Compost Improving Morphophysiological and Biochemical Traits, Seed Yield, and Oil Quality of Nigella sativa under Drought Stress

Abstract

This study aimed to determine the effects of compost amendment on the soil properties, as well as the morphophysiological responses, seed yield, oil content, and fatty-acid profile. of Nigella sativa plants under drought stress conditions. In a split-plot design, the field experiment was carried out during two seasons (2020/2021 and 2021/2022), involving three irrigation regimes (named I₁₀₀, I₇₅, and I₅₀ of crop evapotranspiration) with three levels of compost application (C₀, C₁₅, and C₃₀). Soil porosity, permeability, pore geometry, water-holding capacity, organic content, and soil cation exchangeable capacity were improved in response to applied compost levels. The growth, physiology, biochemistry, and yield characteristics of Nigella sativa plants were positively affected by compost addition but negatively affected by increasing water stress severity. Deficit irrigation regimes increased osmoprotectant substances (i.e., proline, total free amino acids, carbohydrates, and total soluble sugar). Compared to the control (I₁₀₀), deficit irrigation (I₅₀) reduced fixed and essential oil by 16.64% and 39.57% over two seasons. Water stress increased the content of saturated fatty acids, while unsaturated fatty acids decreased. Compost application of (C₃₀) resulted in a significant increase in seed yield, fixed oil, and essential oil of Nigella sativa plants by 34.72%, 46.55%, and 58.11% respectively, compared to the control (C₀). Therefore, this study concluded that compost amendment improved soil properties and significantly mitigated the detrimental effects of drought on Nigella sativa plants, resulting in a considerable increase in seed yield and its oil content, particularly polyunsaturated fatty acids, which are distinguished by their beneficial effects on human health.

1. Introduction

Medicinal and aromatic plants (MAPs) have considerable economic importance due to the increasing demand for their products by domestic and international markets [1]. Black cumin (Nigella sativa L.), also called black seed, is an annual medicinal plant belonging to the Ranunculaceae family [2]. It is extensively cultivated in Syria, Egypt, Saudi Arabia, Iran, Pakistan, India, and Turkey to produce oil and seeds [3]. It is widely used as a condiment on bread and pickles, and it has important components such as metarbin, nigellin, glycosides, anthraquinones, saponines, melanthin, fixed oils, volatile oils, tannin, albuminous, proteins, glucose, and mucilage resins, which are known to promote human health [4]. Seeds of Nigella sativa are a potent antibacterial, anti-inflammatory, antihistamine, and antitumor agents, alleviating the symptoms of a wide range of ailments and disorders [5].

Water stress is one of the most critical factors impacting agricultural production, especially in irrigated agriculture in dry and semi-arid areas [6]. Drought stress reduces the yield of medicinal and aromatic plants. It might impair the output of medicinal and aromatic plants if a brief stress period coincides with the crucial developmental stage around the flowering stage [7]. Plants under water stress have lower water potential and turgor, increasing the solute levels in the cytosol and extracellular matrix; consequently, cell enlargement declines, which inhibits plant growth [8]. Secondary metabolites are synthesized by plants (i.e., accumulation of abscisic acid (ABA) and compatible osmolytes) as an adaptation mechanism in response to biotic and abiotic stresses (i.e., water stress, cold stress, and high visible light) [9,10].

Due to rising industrialization and urbanization, increased cost of irrigation pumping, insufficient irrigation scheme capacity, and scarce water resources, it is necessary to maximize the utility of irrigation water to enhance agricultural productivity. Therefore, the use of deficit irrigation is an urgent demand due to water shortage when targeting appropriate yield. In addition, the ability of aromatic and medicinal plants to adapt to water stress and their economic importance make them excellent alternatives to conventional crops, particularly in arid regions such as Egypt.

Fertilization is a crucial element for plant nutrition [11]. Chemical fertilizers are widely utilized to treat plant nutrient deficiencies. Meanwhile, high-input practices of chemical fertilizers have produced a range of economic, ecological, environmental, and social problems. Furthermore, prolonged usage of chemical fertilizers changes the pH of the soil, lowers the beneficial soil flora, contaminates water supplies, and inhibits the biological systems in the soil [12,13]. Furthermore, to ensure quality and safety for humans and the environment, many researchers and manufacturers have recently focused on producing chemical-free medicinal and aromatic plants [14,15]. The request for organic amendments has increased recently due to their health and environmental benefits, particularly for pharmaceutical products [12]. In this circumstance, organic fertilizers are vital in achieving sustainable agriculture. Enriching the soil with organic fertilizer satisfies the need for macro- and micronutrients. Hence, applying organic materials to agricultural soils significantly increases their fertility [16,17]. Organic substances enhance the soil physical properties (bulk density, aggregation, aeration, water-holding capacity, and water permeability), in addition to its chemical features (reducing soil pH, increasing cation exchange capacity, and improving nutritional status), which are essential for the growth and development of grown plants [18,19]. Organic fertilizers such as compost represent a practical technique to increase the levels of organic substances in soils. The addition of composting materials modifies soil characteristics and increases the availability of water and nutrients in soils, which could be a potentially helpful technique to motivate plant development and improve stress tolerance [20,21]. The use of compost positively influenced some traits, such as the growth, yield, oil content, and oil compounds of black cumin plants [22].

However, the data concerning the effects of organic fertilization (compost) and water stress on the physiological, biochemical, and morphological response, seed oil content, and fatty-acid profile of black cumin (N. sativa) in an arid environment such as Egypt is still insufficient. Therefore, the present study aimed to examine the impact of compost application and drought stress on the performance of N. sativa under arid and water-scarce conditions.

2. Materials and Methods

This experiment was carried out during two seasons (2020/2021 and 2021/2022) at the experimental station of the Faculty of Agriculture, Fayoum University, Fayoum (latitude of 29°17′ N and longitude of 30°53′ E) to study the influence of compost on the performance of Nigella sativa under water stress conditions.

2.1. Climate Conditions during the Experiments

Weather parameters during the study period such as minimum and maximum temperature, pan evaporation, and relative humidity around the experiment site were recorded by the Fayoum meteorological station, Fayoum (Supplementary Figure S1).

2.2. Experimental Site and Soil Characteristics

Data in

Table 1. Initial soil physical characteristics of the experimental site.

Depth (cm)	Particle Size Distribution				ρb (g cm−3)	Porosity (%)	Ksat (cm h−1)	θ Fc. (%)	θ W.P. (%)	A.W. (%)
	Sand (%)	Silt (%)	Clay (%)	Texture
0–25	9.16	19.66	71.18	Clay	1.23	53.58	0.91	43.12	22.67	20.45
25–50	8.11	20.84	71.05	Clay	1.24	53.21	0.92	41.30	22.58	18.72
Mean	8.64	20.25	71.12	Clay	1.24	53.40	0.92	42.21	22.62	19.585

ρ_b, bulk density; K_sat, hydraulic conductivity; θ_Fc., volumetric water content at field capacity; θ_W.P., volumetric water content at wilting point; A.W., available water.

and

Table 2. Chemical analysis of soil and used compost.

Soil		Compost
Properties	Value	Properties	Value
pH [at a soil-to-water(w/v) ratio of 1:2.5]	7.62	pH [at a soil-to-water(w/v) ratio of 1:2.5]	7.31
ECe (dS·m−1; soil paste extract)	2.89	Ece (dS·m−1; soil paste extract)	2.97
CEC (cmol·kg−1)	33.20
CaCO3 (g·kg−1)	3.54	CaCO3 (g·kg−1)	1.60
O.M (%)	1.44	O.M (%)	57.46
O.C (%)	0.84	O.C (%)	33.00
Available nutrients		Total nutrients
N (%)	0.07	N (%)	1.30
P (mg·kg−1 soil)	6.02	P (g·kg−1)	6.30
K (mg·kg−1 soil)	55.70	K (g·kg−1)	8.50

demonstrates the average physical and chemical characteristics of the initial soil at the depth of 0–0.5 m. The soil was clay in texture with the following properties: saturated hydraulic conductivity, 0.92 cm·h⁻¹; bulk density, 1.24 g·cm⁻³; water constants at 0.33 bar and 15 bar, 42.21% and 22.62%, respectively; pH, 7.62; available N, 0.07%; available P, 6.02 mg·kg⁻¹; available K, 55.70 mg·kg⁻¹; EC_e, 2.89 dS·m⁻¹; CaCO₃, 3.54 g·kg⁻¹; organic matter content, 1.44%; cation exchangeable capacity, 33.20 cmol·kg⁻¹. All soil samples collected before and after the experiment were evaluated for physical properties according to [23], whereas chemical soil analysis was conducted according to [24].

2.3. Plant Material and Growth Conditions

Seeds of black cumin (Nigella sativa) were procured from the Department of Medicinal and Aromatic Plants, Ministry of Agriculture, Giza, Egypt and sown at the rate of 10 kg·ha⁻¹ in the third week of October during two seasons (2020/2021 and 2021/2022). The area of experimental units was 10 m² (4 × 2.5 m), comprising four planting rows each of (4 m length × 0.6 m width). Seeds were sown in hills 25 cm apart with 4–5 seeds per hill, and thinning was applied to one plant per hill at (30 DAS). Before sowing, all experimental plots received the organic fertilizer (compost) at different rates (C₀, C₁₅, and C₃₀). Recommended doses of N, P, and K were applied at rates equivalent to 150, 70, and 60 kg·ha⁻¹, respectively. Phosphorus was added as calcium super phosphate (15.5% P₂O₅) during soil preparation. Nitrogen was applied as (NH₄)₂SO₄ with 20.6% N, along with potassium (K₂SO₄ containing 48% K₂O), at three timepoints: at sowing, at 4 weeks from sowing, and at flowering.

2.4. Experimental Design and Treatment Distribution

The experimental treatments were carried out in a complete randomized block design (RCBD) using a split-plot arrangement and performed in triplicate. The applied water deficit levels were installed in main plots; however, the compost application rates were imposed on the subplots of the experimental soil.

2.4.1. Irrigation Treatments and Irrigation Water Applied

Water stress levels were 100%, 75%, and 50% of Etc (crop evapotranspiration) as described in [25].

$$\mathrm{ETc}=\mathrm{Epan}\times \mathrm{Kpan}\times \mathrm{Kc},$$

(1)

where ETc is the crop water consumption (mm·day⁻¹), Epan is the pan evaporation from class A (mm·day⁻¹), Kpan is the pan evaporation coefficient, and Kc is the crop coefficient.

The amount of applied irrigation water was computed as follows:

$$\mathrm{IWA}=\frac{(\mathrm{A}\times \mathrm{ETc}\times \mathrm{Li}\times \mathrm{Kr})}{(\mathrm{Ea}\times 1000)},$$

(2)

where IWA is the irrigation water demand (m³), A is the area (m²), Li is the irrigation interval (days), Etc is the crop water evapotranspiration (mm·day⁻¹), Kr is the covering factor, and Ea is the application efficiency (%).

To manage the irrigation water for each experimental plot, one spile (plastic pipe 2 inches in diameter) was used, and the irrigation schedule was adjusted on the basis of the spile discharge, estimated as follows [26]:

$$\mathrm{Q}=\mathrm{CA}\sqrt{2\mathrm{gh}}\times {10}^{-3},$$

(3)

where Q is the discharge of the plastic pipe (L·s⁻¹), C is the discharge coefficient, A is the irrigation pipe’s cross-section area (cm²), g is the gravity acceleration (cm·s⁻²), and h is the effective head of water as an average (cm).

2.4.2. Compost Treatments

Before sowing, the field was disced and harrowed; then, the application of compost levels in subplots was manually incorporated into the surface layers (0–20 cm, soil depth). The applied compost treatments were as follows: C0 = zero addition (control); C15 = 15 t·ha⁻¹; C30 = 30 t·ha⁻¹. The chemical analysis of the compost used is shown in Table 2.

2.5. Methodologies and Observations Recorded

2.5.1. Morphological Characteristics

Ten plants (n = 10) were randomly collected from each treatment at full bloom (120 DAS), to evaluate the required traits of plant height (cm), stem diameter (cm), and plant fresh and dry weight (g·plant⁻¹).

2.5.2. Yield and Its Attributes

At the maturity stage (180 DAS from sowing), for estimating seed yield (kg·ha⁻¹) of Nigella sativa, as well as yield attributes, i.e., capsules weight (g·plant⁻¹), capsule weight (g), and seed weight (g·capsule⁻¹), all plants in each experimental unit were harvested.

2.5.3. Determinations of Relative Water Content (RWC%) and Membrane Stability Index (MSI%)

RWC (%) was calculated using the following formula [27]:

$$\mathrm{RWC}\left(\%\right)=\left[\frac{\left(\mathrm{FreshMass}-\mathrm{DryMass}\right)}{\left(\mathrm{TurgidMass}-\mathrm{DryMass}\right)}\right]\times 100.$$

(4)

MSI (%) was estimated as follows [28]:

$$\mathrm{MSI}\left(\%\right)=\left[1-\frac{\left({\mathrm{C}}_1\right)}{\left({\mathrm{C}}_2\right)}\right]\times 100,$$

(5)

where C₁ is the conductivity of the solution at 40 °C, and C₂ is the conductivity of the solution at 100 °C.

2.5.4. Estimation of Chlorophyll Content and Total Carbohydrates

The contents of chlorophyll a and b, as well as total carotenoids, in fresh leaves (at 120 DAS) were determined using the method of [29]. The total carbohydrates in dried leaves and stems were determined following the method described by [30].

2.5.5. Determination of Total Soluble Sugar Concentrations, Total Free Amino Acids, and Free Proline

Total soluble sugars (TSS) were extracted and estimated using anthrone reagent according to the method described in [31]. Total free amino acids were determined using ninhydrin reagent according to the method of [32]. The proline content in the leaves was determined colorimetrically using acid-ninhydrin reagent, as described by [33].

2.5.6. Determinations of Leaf Nutrients Content (N, P, and K⁺)

At the bloom stage, the nutrient content (N, P, and K⁺) of black cumin leaves esd estimated after being oven-dried and wet-digested in a mixture (HclO₄ and H₂SO₄, 1:3, v/v, respectively). The prepared samples were analyzed using a spectrophotometer for P [34] and flame photometry (Gallenkamp Co., London, UK) for K⁺ content. The nitrogen content was determined by Kjeldahl digestion (Ningbo Medical Instruments Co., Ningbo, China).

2.5.7. Seed Fixed Oil and Essential Oil Content

The Soxhlet method [35] was used to estimate the percentage of fixed oil in seed samples. The essential oil was extracted from seeds (100 g) using a Clevenger-type apparatus. The oil was then dried with anhydrous sodium sulfate and stored in darkness at −20 °C until used.

2.5.8. Fatty Acid Methyl Ester (FAMEs) Preparation and Fatty-Acid Profile by GC

Before GC analysis, black cumin samples were converted to FAMEs. The evaluation of FAMEs was carried out in accordance with [36]. Each sample was dissolved in hexane (1.5 mL) and boron-trifluoride in methanol (8%, w/vol) for 1 h at 100 °C. FAME samples were extracted in hexane under nitrogen after cooling, and their concentrations were determined using a Shimadzu gas chromatograph equipped with a flame ionization detector (FID) and a fused-silica capillary column (25 m × 0.25 mm × 50 µm; BPX70 SGE Australia PtyLtd.). The following temperature regime was set: 2 min initial period at 70 °C, then increasing at 4 °C·min⁻¹ for 28 min to reach a second step at 180 °C, then flowing out at 3 °C·min⁻¹ to the final period (220 °C, 45 min). The results are presented as a percentage of the total number of identified FA.

2.6. Statistical Analysis

The data were statistically verified by analysis of variance (ANOVA) for a split-plot arrangement in RCBD design, after testing for homogeneity of error variances according to the procedure outlined by [37] using Info Stat software estadistico (2016). Significant differences between means were compared at (p ≤ 0.05) following Duncan’s multiple range test (DMRT).

3. Results

3.1. Soil Physicochemical Properties

Table 3. Effect of compost applications on soil physicochemical characteristics.

Soil Physical Properties (Average of Two Seasons) $
Compost Application	ρb	Ksat	T.P	Q.D.P >30 µ	S.D.P 30–9 µ	W.H.P 9.02 µ	θFc	A.W. (%)
C0	1.23 a	1.11 c	53.58 c	8.66 c	9.34 c	20.53 c	42.68 c	20.53 c
C15	1.19 b	1.35 b	55.09 b	9.51 b	10.84 b	22.73 b	44.87 b	22.73 b
C30	1.14 c	1.65 a	56.98 a	11.13 a	12.64 a	23.88 a	46.21 a	23.88 a
Soil Chemical Properties (Average of Two Seasons)
Compost Application	pH	ECe (dS·m−1)	CEC (cmol·kg−1)	O.M (%)	N (%)	P (mg·kg−1)	K (mg·kg−1)
C0	7.71 a	2.55 c	34.18 c	1.42 c	0.06 c	7.22 c	48.65 c
C15	7.65 b	2.87 b	36.98 b	1.66 b	0.08 b	9.18 b	86.37 b
C30	7.64 b	3.12 a	39.64 a	1.74 a	0.09 a	11.34 a	102.17 a

^$ The values shown in the table are means; those followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test (DMRT). C₀, zero addition (control); C₁₅, 15 t·ha⁻¹; C₃₀, 30 t·ha⁻¹. T.P: total drainable pores, Q.D.P: quickly drainable pores, S.D.P: slowly drainable pores, W.H.P: water-holding pores, O.M: organic matter content.

demonstrates the impact of compost as an organic amendment on the soil physicochemical characteristics. Compost application improved soil bulk density, porosity, water permeability, and water-holding capacity. Increasing the level of applied compost by C₁₅ and C₃₀ led to a significant decrease in soil dry bulk density by 3.25% and 7.32%, respectively, compared to the control (C₀). Significant increases in total porosity by 2.82% and 6.34% were associated with compost application rates C₁₅ and C₃₀ relative to the control. Due to enhanced soil porosity, soil hydraulic conductivity was improved and recorded considerable increases by 21.62% and 48.65%, respectively. Quickly and slowly drainable and water-holding pores were increased by 28.52%, 35.33%, and 16.32% in response to compost addition with C₃₀ compared to the control. The application of compost reduced soil pH; meanwhile, the soil cation exchangeable capacity and soil nutrient content were significantly increased. The greatest values of CEC (39.64), O.M (1.74), N (0.09), P (11.34), and K (102.17) were recorded under high compost application rate (C₃₀).

3.2. Growth Attributes

The applied deficit irrigation regimes and compost application treatments significantly affected the growth aspects of black cumin plants (

Table 4. Effect of compost application and deficit irrigation regimes on growth attributes of black cumin over two seasons.

Treatments		Plant Height (cm)	Stem Diameter (cm)	Fresh Weight (g)	Dry Weight (g)
Irrigation Levels		**	**	**	**
I100		55.87 a	0.90 a	72.05 a	33.26 a
I75		47.48 b	0.66 b	50.85 b	23.44 b
I50		38.03 c	0.51 c	34.88 c	16.24 c
Compost (t·ha1)		**	**	**	**
C0		43.09 c	0.60 c	44.07 c	20.46 c
C15		47.20 b	0.67 b	53.15 b	24.63 b
C30		51.11 a	0.80 a	60.58 a	27.85 a
Interaction: I × C		**	**	**	**
I100	C0	52.67 c	0.73 c	60.26 c	27.77 c
	C15	54.76 b	0.85 b	73.22 b	33.90 b
	C30	60.19 a	1.11 a	82.67 a	38.10 a
I75	C0	43.79 e	0.59 e	43.02 e	20.11 f
	C15	46.50 d	0.66 d	50.37 d	23.32 e
	C30	52.17 c	0.72 c	59.18 c	26.90 d
I50	C0	32.81 g	0.46 g	28.91 h	13.51 i
	C15	40.33 f	0.52 f	35.85 g	16.67 h
	C30	40.95 f	0.56 ef	39.88 f	18.55 g

The values shown in the table are means; those followed by different letters are significantly different (p ≤ 0.05) according to Duncan’s multiple range test (DMRT). ** refer to significant differences at p ≤ 0.01; I₁₀₀, I₇₅, and I₅₀ are irrigation levels at 100%, 75%, and 50% of crop evapotranspiration (ETc), respectively; C₀, C₁₅, and C₃₀ are compost application levels at 0, 15, and 30 t·ha⁻¹, respectively.

). Plant height, stem diameter, and fresh and dry weight per plant were decreased by 31.93%, 43.33%, 51.59%, and 51.17%, respectively, at I₅₀ as the average of two seasons, compared with full irrigation I₁₀₀. However, enriching the soil with compost amendment positively enhanced these parameters. Under compost application (C₃₀), the abovementioned parameters increased by 18.61%, 33.33%, 37.46%, and 36.12%, respectively, compared to the control (C₀). The highest values of plant height (60.19 cm), stem diameter (1.11 cm), fresh weight per plant (82.66 g), and dry weight per plant (38.10 g) were investigated for fully irrigated (I₁₀₀) and compost-amended (C₃₀) plants. On the contrary, the nonfertilized plants by compost subjected to severe deficit irrigation conditions resulted in the lowest estimations for growth parameters.

3.3. Relative Water Content (RWC) and Membrane Stability Index (MSI)

Leaf integrity parameters (RWC and MSI) represented in Figure 1a,b were significantly affected in correspondence to the water and compost treatments. The maximum values of RWC (83.34%) and MSI (73.84%) were observed for non-drought-stressed and high-rate compost-amended plants. The reductions in RWC and MSI were 12.75% and 28.35%, induced by increasing water stress severity from I₁₀₀ to I₅₀. On the other hand, compost application positively led to a significant improvement in leaf integrity. Under compost application (C₃₀), the RWC and MSI were increased by 6.20% and 12.74%, respectively, compared to the control (C₀).

3.4. Photosynthetic Pigments

Water stress negatively impacts the photosynthetic pigments of the leaf. Under drought conditions, photosynthetic pigment (chlorophyll a, b, and carotenoids) decreased, and a higher reduction was recorded at irrigation with I₅₀ (

Table 5. Effect of compost application and deficit irrigation regimes on photosynthetic pigments of black cumin over two seasons.

Treatments		Ch a (mg·g−1)	Ch b (mg·g−1)	Carotenoids (mg·g−1)
Irrigation Levels		**	**	**
I100		1.45 a	0.75 a	0.70 a
I75		1.31 b	0.66 b	0.61 b
I50		1.21 c	0.54 c	0.48 c
Compost (t ha1)		**	**	**
C0		1.29 c	0.62 c	0.57 c
C15		1.32 b	0.64 b	0.59 b
C30		1.36 a	0.68 a	0.63 a
Interaction: I × C		**	**	**
I100	C0	1.41 b	0.72 b	0.67 b
	C15	1.45 b	0.73 b	0.68 b
	C30	1.50 a	0.79 a	0.74 a
I75	C0	1.27 d	0.63 d	0.58 d
	C15	1.31 c	0.66 cd	0.59 d
	C30	1.35 c	0.67 c	0.64 c
I50	C0	1.18 f	0.51 g	0.45 g
	C15	1.21 ef	0.54 f	0.49 f
	C30	1.23 de	0.57 e	0.51 e

The values shown in the table are means; those followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test (DMRT). ** refer to significant differences at p ≤ 0.01; I₁₀₀, I₇₅, and I₅₀ are irrigation levels at 100%, 75%, and 50% of crop evapotranspiration (ETc), respectively; C₀, C₁₅, and C₃₀ are compost application levels at 0, 15, and 30 t·ha⁻¹, respectively.

). The reduction in Chlorophyll a, b, and carotenoids induced under deficit irrigation I₅₀ were 16.55, 28.00, and 31.43%, respectively, compared with sufficient irrigated plants at I₁₀₀. On the contrary, photosynthetic pigments were increased with increasing the compost addition. Compost application (C₃₀) significantly increased chlorophyll a, b, and carotenoids by 5.43, 9.68 and 10.53% respectively, over control (C₀). The maximum values of chlorophyll a and b, and carotenoids amounted 1.50, 0.79, and 0.74 (mg/g) were recorded for fully irrigated plots and received the high level of compost application C₃₀; meanwhile, the lowest estimation for these parameters was shown with limited water supply at I₅₀ in non-amended plots.

3.5. Yield Components, Seed Yield, and Oil Yield

Yield components (capsule weight in g·plant⁻¹, capsule weight in g, seed weight in g·capsule⁻¹, and seed yield in kg·ha⁻¹) indicated significant effects at (p ≤ 0.05) due to applied irrigation and compost treatments (

Table 6. Effect of compost application and deficit irrigation regimes on yield components, seed yield, and oil yield of black cumin over two seasons.

Treatments		Capsules Weight (g plant−1)	Capsule Weight (g)	Seeds Weight (g·capsule−1)	Seed Yield (kg·ha−1)	Fixed Oil (kg·ha−1)	Essential Oil (kg·ha−1)
Irrigation Levels		**	**	**	**	**	**
I100		18.49 a	0.38 a	0.23 a	749.00 a	219.45 a	1.60 a
I75		12.80 b	0.30 b	0.18 b	502.22 b	129.54 b	0.85 b
I50		7.58 c	0.22 c	0.13 c	296.52 c	68.74 c	0.39 c
Compost (t ha1)		**	**	**	**	**	**
C0		10.91 c	0.27 c	0.17 c	439.08 c	113.39 c	0.74 c
C15		12.90 b	0.30 b	0.18 b	517.13 b	138.17 b	0.94 b
C30		15.04 a	0.33 a	0.20 a	591.53 a	166.17 a	1.17 a
Interaction: I × C		**	**	**	**	**	**
I100	C0	16.18 c	0.36 c	0.21 c	651.82 c	179.34 c	1.28 c
	C15	18.00 b	0.38 b	0.23 b	731.22 b	213.77 b	1.54 b
	C30	21.29 a	0.40 a	0.24 a	863.97 a	265.24 a	1.99 a
I75	C0	10.22 f	0.26 f	0.15 f	410.04 f	104.36 f	0.64 e
	C15	13.18 e	0.31 e	0.18 e	521.44 e	131.87 e	0.89 d
	C30	15.01 d	0.34 d	0.20 d	575.19 d	152.40 d	1.04 d
I50	C0	6.33 i	0.19 i	0.12 h	255.40 i	56.47 i	0.30 g
	C15	7.53 h	0.22 h	0.13 g	298.73 h	68.88 h	0.39 fg
	C30	8.88 g	0.24 g	0.14 g	335.43 g	80.87 g	0.47 f

The values shown in the table are means; those followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test (DMRT). ** refer to significant differences at p ≤ 0.01; I₁₀₀, I₇₅, and I₅₀ are irrigation levels at 100%, 75%, and 50% of crop evapotranspiration (ETc), respectively; C₀, C₁₅, and C₃₀ are compost application levels at 0, 15, and 30 t·ha⁻¹, respectively.

). The higher values of yield characteristics were produced under full irrigation (I₁₀₀) and compost (C₃₀). Water stress significantly reduced the capsule weight in g·plant⁻¹, capsule weight in g, seed weight in g·capsule⁻¹, and seed yield in kg·ha⁻¹ by 59.00%, 42.11%, 43.48%, and 60.41%, respectively as water stress increased from I₁₀₀ to I₅₀. However, compost application enhanced plant productivity and alleviated the adverse effects induced by drought stress. Applying compost (C₃₀) increased the capsule weight (g·plant⁻¹), capsule weight (g), seed weight (g·capsule⁻¹), and seed yield (kg·ha⁻¹) by 37.86%, 22.22%, 17.65%, and 34.72%, respectively, compared to the control (C₀).

Furthermore, fixed oil (kg·ha⁻¹) and essential oil content (kg·ha⁻¹) decreased as the severity of drought stress increased (Table 6). Under irrigation (I₅₀), the reductions in fixed oil (kg·ha⁻¹) and essential oil content (kg·ha⁻¹) were 68.68% and 75.63%, respectively, lower than optimally irrigated plants. The positive effect of compost on these yield traits was significant. Plants that received compost (C₃₀) recorded the highest values of seed yield (kg·ha⁻¹), fixed oil content (kg·ha⁻¹), and essential oil content (kg·ha⁻¹) with increases of 46.55% and 58.11% compared to the control (C₀). The best yield indices were recorded under non-deficit irrigation treatments with compost application (C₃₀). However, the lowest yield estimations were observed in the I₅₀ × C₀ treatment.

3.6. NPK Content

As shown in

Table 7. Effect of compost application and deficit irrigation regimes on NPK (mg·g⁻¹ DW) content of black cumin over two seasons.

Treatments		N (mg·g−1 DW)	P (mg·g−1 DW)	K (mg·g−1 DW)
Irrigation Levels		**	**	**
I100		1.53 a	0.42 a	1.60 a
I75		1.39 b	0.34 b	1.37 b
I50		1.26 c	0.27 c	1.18 c
Compost (t·ha−1)		**	**	**
C0		1.35 c	0.31 c	1.32 c
C15		1.39 b	0.35 b	1.39 b
C30		1.44 a	0.38 a	1.44 a
Interaction: I × C		**	**	**
I100	C0	1.50 c	0.38 bc	1.53 c
	C15	1.53 b	0.41 b	1.60 b
	C30	1.58 a	0.47 a	1.66 a
I75	C0	1.34 f	0.31 de	1.31 f
	C15	1.37 e	0.35 cd	1.37 e
	C30	1.44 d	0.36 c	1.43 d
I50	C0	1.21 i	0.25 f	1.12 i
	C15	1.27 h	0.28 ef	1.19 h
	C30	1.31 g	0.30 e	1.23 g

The values shown in the table are means; those followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test (DMRT). ** refer to significant differences at p ≤ 0.01; I₁₀₀, I₇₅, and I₅₀ are irrigation levels at 100%, 75%, and 50% of crop evapotranspiration (ETc), respectively; C₀, C₁₅, and C₃₀ are compost application levels at 0, 15, and 30 t·ha⁻¹, respectively.

, the leaf NPK content was adversely affected by applied water stress levels; meanwhile, it was positively enhanced by compost application compared to the control in both growing seasons. In response to applied irrigation treatments, N, P, and K contents were recorded for the following treatments in descending order: I₁₀₀, I₇₅, and I₅₀. Compared with non-amended cumin plants by compost, the NPK content increased by 2.96%, 12.90%, and 5.30% for the moderate compost application (C₁₅) and by 6.67%, 22.58%, and 9.09% for high compost application (C₃₀), respectively. The interaction effect of applied water deficit levels and compost fertilizer application rates on nutrient acquisition was significant. As shown in Table 7, the highest NPK content in leaves of Nigella sativa plants was determined in response to the I₁₀₀ × C₃₀ combined treatment. On the other hand, reducing the applied irrigation water to the level of I₅₀ for non-compost amended units resulted in the lowest contents of theses macro-nutrients.

3.7. Total Free Amino Acids (TFAA), Total Soluble Sugar (TSS), Carbohydrate Percentage, and Proline

Figure 2a–d illustrate the influence of drought stress and compost treatments on compatible solutes (i.e., total free amino acids (TFAA), total soluble sugar (TSS), carbohydrate percentage, and proline) in leaves of black cumin plants. The highest contents of TFAA (135.5 mg·g⁻¹), TSS (377.66 mg·g⁻¹), carbohydrates (2.76%), and proline (0.73 µmol·g⁻¹) resulted from plots treated with compost (C₃₀) under a drought stress regime (I₅₀). However, the lowest content of these osmoprotectants were recorded for non-drought-stressed treatments (I₁₀₀) without compost addition. Under drought stress conditions, TFAA, TSS, carbohydrates, and proline significantly increased by 21.04%, 14.49%, 10.92%, and 47.48%, respectively. However, compost application showed positive effects on these traits.

3.8. Fixed Oil and Essential Oil Percentages

The fixed and essential oil (%) in seeds of Nigella sativa plants showed a significant response due to irrigation and compost application in two growing seasons (Figure 3a,b). Water stress treatment (I₅₀) caused a reduction in fixed and essential oil by 16.64% and 39.57%, respectively, compared with fully irrigated plants (I₁₀₀). Moreover, compost application resulted in a significant increase in fixed and essential oil. Compared with non-compost-treated plants (C₀), compost application (C₃₀) increased fixed and essential oil by 10.91% and 12.55%, respectively. Furthermore, the interaction effect of irrigation and compost application (I₁₀₀ × C₃₀) improved seed oil status. On the other hand, fixed and essential oil (%) declined when plants were subjected to non-compost-amended (C₀) and severe water deficit regimes (I₅₀).

3.9. Fatty-Acid Composition of Fixed Oil

As shown in Figure 4a–c and

Table 8. Effect of compost application and deficit irrigation regimes on the fatty-acid profile of black cumin over two seasons.

Treatments		SFA				USFA
						M-USFA			P-USFA
		Myristic	Palmitic	Stearic	Arachidic	Palmitoleic	Oleic	Eicosenoic	Linoleic	Linolenic	Eicosadienoic
		(14:0)	(16:0)	(18:0)	(20:0)	(16:1)	(18:1)	(20:1)	(18:2)	(18:3)	(20:2)
Irrigation Levels		**	**	**	**	**	**	**	**	**	**
I100		0.17 c	12.29 c	3.64 a	0.16 b	0.18 a	21.82 a	0.38 a	48.47 a	0.57 a	3.25 a
I75		0.19 b	12.74 b	3.27 b	0.17 a	0.16 b	20.24 b	0.35 b	45.30 b	0.51 b	2.77 b
I50		0.21 a	12.93 a	2.83 c	0.17 a	0.16 b	19.27 c	0.32 c	41.66 c	0.47 c	2.51 c
Compost (t ha1)		**	**	**	**	**	**	**	**	**	**
C0		0.18 c	12.63 b	3.10 c	0.16 b	0.16 b	19.70 c	0.34 c	46.63 a	0.51 b	2.58 c
C15		0.19 b	12.56 b	3.21 b	0.17 a	0.17 a	20.48 b	0.35 b	45.17 b	0.51 b	2.66 b
C30		0.20 a	12.76 a	3.43 a	0.17 a	0.17 a	21.16 a	0.37 a	43.62 c	0.54 a	3.08 a
Interaction: I × C		**	**	**	**	**	**	**	**	**	**
I100	C0	0.16 f	12.24 d	3.50 b	0.16 c	0.17 b	20.35 c	0.37 b	49.34 a	0.56 c	2.67 c
	C15	0.17 e	12.27 d	3.54 b	0.16 c	0.18 a	21.91 b	0.37 b	48.73 a	0.57 b	2. 78 c
	C30	0.17 e	12.35 cd	3.88 a	0.16 c	0.18 a	23.21 a	0.41 a	47.33 b	0.58 a	3.82 a
I75	C0	0.18 d	12.78 c	3.12 e	0.16 c	0.16 c	20.22 c	0.33 c	47.32 b	0.52 e	2.65 d
	C15	0.18 d	12.56 b	3.25 d	0.17 b	0.16 c	20.23 c	0.36 c	45.34 c	0.49 g	2.78 c
	C30	0.20 c	12.87 b	3.44 c	0.17 b	0.17 b	20.27 c	0.37 b	43.24 d	0.53 d	2.87 b
I50	C0	0.21 b	12.87 b	2.69 h	0.17 b	0.16 c	18.53 e	0.32 e	43.24 d	0.44 i	2.43 f
	C15	0.21 b	12.86 b	2.84 g	0.17 b	0.16 c	19.30 d	0.32 e	41.45 e	0.47 h	2.54 e
	C30	0.22 a	13.07 a	2.97 f	0.18 a	0.16 c	19.99 c	0.33 c	40.30 f	0.51 f	2.55 e

The values shown in the table are means; those followed by different letters are significantly different (p < 0.05) according to Duncan’s multiple range test (DMRT). ** refer to significant differences at p ≤ 0.01; I₁₀₀, I₇₅, and I₅₀ are irrigation levels at 100%, 75%, and 50% of crop evapotranspiration (ETc), respectively; C₀, C₁₅, and C₃₀ are compost application levels at 0, 15, and 30 t·ha⁻¹, respectively.

, fatty acids were represented in three groups: saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), and poly-unsaturated fatty acids (PUFAs). SFAs included myristic, palmitic, stearic, and arachidic acids. Among SFAs, the content of palmitic acid was the highest. The SFAs (%) significantly increased by 5.63% in correspondence with drought stress, except for stearic acid, which decreased by 22.25%. Meanwhile, the contents of MUFAs (palmitoleic, oleic, and eicosenoic acids) and PUFAs (linoleic, linolenic, and eicosadienoic acids) were adversely affected by deficit irrigation and recorded their lowest values under I₅₀.

The reduction in MUFAs and PUFAs was 17.7% and 14.6% under drought levels (I₅₀) compared with fully irrigated plots (I₁₀₀). Compost application at rates C₁₅ and C₃₀ resulted in significant increases in SFAs, MUFAs, and PUFAs compared with the control (C₀). Except for all determined fatty acids, linoleic acid exhibited a reduction of 6.46% in response to compost application (C₃₀) relative to control. The highest values of myristic (0.22), palmitic (13.07), and arachidic (18) acids were recorded for a combined effect of I₅₀ × C₃₀. However, the integration of I₁₀₀ × C₃₀ gave greater stearic, palmitoleic, oleic, eicosenoic, linolenic, and eicosadienoic acid values by 3.88%, 0.18%, 23.21%, 0.41%, 0.58%, and 3.82%, respectively.

4. Discussion

Compost amendment influences and improves the physical and chemical properties of the soil. It increases soil porosity, permeability, and water-retaining capacity, but reduces soil bulk density (Table 3). Compost application at levels of 15 and 30 t·ha⁻¹ resulted in a notable increase in the soil organic content. Mixing compost as a low-density organic material with the mineral soil component may have been responsible for the recorded reduction in soil bulk density and increased soil porosity. In addition, increasing macro- and mesopore fractions (Table 3) indicate that soil aggregation, soil porosity, and soil hydraulic properties were improved in response to supplied compost. Therefore, the highest Ksat values were associated with high compost application rates and significantly differed from non-amended plots. Available water improved and positively correlated with compost application. This improvement might have resulted from modified pore geometry, particularly in water-holding pores (Table 3). These observations align with those mentioned by [18,38,39,40].

Similarly, soil chemical properties were enhanced due to compost application (Table 3). Soil nutrient content (NPK) showed positive alterations due to compost application. This increase in soil nutrient content could be attributed to the role of compost amendment as a beneficial source of numerous macro- and micronutrients necessary for plant growth. Soil cation exchangeable capacity (CEC) is one of the most significant indices for assessing soil fertility. Compost application increases soil CEC by enriching the soil with organic matter rich in functional groups. These findings agree with [17,41,42,43,44,45].

Water stress and compost application significantly affected the morphophysiological properties, yield, and oil content of black cumin plants. Deficit irrigation treatments at levels I₇₅ and I₅₀ inhibited plant growth traits (i.e., plant height, stem diameter, and fresh and dry weight per plant) (Table 4), photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoids) (Table 5), leaf integrity traits (i.e., RWC% and MSI) (Figure 1a,b), yield components (i.e., capsule weight per plant, capsule weight, seed weight per capsule, and seed yield in kg·ha⁻¹) (Table 6), leaf NPK content (Table 7), fixed oil content and essential oil percentage in seeds (Table 6 and Figure 3a,b), and fatty-acid content (Table 8 and Figure 4a–c) relative to the control (full irrigation, I₁₀₀). On the contrary, compatible solutes (i.e., total free amino acids, total soluble sugar, carbohydrate percentage, and proline) (Figure 2a–d) in leaves of black cumin plants were significantly increased upon increasing the severity level of drought stress. Increasing the content of osmoprotective substances in plant tissues is a protective mechanism that plants create to encounter detrimental environmental conditions [46,47,48,49,50]. Therefore, accumulating these osmoprotectants could help black cumin plants survive under adverse conditions (drought stress) but does not guarantee appropriate yield. Hence, in the current experiment, compost amendment was incorporated with soil to be used as a helpful approach to encounter the detrimental effects of drought stress on black cumin plants. Compost maintains soil quality by diminishing losses of moisture and nutrient from the root zone, providing the soil with sufficient nutrient content and improving the soil’s water-retaining capacity [18]. Consequently, compost-treated experimental plots showed considerable improvements in plant growth, physiological responses, seed yield, and oil yield for both deficit and sufficiently irrigated cumin plants.

The growth of Nigella sativa plants was reduced under deficit irrigation treatment (I₇₅); however, the lowest growth attributes were recorded for drought levels (I₅₀) (Table 4). The restricted growth under irrigation levels (I₇₅ and I₅₀) is illustrated in Figure 1a,b; this may have been a consequence of inhibited leaf relative turgidity [51,52]. In addition, water stress limits nutrient uptake by the roots [53,54,55]. Hence, decreases in the NPK content (Table 7) and water in plant tissues (Figure 1a,b) may disrupt the accumulation of photosynthetic pigment [56] (Table 5) and restrict the division and enlargement of plant cells [6,57,58]. Drought increases ROS production, causing damage to cell structures, DNA, nucleic acids, and proteins, damaging the peroxidation of the membrane’s lipids, and leading to cellular death [9,59,60]. Similar results have demonstrated the detrimental impacts of drought on the growth of these crops, as recorded in [61] on cumin, [62] on safflower, and [63] on Nigella sativa.

Compost application enhances soil characteristics (Table 3), improves root growth [17], and increases the extraction of nutrients and water, leading to better growth than nontreated ones. Furthermore, compost also generated the production of compatible solutes (TFAA, TSS, carbohydrate percentage, and proline; Figure 2a–d), which enabled the stressed Nigella sativa plants to maintain turgor pressure and higher growth. These findings parallel those previously reported by [63,64,65].

Drought stress reduced the seed yield of cumin plants (Table 6 and Table 7), which may have been a result of growth inhibition (Table 4), reduced turgor pressure (Figure 1a,b), a disruption in photosynthesis (Table 5), or decline in nutrition absorption (Table 8); alternatively, it may have been related to a disorder in metabolic activities and accumulation of ROS [6,54,55,60,66,67]. Furthermore, the increase in seed yield could be attributed to enhanced vegetative growth (Table 4) and yield traits (Table 6) resulting from favorable nutrient and water uptake conditions associated with compost addition. Compost fertilizers positively modulate soil physical, chemical, and biological properties, leading to better growth and yield [18,68]. The release of macro- and micronutrients increases with compost, resulting in a better crop yield for grown plants. Furthermore, chlorophyll biosynthesis and CO₂ assimilation were observed to be increased due to compost application (Table 5). These findings are closely linked to those in [67,69,70]. The decrease in photosynthetic pigment observed under insufficient water supply (I₇₅ and I₅₀) may have significantly reduced oil content. The obtained results agreed with [60,71,72].

Compost application showed a favorable effect on both essential and fixed oil content in cumin seeds. This could be attributed to compost’s stimulating effect on metabolism reactions and activation of the enzyme reactions for oil biosynthesis [65,67]. Since plant nutrition status is a crucial factor influencing the assimilation of secondary metabolites in plants, adding compost with sufficient rates may stimulate all vegetative growth and accelerate oil accumulation in Nigella sativa plants. Previously, many researchers indicated that the use of organic fertilizers enhanced the oil percentage in medicinal plants [67,73]. Earlier studies also concluded that organic manure application positively affected seed yield and essential oil in Dysphania ambrosioides L. [74], Carum copticum L., and Trigonella foenum-graecum L. [75].

Saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), and polyunsaturated fatty acids (PUFAs) were significantly affected by water stress treatment. This led to a substantial increase in SFA content (i.e., palmitic, myristic, and arachidic)s except stearic acid. Considering prior findings in soybean [76], the increase in palmitic acid was combined with a decrease in stearic acid under deficit irrigation. Meanwhile, water stress severity coincided with a considerable reduction in the concentration of MUFAs (palmitic, oleic, and eicosenoic acids) and PUFAs (linoleic, linolenic, and eicosadienoic acids). Among estimated fatty acids, linolenic acid was the primary fatty acid recorded (40.30–50.34%). Our results agree with previous results showing a reduction in unsaturated fatty acid content in response to water stress [60,62]. The decrease in unsaturated fatty acids induced by drought stress may have been due to stimulation of the lipase enzyme (as an adaptation mechanism) under limited water supply [71].

Furthermore, plants grown under water stress activate desaturase enzymes which are closely linked with PUFA synthesis and have stability under biotic and abiotic stress [60,73,77]. The increase in the content of saturated fatty acids under water stress conditions results from the myristic acid, palmitic acid, and arachidic acid content (Table 8). However, the decrease in stearic acid content under water stress might be due to desaturase activity and related gene expression in the synthesis pathway of stearic acid. A similar trend was recorded for canola under water stress [78]. Compost application improved and increased fatty-acid content for stressed and non-stressed Nigella plants. Compost addition combined with an increase in PUFAs and MUFAs and a decrease of SFAs (Table 8). These results parallel those reported by [79] in soybean seed and [80] in safflower seed. They identified reduced SFAs (palmitic acid and stearic acid) for compost-amended plots, along with the previously proven role of compost in increasing growth, seed yield, and chemical constitution of oil [61,64,65,67,79,80,81].

5. Conclusions

The present study concluded that water stress regimes (I₇₅ and I₅₀ of ETc) inhibited growth, a physiological response and seed yield of Nigella sativa plants. The most significant values of fixed and essential oil yield were observed with fully irrigated plants (I₁₀₀). Saturated fatty acids (SFAs) increased, whereas unsaturated fatty acids (UFAs) decreased at high water stress levels. Due to its ameliorative effect on the soil, compost improved plant uptake of water and essential nutrients, resulting in better growth and yield for stressed and fully irrigated Nigella sativa plants. The maximum seed yield (863.96 kg·ha⁻¹), fixed oil yield (256.33 kg·ha⁻¹), and essential oil yield (1.99 kg·ha⁻¹) were recorded for experimental plots treated with I₁₀₀ × C₃₀. Furthermore, compost amendment contributed to a significant increase in the levels of both SFAs and UFAs.

Therefore, adding compost amendment could save irrigation water and relatively enhance the productivity of moderate water-stressed Nigella sativa plants without a considerable reduction compared with fully irrigated plants, ensuring the sustainability of water use in areas of limited water resources.