting point (PWP) values varied between 4.83 - 8.67%.
Soil bulk density (BD) values varied between 1.36 - 1.59
g/cm3.
Halisbey, NC-7, and Sultan cultivars with Virginia-type seeds and
commonly grown in Turkey were used as the plant material of the study.
Two different irrigation intervals (2 and 4 days) and four irrigation
levels (I50, I75, I100,and I125) were applied. Experiments were conducted in
split-split plots experimental design with three replications. The main
plots included cultivars, sub-plots included irrigation intervals, and
sub-sub-plots included irrigation levels. A 4 m spacing was provided
between sub-plots, and 2 m spacing was provided between sub-sub-plots to
prevent transitions between the treatments. Plots were 5 m long and
composed of 4 rows. Row spacing was 70 cm, and on-row plant spacing was
20 cm.
Sowing was performed on 20 April 2017 in the first year and 23 April
2018 in the second year. Two seeds were sown manually into each seedbed,
and following the emergence, the number of plants was thinned to one in
each seedbed. Before sowing, 250
kg/ha DAP (45 kg/ha N, 115 kg/ha
P2O5) fertilizer was provided, and 50
kg/ha ammonium nitrate was applied twice through 3rd and 4th
irrigations. Befuraline active-ingredient herbicide (60% w/w) was
applied before sowing, and hoeing was practiced after emergence for weed
control.
Harvest was performed through manual pull-out on 15 October 2017 and 20
October 2018. For oil content and fatty acid analyses, 100 g seeds were
sampled from 10 plants of each plot. Seeds were dried to constant
moisture before the analyses.
Oil extraction and preparation of fatty acid methyl esters
(FAME): Impurities were removed from the seeds, and the cleaned seeds
were ground using a ball mill into powder. Lipids were extracted with
hexane/isopropanol (2 v/v) solution overnight in a laboratory-type
shaker. The lipid extracts were centrifuged at 10000 g for 5 min and
filtered after that solvent was removed on a rotary evaporator at 40 ºC.
After the extraction procedure, fatty acids in the lipid extracts were
converted into methyl esters employing 2% sulphuric acid (v/v) in
methanol (Christie, 1990).
Determination of fatty acid composition by GC system: Lipid fatty
acid methyl esters (FAME) in seed samples were prepared using 1-step
extraction-trans esterification. The FAME profile for a 0.6-µL sample at
a split ratio of 1:50 was generated using a gas chromatograph
(Schimadzu, GC 2010 plus) equipped with a flame ionization detector
(Schimadzu, Kyoto, Japan), a 100-m fused silica capillary column (i.d.
0.25 mm) and H2 as the carrier and fuel gas. The FAME was separated
using a temperature gradient program (Chilliard et al., 2013), and the
peaks were identified based on comparing retention times with authentic
standard (Supelco #37, Supelco Inc., Bellefonte, PA, USA; L8404 and
O5632; Sigma).
Resultant data were subjected to variance analysis. Then, regression
analyses were conducted for each fatty acid to assess years, cultivars,
irrigation intervals, and irrigation levels visually. In regression
analysis, the cubic model was used to see the effects, especially of
irrigation levels. Variance analysis and regression graphs were
generated with the use of JMP software (SAS Institute, 2014). Finally,
biplot graphs were generated with the aid of GGE biplot software to see
the effects of different irrigation interval x irrigation level
combinations on the fatty acid composition of different cultivars (Yan,
2001).
Results and Discussion
Initially, oil contents were determined, then unsaturated fatty acids
(oleic and linoleic) and saturated fatty acids (palmitic, myristic,
arachidic, behenic, and lignoceric) were determined. Oil contents and
fatty acid compositions of peanut cultivars varied with the years,
cultivars, irrigation intervals, irrigation levels, and interactions of
these factors (Table 2).
Effects of years on total oil, saturated and unsaturated fatty acid
contents and fatty acids, except for linoleic acid; effects of cultivars
on total oil, saturated and unsaturated fatty acid contents and fatty
acids, except for arachidic and behenic acid; effects of irrigation
levels on total oil, saturated and unsaturated fatty acid contents and
fatty acids, except for arachidic acid; effects of irrigation intervals
on total oil, saturated and unsaturated fatty acid contents and fatty
acids, except for palmitic, arachidic and behenic acids were found to be
highly significant (P<0.01).
Of double interactions, year x irrigation interval and year x irrigation
level interactions were found to be significant for total oil content,
but insignificant for all fatty acids. Irrigation interval x cultivar
interactions were found to be significant for total oil, saturated and
unsaturated fatty acid contents, myristic, arachidic and lignoceric
acids; cultivar x irrigation level interactions were found to be
significant for total oil, saturated and unsaturated fatty acid
contents, myristic, palmitic, behenic, oleic and linoleic acids;
irrigation interval x irrigation level interactions were found to be
significant for total oil, saturated and unsaturated fatty acid
contents, myristic, oleic and linoleic acids (P<0.01).
Of triple interactions, irrigation interval x cultivar x irrigation
level interactions were found to be significant for total oil, saturated
and unsaturated fatty acid contents and all fatty acids, except for
linoleic acid; year x irrigation interval x irrigation level
interactions were found to be significant for total oil contents and
myristic acid (P<0.01).
Since the effects of the years on oil content and fatty acid
compositions were found to be significant, assessments were made
separately for fatty acids through distributing the effects of
cultivars, irrigation intervals over the year. Resultant averages are
provided in Tables 3 and 4; regression graphs are presented in Figures
1, 2, and 3; biplot graphs are presented in Figure 4.
Total oil, saturated, and unsaturated fatty acid contents of peanut
cultivars were significantly influenced by the years and irrigation
treatments. Effects of irrigation intervals and irrigation levels on
total saturated and unsaturated fatty acid contents significantly varied
with the cultivars. Generally, the saturated fatty acid ratio increased,
and the unsaturated fatty acid ratio decreased (Figure 1e-h). In
general, the total oil ratio at a two-day irrigation interval was lower
in the first year (40.8%) than in the second year (43.6%). In the
four-day irrigation interval, the oil ratio of the second year (42.8%)
was lower than the oil ratio of the first year (43.5%). As can be seen
in regression analyses, peak levels were determined for the greatest oil
ratios at two and four-day irrigations of both years (Figure 1a and b).
The lowest oil ratios of all cultivars were obtained from
I50 and I125 irrigation levels. Since
the soil texture of the experimental site was sandy, plants were highly
influenced by water stress in deficit irrigation treatment and
negatively influenced by excessive water in I125treatments. The greatest oil ratios were obtained from the
I75 irrigation level. In general, at both irrigation
intervals, oil ratios significantly increased from I50to I75 level, but a significant change was observed at
the I100 level in Halisbey and Sultan cultivars. On the
other hand, NC-7 exhibited a different response to I100treatment from the other two cultivars (Figure 1a and b). Aydınşakir et
al. (2019) reported the greatest crude oil ratio of NC-7 cultivar under
clay-loam soil conditions at I100 and
I125 irrigation levels.
Considering the entire factors of the years together, it was observed
that the values of oleic acid and total unsaturated fatty acids were
greater in the first year than the second year and a reverse case was
valid for the total oil ratio, saturated fatty acid ratio and the other
fatty acids (Table 3 and 4).
The major unsaturated fatty acids of peanuts were reported as oleic and
linoleic acids, and the major saturated fatty acid was reported as
palmitic acid (Janila et al., 2016). Therefore, these fatty acids were
separately assessed from the others, and regression graphs, including
years, irrigation intervals, cultivars, and irrigation levels, are
presented in Figure 2.
As can be seen in Figure 2, while regression curves of oleic acid
concentrations were similar for the years and irrigation intervals, they
were significantly different for cultivars and irrigation levels.
In Halisbey cultivar, the greatest oleic acid content was obtained from
the two-day interval of I50 irrigation level (48.74% in
2017 and 48.46% in 2018), and the lowest value was obtained from
I125 irrigation level (45.42% in 2017 and 45.22% in
2018). In the two-day irrigation interval of Halisbey cultivar, changes
in oleic acid contents were similar in I75 and
I100 treatments (Figure 2a). In the four-day irrigation
interval of Halisbey cultivar, oleic acid content was 46.39% in 2017
and 47.01% in 2018. As it was in the two-day irrigation interval, the
greatest oleic acid content was observed in I50treatment, and the lowest oleic acid content was observed in
I125 treatments of both years (Figure 2a and b).
In a two-day irrigation interval of NC-7 cultivar, the greatest oleic
acid content was measured as 50.04% in 2017 and 56.03% in 2018.
Despite this difference, regression curves were quite similar in both
years (Figure 2 a and b). The lowest oleic contents were observed in
I75 and I100 irrigation levels of both
years, and slightly greater oleic acid contents were observed in the
I125 irrigation level than the previous two doses. In
the four-day irrigation interval of NC-7 cultivar, the greatest oleic
acid contents were observed in I75 and
I50 irrigation levels (51.05 and 50.49% in 2017 and
56.89 and 56.46% in 2018). The lowest oleic acid contents in this
irrigation interval were observed in the I125 irrigation
level (46.67% in 2016 and 52.61% in 2018) (Table 3 and 4).
In a two-day irrigation interval of Sultan cultivar, the greatest oleic
acid contents were obtained from the I75 irrigation
level (48.68% in 2017 and 49.46% in 2018), and the lowest oleic acid
contents were obtained from I125 treatments (45.26% in
2017 and 43.56% in 2018). In a two-day irrigation interval of Sultan
cultivar, years had slight effects on oleic acid contents. Consequently,
resultant regression curves were quite similar to each other (Figure 2 a
and b). In a four-day irrigation interval of Sultan cultivar, oleic acid
contents of 2017 varied between 46.19 – 47.35% based on irrigation
levels. In 2018, the greatest oleic content was obtained from
I75 (50.28%), and the lowest oleic acid content was
obtained from I50 (44.90%) treatment (Table 3 and 4).
The regression curves generated based on irrigation intervals, and
irrigation levels of Sultan cultivar were significantly different in
2017 and 2018 (Figure 2a and b).
In previous studies, oleic acid contents of peanuts were reported as
between 35 – 72% and such differences were attributed to cultivars and
environmental factors (Liu et al., 2008; Abbas et al., 2 018).
Significant effects of irrigation levels and drought treatments on oleic
acid contents were also reported (Dwivedi et al., 1996).
For linoleic acid contents, in a two-day irrigation interval, Halisbey
and NC-7 cultivars exhibited similar responses to irrigation levels in
2017 and 2018 (Figure 2c and d). In two-day irrigation interval of 2017,
the lowest linoleic acid contents of these two cultivars were obtained
from I50 irrigation level (32.32% for Halisbey and
30.17% for NC-7), and the greatest linoleic acid contents were obtained
from I100 irrigation level (33.60% for Halisbey and
32.62% for NC-7) (Table 3). In 2018, Halisbey and NC-7 cultivars also
exhibited the same responses. The lowest linoleic acid content was
observed in the lowest irrigation level (I50). The
greatest linoleic acid content was observed in I75treatment of Halisbey (36.43%) and I100 treatment of
NC-7 (29.37%). In the four-day irrigation interval, linoleic acid
contents of these two cultivars had similar variations based on
irrigation levels. The lowest linoleic acid content was obtained from
I75 treatment (Figure 2c and d).
In Sultan cultivar, similar regression curves were obtained for linoleic
acid contents in both years and irrigation intervals. The lowest
linoleic acid contents at a two-day irrigation interval of 2017 and both
two and four-day irrigation intervals of 2018 were observed in the
I75 irrigation level (Figure 2c and d).
The total saturated fatty acid content of the cultivars was greater in
2018 than in 2017. Among the present cultivars, NC-7 had the greatest
total saturated fatty acid content. Considering the year x cultivar x
irrigation interval x irrigation level interactions, the greatest total
saturated fatty acid ratio (82.74%) was obtained from the
I100 irrigation level of the second year at a two-day
irrigation interval of NC-7 cultivar (Table 3).
Oleic acid is the second unsaturated fatty acid of peanuts, and oleic
acid contents of peanuts were reported as between 20 – 45% by Liu et
al. (2008) and between 20.9 – 43.2% by Carrin and Carelli (2010). The
change in linoleic acid content based on genetics and environmental
factors (drought stress) directly reflects on oleic acid content. In
other words, decreasing oleic acid contents were observed with
increasing linoleic acid contents (Akram et al., 2018). Just because of
significant correlations between these fatty acids, some researchers
indicated oleic/linoleic acid ratio (O/L) as a significant quality
parameter. It was reported in previous studies that plants under drought
stress and plants grown in cool regions generally had low O/L ratios
(Hashim et al., 1993; Dwivedi et al., 1993).
Palmitic acid was the major saturated fatty acid of peanut cultivars.
For palmitic acid contents, Halisbey cultivar exhibited similar
reactions against different irrigation levels in two and four-day
irrigation intervals of 2017 and 2018 (Figure 2 and f). In a two-day
irrigation interval of Halisbey cultivar, the lowest palmitic acid
content was obtained from the I50 irrigation level
(10.28% in 2017 and 10.21% in 2018) (Table 3 and 4). The greatest
palmitic acid ratios of the same cultivar were obtained from
I75 treatment in a two-day irrigation interval and
I100 treatment in a four-day irrigation interval. For
palmitic acid content, the most extreme reactions to entire experimental
factors were presented by NC-7 and Sultan cultivars. In regression
analysis, the lowest coefficient of determination values was identified
in two and four-day irrigation intervals of these two cultivars (Figure
2e and f).
In both years, in a two-day irrigation interval, the palmitic acid
content of NC-7 cultivar initially increased, then decreased with
increasing irrigation levels. In the four-day irrigation interval,
palmitic acid contents increased in both years with increasing
irrigation levels. The lowest palmitic acid content in this irrigation
interval was obtained from I50 and the greatest from
I125 treatments. In Sultan cultivar, in a two-day
irrigation interval, palmitic acid contents increased in both years with
increasing irrigation levels, however, in four-day irrigation interval,
palmitic acid contents decreased with increasing irrigation levels
(Figure 2 e and f).
The other unsaturated fatty acids of peanut seeds included arachidic,
behenic, lignoceric, and myristic acids. The averages for these fatty
acids based on experimental factors are provided in Tables 3, and 4 and
regression curves are presented in Figure 2.
For arachidic acid, regression curves were similar to each other
(Figures 3a and b). In both years of the experiments, in two-day
irrigation interval, arachidic acid contents decreased in NC-7 cultivar
and increased in Halisbey and Sultan cultivars with increasing
irrigation levels. In the four-day irrigation interval, arachidic acid
contents increased in NC-7 and Sultan cultivars with increasing
irrigation levels in both years. In Halisbey cultivar, in the first
year, arachidic acid contents decreased with I75 and
I100 treatments, but increased with I125irrigation level; in the second year, arachidic acid contents decreased
with increasing irrigation levels.
In the first year, the lowest behenic acid contents of two-day
irrigation intervals were obtained from I75 treatments
of Halisbey and Sultan cultivars, and the lowest behenic acid contents
of four-day irrigation intervals were obtained from I50and I125 treatments. In 2018, in a two-day irrigation
interval, the lowest behenic acid contents were observed in Halisbey and
Sultan; in a four-day irrigation interval, the lowest values were
obtained from the I75 irrigation level of Halisbey and
NC-7 cultivars. In both years, the greatest behenic acid contents of
two-day irrigation intervals were obtained from I125treatment of Halisbey cultivar, and the greatest behenic acid contents
of four-day irrigation intervals were obtained from I125treatment of NC-7 cultivar (Figure 3c and d).
For lignoceric acid contents, Halisbey and Sultan cultivars had similar
reactions in 2017 and 2018 against irrigation intervals and irrigation
levels. In both years, the lignoceric acid content of both cultivars
increased in two-day irrigation intervals and decreased in four-day
irrigation intervals with increasing irrigation levels. In NC-7
cultivar, lignoceric acid contents decreased in two-day irrigation
intervals and increased in four-day irrigation intervals with increasing
irrigation levels in both years (Figure 3e and f).
Myristic acid was the least common unsaturated fatty acid in present
experiments. NC-7 and Sultan cultivar and similar changes in myristic
acid contents with the years and irrigation intervals. In NC-7 cultivar,
in a two-day irrigation interval of both years, myristic acid contents
initially decreased with irrigation levels but reached the maximum in
I125 treatment. In four-day irrigation intervals of the
same cultivar, a reverse case was evident, there was an increase in
I50, I75, and I100treatments, but a decrease in I125 treatment (Table 3
and 4). In two-day irrigation intervals of Halisbey cultivar, myristic
acid contents increased in I50, I75, and
I100 treatments and decreased in I125treatment in both years (Figure 3g and h). In Sultan cultivar, myristic
acid contents did not change significantly with irrigation levels in two
and four-day irrigation intervals of the first year; significant
decreases were observed in myristic acid content, especially in four-day
irrigation intervals of the second year with increasing irrigation
levels.
The biplot graphs generated for visual assessment of the relationships
between the fatty acids of the cultivars based on irrigation intervals
and irrigation levels are presented in Figure 4. Each one of the present
biplots explained over 60% of total variations. Since Singkham et al.
(2010) used the oleic/linoleic acid ratio as a quality criterion for
peanuts, O/L ratio is presented in Figure 4.
As can be seen in Figure 4a, investigated fatty acids formed four groups
in Halisbey cultivar in the first year. There were significant negative
correlations between G-IV (linoleic acid) and G-II (oleic acid and
oleic/linoleic acid ratio); between G-III (arachidic acid, TUSFA) and
G-I (the other saturated fatty acids, TSFA, and oil ratio) (Figure 4a).
In the second year, TUSFA alone formed Group-IV and had negative
correlations with Group-II (lignoceric, palmitic acid, oil ratio, and
TSFA). Group-III was composed of oleic acid and oleic/linoleic acid
ratio and had negative correlations with Group-I (Figure 4b). In both
years of Halisbey cultivar, the group including oleic acid (G-II in the
first year and G-III in the second year) negatively correlated with the
group, including oil ratio (G-I in the first year and G-II in the second
year). Such a finding indicated that the treatments to increase the oil
ratio of this cultivar would reduce the oleic/linoleic acid ratio. Like
this, in both years, 2D-I50, 4D-I50, and
4D-I75 treatments increased oleic acid contents, and
4D-I100 treatment increased oil content in both years
(Table 3 and 4).
In NC-7 cultivar, the interactions between oil ratio and fatty acids
were similar in both years. In the first year, TUSFA and myristic acid
alone formed single groups. In the second year, myristic acid and TUSFA
were placed in the same group. In the first year, there were significant
negative correlations between Group-V (oleic acid, oil content,
oleic/linoleic acid ratio) with the greatest vector length and Group-II
(linoleic acid). There were significant negative correlations between
Group-IV (TUSFA) and Group-I (Arachidic, Palmitic, Lignoceric, Behenic
acids, and TSFA) (Figure 4c). In the second year, there were significant
negative correlations between Group-V (Oleic and oleic/linoleic acid
ratio) and Group-II (linoleic acid) and between Group-III (TUSFA and
Myristic) and Group-I (oil content, Arachidic, Palmitic, Lignoceric,
Behenic acids, and TSFA) (Figure 4d). In NC-7 cultivar, the oil ratio
had significant positive correlations with oleic acid and oleic/linoleic
acid ratios. Thus they were placed into the same attribute group (G-V)
in the first year. In the second year, the level of correlation of oil
ratio with oleic acid and oleic/linoleic ratios was lower. Thus oil
ratio was placed into a different group. In NC-7 cultivar,
4D-I50, 2D-I50, and
4D-I75 treatments were prominent for high oil and oleic
acid content in the first year, and besides these treatments,
4D-I100 was also prominent for both oil ratio and oleic
acid content in the second year (Figure 4c and d).
In Sultan cultivar, there were significant negative correlations between
Group-IV (TUSFA and Myristic) and Group-II (Behenic, TSFA, and
Arachidic) and between Group-III (Oleic, Oil content, Oleic/Linoleic
acid ratio) and Group-I (Linoleic, Lignoceric, and Palmitic) in the
first year (Figure 4e). In the second year, Arachidic acid alone formed
a group (Group-3), and this group had significant negative correlations
with Group-V (TUSFA, Myristic) and Group-I (linoleic). The greatest
negative correlation was observed between Group-I (linoleic) and
Group-IV (Oil content, oleic acid, and oleic/linoleic acid ratio)
(Figure 4f). The most stable and positive correlations between oil
content and oleic acid were observed in the Sultan cultivar. In this
cultivar, oil content, oleic acid, and oleic/linoleic acid ratio were
placed in the same attribute group. The 2D-I75 treatment
had positive correlations with this attribute group and placed in the
closest position. Such a case indicated that 2D-I75 was
the best irrigation combination for high oil content and thus high oleic
acid content in Sultan cultivar (Figure 4e and f).
High oleic acid content is the primary quality attribute in peanuts, as
can be inferred from the present findings that oleic acid contents
varied with the cultivars and environmental conditions.
When an appropriate irrigation
program was applied, high oleic acid contents could be achieved based on
the genetic potential of the cultivar. As can be seen in Figure 4, the
greatest oleic contents in
Halisbey and NC-7 cultivars were
obtained from I50 treatment of two and four-day
irrigation intervals. In Sultan cultivar, the greatest oleic acid
content was obtained from I75 treatment of two and
four-day irrigation intervals of the second year.
Three peanut cultivars used in the present experiments were the most
common cultivars of Turkey. As a general average, the greatest oleic
acid content (51.90%) was obtained from NC-7 cultivar, and it was
followed by Sultan (47.12%) and Halisbey (46, 40%) cultivars.
Similarly, in peanut samples taken from common peanut-cultivated
regions, NC-7 cultivar had greater oleic acid content than Sultan and
Halisbey cultivars (Kalefetoğlu et al., 2018).
Nutritional quality, taste, and shelf life of peanut products and seeds
largely depend on relative quantities of saturated fatty acids (SFA),
mono-unsaturated fatty acids (MUFA) and poly-unsaturated fatty acids
(PUFA). High oleic acid contents play a significant role both in human
health and thermo-oxidative stability of peanuts for industrial purposes
(Nawade et al., 2018). Singkham et al. (2010) indicated that genetics
was the most significant factor affecting the fatty acid composition of
peanuts, genotype x environment interactions were also significant for
oleic, linoleic acids and O/L ratio and genetic effects especially on
oleic acid contents may decrease based on (GxE) interactions. Climate,
irrigation, and soil conditions are effective primary factors in peanut
cultivation. During the seed ripening stage, environmental factors
generally increase oleic acid contents and decrease the other fatty acid
quantities (Sanders, 1980). In the present study, significant variations
were observed in the oleic contents of three peanut cultivars with
irrigation intervals and irrigation levels. In the two-day irrigation
interval, oleic acid contents decreased in NC-7, and Halisbey cultivars
with increasing irrigation levels and the greatest oleic acid content in
Sultan cultivar were achieved in the I75 irrigation
level. In a four-day irrigation interval, the response of cultivars for
oleic acid content varied with the years and irrigation levels. In this
irrigation interval, the greatest oleic acid content was obtained
fromI75 treatment of NC-7 cultivar. The oleic acid
contents were generally greater in 2018 than in 2017. Such a case was
mostly attributed to climate factors. It was reported in a previous
study that high temperatures during the seed development phase reduced
linoleic acid content and increased oleic acid content of peanut oil
(Casini et al., 2003). Plant tolerance of high temperatures largely
depends on soil moisture, thus irrigation treatments. The oleic and
stearic acid content of peanuts grown in rainy seasons was greater than
the peanuts grown in dry seasons and linoleic, and palmitic acid
contents also increased in rainy seasons (Singkham et al. 2010).
Chaiyadee et al. (2013) reported that droughts improved the oil quality
of peanuts, increased oleic acid contents, genotype x environment
interactions influenced fatty acid composition, resistant and sensitive
genotypes exhibited similar response to droughts and recommended further
research about the effects of droughts on oleic acid content.
Since irrigations have quite significant effects on peanut oil quality,
effects of irrigation intervals and irrigation levels on oil content and
fatty acid composition of 3 commonly cultivated peanut cultivars of
Turkey were put forth in this study. Especially the effects of
irrigation interval x irrigation level interactions on fatty acid
composition were investigated. In previous studies conducted on peanuts,
either irrigation intervals or irrigation levels, were investigated
alone. Together with different peanut cultivars, two factors were not
considered together in previous studies. Previous studies mostly focused
on a single cultivar with different irrigation intervals or with
different irrigation levels. Therefore, the present findings were not
able to be sufficiently discussed with the findings of earlier studies.
Aydınsakir et al. (2019) applied different water-deficit levels to NC-7
peanut cultivar under Mediterranean conditions and reported that oleic
acid contents decreased with increasing irrigation levels, linoleic acid
contents increased until I75 treatment and decreased in
I100 and I125 treatments. In another
study, decreasing oleic contents were reported with increasing
irrigation levels, and the greatest linoleic acid contents were reported
for I75 and I100 treatments (Boydak et
al., 2010).
Regression graphs facilitated visual assessment of oil content, and
fatty acid composition of peanut cultivars and biplot graphs facilitated
the assessment of cultivar x years x irrigation interval x irrigation
level interactions. With the aid of biplot graphs, the best irrigation
interval x irrigation level combination for each attribute group was
tried to be identified. In two and four-day irrigation intervals, the
greatest oil contents were generally achieved in I75 and
I100 irrigation levels. Recommendations of cultivar,
irrigation interval, and irrigation level were provided for high oleic
acid contents. It was concluded based on present findings that high
oleic acid contents could be achieved based on genetic potential of the
cultivars with a proper irrigation program; I50treatment could be used at two or four-day irrigation intervals in
Halisbey, and NC-7 cultivars and I75 treatment could be
used in four-day irrigation interval in Sultan cultivar to get high
oleic acid contents under sandy soil conditions.