3.1 Biological yield (BY)
Figure 1 shows the variation of BY under different irrigation regimes and Si application. The BY decreased by progressing the drought stress but both Si and Si-NPs increased it (P ≤ 0.05). The highest BY was reported in Si-NPs treated plants under well-water conditions to be 7120 kg ha− 1. The significant reduction of BY was observed in the plants experiencing severe drought stress. Under non-Si application, a 37% decline of BY was observed under severe water stress compared to well-watered conditions. Thus, both Si and Si-NPs alleviate the adverse effects of drought stress on BY.
Growth (biomass) is a primary process to be changed by water shortage [24]. We observed a decline of growth under moderate and severe stresses. It shows that even at decreased soil water availability, coriander plants are able to grow but in the slow rate. These findings agree with those obtained by Achten et al. [25] (2010) on Jatropha curcas and Sarker and Oba [24] on Amaranthus tricolor, who reported plants are able to grow under deficit water stress but at the slower rate than fully irrigated condition. The decline of BY with progressing drought could be attributed to inhibition of cell elongation and expansion, reduced turgor pressure, use of the energy to conserve cell turgor, which results in reduction of photo-assimilation and metabolites required for cell division [24]. However, both Si and Si-NPs improved the BY at well-watered and water-stressed conditions. Bukhari et al. [1] represented the higher effective role of Si under water stress condition compared to well-watered situation, which it suggests the principal role of Si in plant growth and development at water deficit conditions. Compared to Si, Si-NPs was significant more effective, which is related to the higher availability of Si-NPs with coriander plants than Si. Similar results for the better effect of Si-NPs compared to Si were reported on wheat seedling under UV-B stress [15]. The similar enhancement in biomass of Si-NPs-treated plants was reported in various studies [5, 7, 26]. The Si-NPs may improve the nutrients uptake by plants, which results in higher biomass. In this regard, Si-NPs increases the uptake of Mg, P, K and S contents in stressed plants [26, 27]. Moreover, plant biomass may be remarkably improved by Si-NPs which might be due to its placement in the leaf bundle that affect the structure of chloroplasts and leaf yield at the unit level and therefore increase the plant’s ability to use light [5].
3.2 Relative water content (RWC) and total soluble sugar (TSS)
Although RWC decreased with advancing the water stress, different applications of Si improved the RWC (P ≤ 0.05). It differed from 55.6% in severe-stressed plants with non-Si application to 81.3% in well-watered plants and Si-NPs treatment (Fig. 2a). Unlike RWC, TSS increased by advancing the rate of drought stress and with Si application. The greatest TSS was obtained in severe stress plants with foliar application of Si-NPs. However, the plants experiencing no drought stress (control) in the absence of Si and Si-NPs had the lowest TSS (Fig. 2b).
The reduction of RWC and enhancement of TSS were the strategy of coriander to cope the stressed conditions. RWC is a useful variable to assesses the physiological water status of plants, which can be used as an important index for determining the tolerant and sensitive plants under stressful environments [24]. Reduced RWC was observed under moderate and severe stresses (Fig. 2, a). The decreases of turgor pressure, water uptake limitation in available water, and prohibition of water uptake by roots lead to reduced RWC in the leaves. Plants exposed to stressed conditions reduce the RWC to reduce the secondary biochemical processes and to maintain the energy on their survival. Moreover, TSS is the highly sensitive variable to environmental stresses [28]. Increased TSS under drought stress has been demonstrated in several studies [29, 30]. In contrast, a few studies have shown that drought significantly decreases the TSS in the whole plant [29]. In this two-year study, moderate and severe drought stresses enhanced sugar accumulation in leaves of coriander plants. Soluble sugar plays the vital role an osmoprotectant, regulating osmotic adjustment, scavenging toxic reactive oxygen species (ROS), and providing membrane protection, and under biotic and abiotic stresses [30]. The accumulation of TSS is an effective strategy of plants to maintain RWC and membrane stability of cells under water stress conditions [30].
3.3 Total phenolic content (TPC) and total flavonoid content (TFC)
Figure 3 shows the response of TPC and TFC to Si application under different irrigation regimes. The TPC increased under moderate drought stress and also it was improved with S application. The plants experiencing severe stress condition showed the lowest TPC. This biochemical trait was obtained in a range of 32.62–41.91 mg GA g− 1 DW in which the highest amount was observed in moderate water stress with Si-NPs spraying. TFC showed the variation under irrigation regimes and Si application. The highest TFC was found in well-watered plants with Si-NPs. The reaction of plants to TFC was different amog the treatments. In non-Si treatments, moderate drought stress increased TPC by 19% compared to well-watered condition. Generally, the optimum TPC and TFC were reported under moderate stress conditions with foliar application of Si-NPs.
Phenolic compound is an eminent antioxidant factor for protecting the plants from dangerous radicals such as ROS. We found the higher TPC and TFC under moderate water stress and with Si and its nanoparticles, but it decreased in plants experiencing the severe drought (Fig. 3). Similar to our results, Gharibi et al. [31] and Krol et al. [32] showed the increased TPC and TFC under moderate drought but they remained unchanged under severe stress conditions. Therefore, it can be concluded that severe drought stress might inhibit the synthesis of TPC and TFC [32]. Plants use different mechanisms to distribute flavonoids among subcellular compartments [31]. The augmentation of TPC and TFC under high limited water is related to accumulation of soluble carbohydrates in plant cells because of reduced transportation of TSS under drought stress [33]. Phenolics and flavonoids are produced by shikimic acid or phenyl propanoid pathways, and as hydroxyl groups can donate hydrogen and react with ROS in the termination reaction, which ultimately breaks the cycle of producing new radicals. The shikimic acid pathway converti simple carbohydrate precursors into aromatic amino acids [31]. Si and particularly its nanoparticles improved the TPC and TFC. Si and Si-NPs due to their role in improving the production of secondary metabolisms such as TPC and TFC can protect the stressful plants from the ROS. The positive role of Si-NPs on polyphenols has been observed on coriander plants grown in lead (Pb)-spiked soil [5].
3.4 Essential oil (EO) percentage and yield
Moderate water stress conditions and foliar application of Si-NPs led to higher EO percentage than other experimental treatments. At non-foliar applied plants, moderate water stress improved EO content by 70% compared to well-watered plants. Under severe drought, Si-NPs and Si increased EO content by 29% and 22% compared to non-foliar application (Fig. 4a). EO yield in moderate drought stress was significantly higher compared to other irrigation regimes. Although, there was no significant difference between Si and Si-NPs in EO yield of coriander plants, these amendments led to increased EO yield (Fig. 4b).
The slight drought stress was effective to reach the optimum EO percentage and yield of aerial parts of coriander. However, the adverse impact of severe drought stress on EO yield of coriander was addressed. Since EO yield is calculated by dry matter and EO percentage, the reduction of shoot biomass and EO percentage results in reduced EO yield [34]. Si and Si-NPs subsided the undesirable effects of severe drought stress and promoted EO yield (Fig. 4). Environmental stresses are important in EO production [35, 36]. Our results revealed that moderate drought stress along with Si and S0-NPs enhanced EO yield, which confirmed the previous investigations with drought [37, 38] or Si application [36]. The enhanced EO amount under slight drought stress may be attributed to the decline in leaf surface and enhancement of oil gland number and density [39]. The abiotic stresses may affect EO production through changing the pathways of secondary metabolites production and their distribution [36]. This study demonstrated the improvement of EO yield by Si and Si-NPs application. Si plays an important role in elicitor-accelerated secondary metabolite production by inducing several transcriptional modifications [40]. Si may increase EO yield via improvement of cell development, ion uptake, and leaf oil gland density and size [41]. Si can enhance the EO production via its different uses such as Si or its nanoparticles. The improvement of EO yield under Si treatment in this work are in agreement with those reported on rosemary [42], geranium [34], and sweet basil [40]. Si and its nanoparticles increased EO production, and probably served as an effective stimulant to increase the production of secondary metabolites [43].
3.5 Essential oil composition
The GC/MS analysis showed 22 compounds in EO of coriander (Tables 2 and 3). The main EO constitutes were n-Decanal, 2E-Dodecanal, 2E-Decanal, 2E-Tridecen-1-al, Dodecanal, and n-Nonane in both 2019 and 2020. N-decanal as a main compound was affected by foliar application of Si-NPs. It differed from 20.80% at severe water stress and Si to 27.60% under well-watered conditions and foliar application of Si-NPs. 2E-Decanal (13.30–16.70%) showed no remarkable variation under irrigation regimes and Si foliar application. Unlike N-decanal, Si and it nanoparticles decreased 2E-Dodecanal in which it ranged from 13.9% in plants treated with Si-NPs under severe stress conditions to 18.7% in well-watered plants with no foliar application. The 2E-Tridecen-1-al was changed under different treatments of irrigation regimes and Si in a range of 7.32–10.50%. Dodecanal (7.2–10.6%) was different under these treatments in which its highest amount was obtained under moderate water stress and Si-NPs application. The maximum n-Nonane amount (8.7%) was obtained in well-watered plants.
Table 2
Essential oil composition of coriander under silicon and drought stress in 2019
Compound
|
RI
|
Non-stressed condition
|
Moderate stress
|
Severe stress
|
Control
|
Si
|
SiNPs
|
Control
|
Si
|
SiNPs
|
Control
|
Si
|
SiNPs
|
n-Nonane
|
900
|
8.70
|
6.70
|
7.60
|
4.60
|
7.80
|
7.30
|
8.50
|
4.30
|
5.70
|
α-Pinene
|
933
|
0.17
|
0.24
|
0.23
|
0.18
|
0.16
|
1.20
|
0.37
|
0.39
|
0.46
|
n-Decane
|
996
|
0.65
|
0.45
|
0.46
|
0.87
|
0.65
|
0.54
|
0.40
|
0.34
|
0.45
|
n-Octanal
|
1005
|
0.56
|
0.43
|
0.56
|
0.32
|
0.12
|
0.00
|
0.45
|
0.56
|
0.00
|
p-Cymene
|
1027
|
0.23
|
0.17
|
0.19
|
0.23
|
0.25
|
0.27
|
0.01
|
0.66
|
0.00
|
Limonene
|
1030
|
0.00
|
0.00
|
0.12
|
0.15
|
0.18
|
0.56
|
0.76
|
0.34
|
0.24
|
n-Undecane
|
1096
|
0.13
|
0.00
|
0.14
|
0.45
|
0.03
|
0.00
|
0.43
|
0.13
|
0.13
|
Linalool
|
1102
|
0.00
|
0.12
|
0.13
|
0.65
|
1.30
|
0.00
|
0.23
|
0.87
|
0.01
|
n-Nonanal
|
1107
|
1.23
|
1.60
|
1.15
|
1.34
|
1.26
|
1.23
|
1.18
|
1.43
|
1.25
|
n-Decanal
|
1219
|
22.50
|
25.50
|
27.60
|
23.50
|
24.45
|
25.40
|
21.20
|
20.80
|
22.30
|
2E-Decanal
|
1273
|
14.70
|
16.70
|
13.30
|
13.80
|
14.20
|
13.50
|
16.40
|
13.40
|
16.20
|
1-Decanol
|
1277
|
0.34
|
0.32
|
2.30
|
0.45
|
0.57
|
0.32
|
2.30
|
0.43
|
0.35
|
Undecanal
|
1311
|
3.12
|
2.70
|
3.29
|
3.40
|
3.29
|
3.20
|
3.60
|
3.70
|
2.70
|
2E-Undecanal
|
1370
|
3.60
|
4.30
|
3.40
|
3.50
|
3.20
|
2.70
|
1.60
|
5.40
|
3.70
|
Dodecanal
|
1413
|
8.70
|
8.70
|
9.70
|
7.60
|
8.70
|
10.60
|
8.80
|
10.20
|
9.90
|
2E-Dodecanal
|
1467
|
17.80
|
16.70
|
14.50
|
18.70
|
16.70
|
14.50
|
17.80
|
15.60
|
13.90
|
Tridecanal
|
1511
|
0.45
|
0.34
|
0.56
|
0.76
|
0.45
|
0.54
|
0.34
|
0.95
|
0.76
|
2E-Tridecen-1-al
|
1527
|
1.05
|
1.04
|
0.00
|
1.02
|
1.12
|
0.00
|
1.50
|
1.70
|
1.40
|
Tetradecanal
|
1614
|
1.16
|
1.02
|
0.12
|
1.27
|
1.28
|
1.87
|
1.22
|
1.45
|
1.28
|
2E-Tridecen-1-al
|
1680
|
10.32
|
8.70
|
9.50
|
9.10
|
7.32
|
7.80
|
8.20
|
9.70
|
9.90
|
Neocnidilide
|
1793
|
2.78
|
2.45
|
1.78
|
3.12
|
2.67
|
3.16
|
2.56
|
2.76
|
4.01
|
Hexahydrofarnesyl
|
1780
|
1.23
|
1.43
|
2.34
|
1.23
|
1.43
|
3.50
|
1.20
|
0.98
|
3.44
|
Total
|
|
99.42
|
99.61
|
98.97
|
96.24
|
97.13
|
98.19
|
99.05
|
96.09
|
98.08
|
Table 3
Essential oil composition of coriander under silicon and drought stress in 2020
Compound
|
|
Non-stressed condition
|
Moderate stress
|
Severe stress
|
RI
|
Control
|
Si
|
SiNPs
|
Control
|
Si
|
SiNPs
|
Control
|
Si
|
SiNPs
|
n-Nonane
|
900
|
8.30
|
7.90
|
7.60
|
5.70
|
7.10
|
5.90
|
8.30
|
5.20
|
4.90
|
α-Pinene
|
933
|
0.18
|
0.26
|
0.26
|
0.22
|
0.27
|
0.98
|
0.40
|
0.43
|
0.33
|
n-Decane
|
996
|
0.67
|
0.65
|
0.68
|
0.65
|
0.56
|
0.76
|
0.28
|
0.67
|
0.67
|
n-Octanal
|
1005
|
0.53
|
0.34
|
0.47
|
0.65
|
0.87
|
0.90
|
0.12
|
0.00
|
0.54
|
p-Cymene
|
1027
|
0.00
|
0.12
|
0.12
|
0.60
|
0.00
|
0.00
|
0.23
|
0.76
|
0.12
|
Limonene
|
1030
|
0.56
|
0.00
|
0.14
|
0.13
|
0.18
|
0.56
|
0.72
|
0.34
|
0.27
|
n-Undecane
|
1096
|
0.00
|
0.00
|
0.13
|
0.12
|
0.16
|
0.00
|
0.45
|
0.16
|
0.98
|
Linalool
|
1102
|
0.01
|
0.30
|
0.13
|
0.12
|
1.30
|
0.00
|
0.23
|
0.65
|
0.76
|
n-Nonanal
|
1107
|
1.26
|
1.25
|
1.34
|
1.37
|
1.02
|
0.00
|
1.23
|
1.86
|
0.98
|
n-Decanal
|
1219
|
22.90
|
23.40
|
26.70
|
22.40
|
24.30
|
25.20
|
22.10
|
20.81
|
22.70
|
2E-Decanal
|
1273
|
14.30
|
16.30
|
13.30
|
15.70
|
14.20
|
14.20
|
15.60
|
14.20
|
15.50
|
1-Decanol
|
1277
|
0.56
|
0.43
|
1.24
|
0.54
|
0.34
|
0.27
|
0.87
|
0.45
|
0.76
|
Undecanal
|
1311
|
3.20
|
2.70
|
3.29
|
3.50
|
3.29
|
3.10
|
3.60
|
3.70
|
1.87
|
2E-Undecanal
|
1370
|
3.50
|
4.30
|
3.60
|
3.20
|
3.80
|
4.30
|
3.20
|
3.90
|
3.80
|
Dodecanal
|
1413
|
7.23
|
8.23
|
7.89
|
7.30
|
8.70
|
9.20
|
7.60
|
8.30
|
7.80
|
2E-Dodecanal
|
1467
|
16.80
|
15.60
|
15.60
|
18.71
|
17.60
|
15.60
|
18.20
|
17.50
|
15.60
|
Tridecanal
|
1511
|
0.54
|
0.57
|
0.23
|
0.87
|
0.23
|
0.54
|
0.23
|
0.87
|
0.59
|
2E-Tridecen-1-al
|
1527
|
0.00
|
1.03
|
1.30
|
1.80
|
0.00
|
1.04
|
1.20
|
1.05
|
1.12
|
Tetradecanal
|
1614
|
1.78
|
1.12
|
1.28
|
1.13
|
1.28
|
1.40
|
1.67
|
1.23
|
1.35
|
2E-Tridecen-1-al
|
1680
|
10.32
|
9.70
|
9.50
|
9.10
|
7.32
|
7.80
|
8.50
|
9.70
|
10.50
|
Neocnidilide
|
1793
|
2.13
|
0.98
|
1.23
|
1.25
|
2.12
|
1.60
|
2.13
|
3.20
|
1.78
|
Hexahydrofarnesyl
|
1780
|
1.45
|
1.02
|
2.54
|
1.02
|
1.43
|
3.21
|
2.45
|
1.12
|
3.76
|
Total
|
|
96.22
|
96.20
|
98.57
|
96.08
|
96.06
|
96.56
|
99.31
|
96.10
|
96.68
|
The main EO constitutes were n-Decanal, 2E-Dodecanal, 2E-Decanal, 2E-Tridecen-1-al, and n-Nonane, which were obtained to be comparable to previous studies with little differences. Pirbalouti et al. [44] showed the main EO profile of coriander leaves were decanal (0–37.5%), cis-phytol (1.0–34.1%), 1-tetradecanol, 2E-dodecenal (8.3–17.2%), dodecanal (0.5–14.8%), n-decanol (0.5–14.8%), trans-2-undecen-1-ol (trace – 12.9%), 2E-decenal (0–11.3%),1-eicosanol (0–6.4%), and methyl chavicol (0–6.0%). Neffati and Marzouk [45] identified 2E-decenal as a main EO compound of Tunisian coriander, followed by decanal, dodecanal, and 2E-Tridecen-1-al. EO profile can be specified by the presence and amount of the compositions, which may be affected by environmental variations [44]. Plants accumulate the secondary metabolisms when exposed to different stresses through changing their cellular metabolism cope with different challenges [3]. Biosynthesis of secondary metabolites is strongly affected by environmental parameters and genetic factors. Environmental challenges such as stresses can influence major compounds of the EOs [41].
3.6 Heat map analysis (HMA)
Figure 5 showed HMA of physiological and biochemical attributes of coriander. The HMA showed severe drought stress was classified in a separated cluster, while well-watered and moderate stress treatments were placed in a distinguished cluster. In Addition, Si and Si-NPs were in a distinguished cluster, while control was classified in another cluster. In both drought and Si classifications, TPC and EO yield were the remarkable variables in distinguishing the clusters; i.e. they covered the wide range of colors, representing their main effect on distinguishing the clusters. According to HMA, TFC and EO yield were the main traits representing the highest variation under stress conditions and Si or Si-NPs foliar application. Therefore, we can remark these variables as the main factor in investigating the response of coriander to these treatments. The main EO compositions were monoterpenes. HMA showed that severe stress significantly differed from well-watered and moderate drought stress. Moreover, both Si and Si-NPs were distinguished from control treatment with respect to the physiological and biochemical attributes of coriander plants.