4. Discussion
Although there is documented that Camelina can well adapted to semi-arid
environmental condition but the present results showed that severe water
deficit (25% FC) in particular during the reproductive phase
significantly disrupted flower bud and declined seed yield. Based on the
recent study, PGPBs are pivotal candidates in sustainable agricultural
management under water deficit conditions. Our results showed that water
deficit and symbiosis association significantly changed the physical
parameters of camelina seed. Exposure to Water deficit during the
reproductive phase resulted in a significant decline in silique number
and length. It can be a part of the stress tolerance mechanism for the
appropriate allocation of assimilate supply to seeds. Seed number
strongly affected by water stress. It related to drought effects on
inaccurate fertility and abortion of flower and or premature seed
production and finally decreasing the yield (Aslan et al., 2009; Rad and
Zandi, 2012). Similar results were recorded for Carthamustinctorius (Istanbulluoglu et al., 2009), Brassica napus(Hatzig et al., 2018), Brassica gunica (Gan et al., 2019). The
drought- induced improvement of seed weight was observed in both
no-inoculated and inoculated plants (Table 2). Seed filling is strongly
dependent on interplant competitions and nutrient accumulation for the
synthesis of metabolites. There was a considerable association between
seed weight and complement of oil and protein content (Fig1A, C).
Additionally, during water deficit stress, the TSC content increased as
compared to control. The increase of seed weight with an associated
increase in protein content and TSC (Fig1B) can be a sustainable
characteristic of water deficit tolerance in plants.
The results of the present study showed the reduction of carbon content
and associated increase nitrogen content in seed grown under mild water
stress in inoculated and non-inoculated plants. Not only, the C: N ratio
is a sustainable indicator in oilseeds to identify the production rate
of protein and oil (Jaradat and Rinke, 2010) but also, the C: N balance
illustrates the rate growth and development of plants (Otori et al.,
2017). There was a positive correlation between C: N ratio and oil
accumulation (Fig 2A). Similar to the present study, reported inVicia faba (Kabbadgi et al., 2017), Tephrosia apollinea(Hussain et al., 2019) that C: N ratio decreased along with oil
accumulation under water deficit stress. Nitrogen accumulation is one of
the defense mechanisms in response to the water deficit stress in the
seed filling stage. Nitrogen not only tends to mobility from vegetative
tissues to seed but also to derivate from free amino acids (Kinugasa et
al., 2012). As in seeds developed underwater deficit, the rate of
protein synthesis infrequently range and or possibly increase. A
positive correlation between N content and protein content in camelina
was reported by Jiang and his workers (2014). Conversely, the reduction
of carbon supply in developing seed under stress is due to the reduction
of the photosynthetic carbon fixation in vegetative tissues and less
remobilization of carbohydrates (Wang and Frei, 2011; Thalmann and
Santelia, 2017; Sehgal et al., 2018). Based on numerous studies,
supplied carbon to developing seeds regulated with carbon partitioning
between oil and carbohydrate synthesis a much higher rate of
carbohydrates than oil synthesis (Schwender et al., 2015). It is in
agreement with our results that water deficit stress– induced
alternation of carbon and nitrogen content coincided with the decrease
of oil and increase of protein. Additionally, rising TSC content is
considerable for acclimation to drought stress and as the main resource
for oil synthesis in the seed (Ni et al., 2019). Results presented inBrassica napus (Aslam et al., 2009), Camelina sativa(Obour et al., 2017; Hossein et al., 2019), Brassica junica(Elferjani and Soolanayakanahally 2018) Brassica napus , andBrassica junicea are similar to the present report for positively
correlation protein with TSC content and negatively correlation oil with
both protein and TSC content under stress and or non-stress.
Additionally, a positive correlation between seed weight and nitrogen
and phosphorus concentration was detected in our study. It can be
related to phosphorous and nitrogen partitioning for seed protein
synthesis. Nakagawa and workers (2018) has reported that water deficit
during the reproductive stage resulted in the reduction of oil and
protein content and the increase of carbohydrate in soybean seed. It is
related to the increase of genes expression involving in lipid
degradation along with the decrease of gene expression being active in
lipid biosynthesis.
S (sulphur) is a required constituent for plants in particular
Brassicaceae family due to participate in the structure of S-containing
proteins and secondary metabolites (like glucosinolates) (Poisson et
al., 2019). The present study found a severe reduction in (10 fold) S
concentration under water stress conditions in the inoculated plants.
The previous finding showed that the reduced S content long with an
increase of N content leads to an increase in the protein. Based on
these facts, the N: S ratio is a suitable criterion for assay of the
S-containing metabolites and oilseed quality (Dhillon et al., 2019,
Poisson et al., 2019) possessing in various environment condition
(Sutradhar et al., 2017). Our results illustrated a positive
relationship between protein content and the N: S ratio under water
stress.
Phosphorus (P) is one of the critical nutrient, responsible for plant
growth and development with the supply of energy and as a macro-molecule
structural component (Cetinkaya et al., 2016). The present study showed
that the enhancement of P content is associated with water stress. The
higher partition of P content in seed under stress may indicate the
possible involvement of P in supplying energy which in turn decreasing
seed filling time. Additionally, our finding showed that the increasing
trend of P in seed was in a line with protein content and subsequently
1000- weight seed similarity nitrogen. It is suggested that water
stress- induced enhancement of phosphorous supply energy for the
synthesis of proteins. Our results confirmed the work of Gaspar and
workers (2017). One well-known effect of PGPB is the improvement of
phosphorus solubility and mobility from soil to root, leave and seed
(Etesami and Maheshwari 2018; Taliman et al., 2019). Our finding is in
agreement with results reported by Khan et al., 2019 and Mogal et al.,
2019.
In as much as the main resources of hydrogen (H) are polymers such as
carbohydrates and proteins, hereupon a positive correlation exists
between C and both termed polymers (Chávez-Mendoza et al., 2019; Reshad
et al., 2019). It is in agreement with our report and Chávez-Mendoza et
al., 2019 for hydrogen content. In the present study, drought and PGPB
had no significant effect on H content.
The antioxidant capacity of camelina seed by methanol extract was
assessed by scavenging DPPH• radicals value and measuring TPC. The
present results showed that antioxidant capacity was enhanced under
drought stress with significantly higher rate to enhance in inoculated
than that in no- inoculated plants. The present study consisted with
those reported in Cuminum cyminum (Rebey et al., 2012). Data on
the total phenolic content in Table 4 showed that under water deficit
stress, TPC increased in inoculated and non-inoculated plants. It is in
agreement with this was reported by Rehman et al., 2018 for wheat when
PGPR induces phenolic content. In plants, accumulation of phenolic
features is a potential response to overcome abiotic and biotic stresses
through to remove reactive oxygen species (ROS) and improve nutrients
uptake (Sharma et al., 2019). Researchers have reported that PGPRs
enhance phenolic content in plants and subsequently increase of
nutrients uptakes (Etesami and Maheshwari, 2018).
The presence of Fe, Mn and Zn in oilseeds are consequential in
translocation of assimilates and enzymatic reactions of oil, protein and
carbohydrate biosynthesis (Goli et al., 2018). The concentration of Fe,
Mn, and Zn of Camelina seeds is presented in Table 3. During water
deficit stress, the enhancement of Zn, Fe, and Mn content was well known
with a higher rate to improve in inoculated than non-inoculated plants.
Our results showed a relatively positive correlation between
micronutrients and protein content. It is in agreement with this is
reported for Phaseolus vulgaris (Ghanbari et al., 2013). In
previous studies, it has been documented drought-induced decrease of
nutrients availability was alleviated by plant- associated bacteria
through siderophores production (high affinity to
Fe3+) and to convert Mn 4+ to Mn2+ and improvement of Zn availability (Etesami and
Maheshwari, 2018). Reports on the effects of water deficit on
micronutrients content are quite varied depending on the plants
potential to stress tolerance (Samarah et al., 2004). Wijewardana and
workers (2018) has reported enhancement of Fe and Mn content during
drought stress and termed as osmoregulation of developing seeds.
A part of the water deficits tolerance mechanism in plants is the
alternation of membrane structure and fluidity with remodeling oil
content and fatty acids profile (Mohamed and Latif 2017). The decrease
of photosynthesis and carbon remobilization lead to the reduction of
seed oil content under drought stress (El Sabagh et al., 2019). The
present study confirmed previous findings indicating the effect of water
deficit stress on the significant reduction of Camelina oil by about
14.74% (Rebey et al., 2012; Elferjani and Soolanayakanahally 2018;
Hatzig et al., 2018). Additionally, the decrease of Camelina oil
coincided with the increase SFA (C16:0, C 18:0 and C20:0) and PUFA
(C18:2 and C18:3) and the decrease MUFA (C18:1, C20:1). In the Camelina
FAMEs, the most abundant fatty acid was linolenic acid at 43.44-
33.40%. Our finding coincides with the previous results that showed
water deficit stress at the reproductive phase increase linoleic and
linolenic acid (Aslan et al., 2009; Gharechaei et al., 2019). The
increase in unsaturated fatty acid (linoleic acid and linolenic acid)
could be due to the activation of the lipase enzyme and followed
developing membrane fluidity for the adaptation to water deficit stress
(Upchurch 2008). On the other hand, desaturase enzymes activating in
PUFA synthesis have stability under the biotic and abiotic stress
(Nayeri and Yarizade 2014). As illustrated in Table5 the increase of the
proportion of SFA under stress condition is due to palmitic acid,
stearic acid and arachidic acid content having a negative correlation
with oil content. The stress-induced increase trend in C16:0 and C18:0
was quite similar in inoculated and non-inoculated plants. These results
were the agreement with the obtained report by Laribi et al., 2009.
Based on previous findings in soybean (Mohamed and Latif 2017) and
sunflower (Petcu et al., 2001) increase of palmitic acid coincided with
the decrease in stearic acid under water deficient stress. In the
present study, oleic acid, gadeolic acid, and erucic acid were detected
as MUFA. In our experiment, MUFAs content correlated positively to
Camelina oil under drought stress. It is suggested that a remarkable
decrease in oleic acid content may be due to water deficit
stress–induced reduction of ∆9 desaturase activity
and related – gene expression in the synthesis pathway of oleic acid
from stearic acid. Additionally, a decrease of oleic acid along with
increase linoleic acid and linolenic acid could be due to the higher
enhancement of ∆6 and ∆12 desaturase
than ∆9 desaturase activity for strong tolerance to
drought stress. Similar results reported for canola under drought stress
(Aslam et al., 2009). On the other part, there is a significant decrease
in gadeolic acid content in the stressed plant compared to control. It
may be due to associate the desaturase activity with elongase enzyme
which convert oleic acid to linoleic acid and or gadeolic acid
respectively. Erucic acid (C22:1), one of the considerable fatty acids
in the Brassicacea family (Velioglu et al., 2017) detected in our
experiment. It was showed that the most abundant erucic acid by about
3.03% in B1D0 is still lower than limited content for food consumption.
Both of water deficit stress and PGPB decreased C22:1 content by about
1.64%. The present study showed that seed inoculation byMicrococcus yunnanensis resulted in the different alterations of
seed composition. According to Table 5 the decrease of oil content
coincided with the increase of PUFA and decrease of SFA and MUFA in the
inoculated in comparison to no-inoculated plant.
Of unsaturated acids fatty acids (USFA), oleic acid and linolenic acid
contents increased with PGPB while linoleic acid decreased. Results are
in a line with those has reported Silva et al., 2013 in soybean seed and
Sharifi et al., 2019 in safflower seed about the effect of PGPB on the
reduction of SFA (palmitic acid and stearic acid ) and enhancement of
USFA (oleic acid and linolenic acid) in soybean seed. Abased on obtained
reports, PGPBs increase seed filling duration increasing involvement of
the nutrients in the synthesis of composition seed (Sharifi et al.,
2019).