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).