Comparative Evaluation of Marine Algae-Based Biostimulants for Enhancing Growth, Physiological Performance, and Essential Oil Yield in Lavender (Lavandula angustifolia) Under Greenhouse Conditions

Abstract

The application of marine algae-derived biostimulants offers a sustainable approach to improving plant performance in aromatic and medicinal crops. This study investigated the effects of four macroalgal extracts and two commercial biostimulant products on the growth, physiology, and essential oil production of Lavandula angustifolia cultivated under greenhouse conditions at CREA, Pescia (Italy). Treatments included extracts from Ascophyllum nodosum (France and Greenland), Laminaria digitata (Iceland), Sargassum muticum (Italy), two commercial formulations (a seaweed-based and an amino acid-based biostimulant), and a control receiving only standard fertilization. Over a 10-week period, plants were evaluated for multiple parameters: plant height, leaf number and area, SPAD index (chlorophyll content), above- and below-ground biomass, flower production, microbial activity in the growth substrate, and essential oil yield. Algae extracts, particularly those from A. nodosum (Greenland) and S. muticum (Venice), significantly enhanced most parameters compared to the control and commercial products. These treatments yielded higher biomass, greater chlorophyll retention, increased flower number, and improved essential oil content. Rhizosphere microbial counts were also elevated, indicating a positive interaction between algae treatments and substrate biology. The study highlights the multifunctional nature of marine algae, whose complex composition of bioactive compounds appears to promote plant growth and secondary metabolism through multiple pathways. The superior performance of cold- and temperate-climate algae suggests a relationship between environmental origin and biostimulant efficacy. Compared to commercial inputs, the tested algae extracts showed broader and more consistent effects. These findings support the integration of macroalgae-based biostimulants into sustainable lavender cultivation strategies. Further research is recommended to optimize formulations, validate field performance, and explore synergistic effects with beneficial microbes or organic inputs.

1. Introduction

In the face of increasing global pressure for sustainable agricultural practices, the cultivation of medicinal and aromatic plants has emerged as a priority area for innovation [1]. Among these species, Lavandula angustifolia—commonly referred to as lavender—has gained prominence due to its diverse uses in cosmetics, perfumery, and natural therapies [2]. Its essential oils are of high economic and pharmaceutical value, yet their yield and quality can be significantly impacted by environmental conditions [3]. Climate change-induced stressors such as drought, salinity, and soil degradation pose growing threats to the reliable cultivation of lavender, especially under greenhouse or pot culture systems [4,5].

To address these challenges, researchers and growers are increasingly turning to biostimulants: substances and microorganisms that enhance plant growth and resilience without acting as direct sources of nutrients [6]. Biostimulants improve metabolism, nutrient use efficiency, and abiotic stress tolerance through natural plant responses [7]. Among the various biostimulant categories, marine macroalgae have emerged as promising candidates due to their rich profile of bioactive compounds [8,9,10]. These compounds include plant hormones, amino acids, vitamins, polysaccharides, and antioxidants [11], all of which contribute to enhanced physiological and biochemical plant responses [12,13].

The appeal of macroalgae as biostimulants lies in their ecological and biochemical complexity [14]. In their native aquatic environments, macroalgae are subjected to intense and fluctuating abiotic stresses such as UV radiation, osmotic changes, and desiccation [15]. These stressors have led to the evolution of protective molecules—many of which, when applied to terrestrial plants, may induce stress-tolerance responses [16]. Consequently, seaweed extracts are considered multifunctional: promoting growth, enhancing resistance to stress, and improving soil–plant interactions [17,18].

While the biostimulant potential of macroalgae has been extensively explored in vegetable and cereal crops, less attention has been paid to their application in aromatic species [19]. In particular, the use of macroalgae to improve secondary metabolites such as essential oils in lavender remains under-researched [20]. In lavender, oil composition and concentration are closely linked to the plant’s physiological status [21]. Stress conditions can reduce oil yield or alter its chemical profile by disrupting the metabolic pathways involved in terpenoid biosynthesis [22,23]. Thus, improving stress tolerance through natural biostimulants may help preserve both yield and oil quality [24].

In addition to promoting vegetative growth, macroalgae-based biostimulants can influence rhizospheric microbial communities [25]. Several studies have shown that seaweed extracts can stimulate beneficial soil microbes, increase enzymatic activity, and enhance nutrient cycling [26,27]. In greenhouse and pot culture systems, where microbial diversity is often limited due to controlled conditions, such microbiota stimulation could offer significant agronomic advantages [28].

In lavender production, biostimulants can influence multiple plant traits critical to commercial success. These include plant height, number and size of leaves, root biomass, chlorophyll content (a proxy for photosynthetic efficiency), number of flowering spikes, and essential oil yield [29,30,31]. Improvements in any of these traits may translate directly into higher productivity and better quality of the final product [32].

Despite the growing interest in seaweed biostimulants, only a few studies have conducted comprehensive, multi-parameter evaluations in lavender cultivation [33]. Many existing investigations focus on single growth outcomes or limited physiological responses [34]. Similarly, the impact of biostimulants on microbial activity in substrates used for lavender has received minimal attention [35]. This represents a critical knowledge gap, particularly as sustainable agriculture increasingly emphasizes the role of soil biology in plant health [36].

To fully understand the potential of macroalgal biostimulants in lavender, controlled studies are required that assess their influence on both primary and secondary growth attributes under semi-protected systems like greenhouses. Such environments allow for reproducibility while mimicking real-world production conditions [37]. Evaluating morphological (plant height, biomass, leaves), physiological (chlorophyll content), reproductive (flower number), and biochemical (oil content) parameters provides a holistic view of the plant’s response [38].

Moreover, lavender’s sensitivity to abiotic stressors like salinity and drought makes it an ideal model species to investigate biostimulant efficacy under suboptimal growing conditions [39]. Water deficit, for example, can reduce essential oil production by inhibiting the mevalonate and MEP pathways involved in terpene biosynthesis [36,40]. Salinity, on the other hand, can lead to ionic toxicity and osmotic stress, reducing both vegetative growth and metabolic activity [28]. Biostimulants capable of modulating plant hormonal balance and antioxidant systems may help mitigate these negative effects [13].

Essential oil synthesis is particularly responsive to changes in the plant’s metabolic and hormonal state [9]. Studies in other aromatic plants have shown that biostimulants can upregulate genes related to secondary metabolite biosynthesis and influence oil composition [15]. If similar effects occur in lavender, biostimulant applications could serve not only to stabilize yield under stress but also to enhance oil quality under optimal conditions [38].

There is also increasing interest in how biostimulants affect the root microbiome, especially in closed systems where natural microbial colonization is restricted [26]. Seaweed extracts may serve as prebiotic inputs that selectively stimulate beneficial microbial communities such as Trichoderma, Pseudomonas, and mycorrhizal fungi [32]. Such interactions can further enhance nutrient uptake and resistance to pathogens [35].

This research was developed to explore the multifaceted role of marine-derived biostimulants in lavender cultivation under greenhouse conditions. The study focuses on measuring plant responses in terms of vegetative development, flowering, chlorophyll content, biomass allocation, microbial activity in the substrate, and essential oil content. These parameters were chosen to represent both plant productivity and biochemical quality, with particular attention to responses under controlled environmental conditions [38,39]. Through an integrative approach, the present study aims to contribute new data on the effectiveness of natural biostimulants in high-value aromatic crops. By evaluating multiple growth and quality metrics, it helps define optimal strategies for enhancing lavender performance using environmentally sustainable inputs [23]. The findings may have broader implications for the use of seaweed extracts in other medicinal and aromatic plant species cultivated under similar conditions [26]. As agriculture shifts toward reduced chemical input systems and circular bioeconomy models, the adoption of seaweed-based biostimulants aligns with both ecological and economic objectives [28]. Their use supports improved crop resilience, increased efficiency in resource use, and enhanced product quality, all of which are essential in adapting to the pressures of climate change and environmental regulation [25]. Ultimately, this study positions macroalgal biostimulants as promising tools in the sustainable intensification of aromatic plant production, particularly in greenhouse or pot-based systems where input control and stress exposure are highly manageable [15].

2. Materials and Methods

2.1. Experimental Site and Conditions

The study was conducted between March and July in the experimental greenhouses of the CREA (Research Centre for Plant Nursery and Ornamental Crops) in Pescia, Italy. The greenhouses provided semi-controlled environmental conditions suitable for potted plant trials. Average temperatures ranged from 20 °C (night) to 28 °C (day), with a relative humidity of 60–75%. Natural daylight was supplemented with LED lighting when photosynthetically active radiation (PAR) dropped below 300 µmol m⁻² s⁻¹ to maintain uniform photoperiod and light intensity across treatments. Ventilation and irrigation systems were automated to ensure consistency across all experimental units, except when stress treatments were intentionally applied.

2.2. Plant Material and Pot Setup

Uniform Lavandula angustifolia plants, approximately eight weeks old, were sourced from a local certified nursery. Prior to treatment application, plants were acclimatized in the greenhouse for two weeks. Each plant was transplanted into 3 L plastic pots filled with a sterile peat-based substrate (pH 6.2; EC 0.4 mS/cm) containing 70% peat moss and 30% perlite. Each pot was fertilized at transplant with a basal slow-release organic fertilizer (NPK 6-5-10) to avoid nutrient deficiency during early establishment.

2.3. Experimental Design

The experiment was conducted in a randomized complete block design (RCBD) with 7 treatments and 4 replicates per treatment, totaling 28 experimental units. Each replicate consisted of 10 uniform lavender plants, resulting in 280 potted plants in total. The seven experimental treatments were as follows:

• Treatment A: Ascophyllum nodosum APP053.1 (France, Brittany) (composition: Fucoidan: 350 mg/g dry weight (DW); Alginic acid: 100 mg/g DW; Laminarin: 52 mg/g DW; Mannitol: 61 mg/g DW) (composition: Alginic Acid: 28% of dry weight (DW); Fucoidan: 6% mg/g DW; Laminarin: 12% mg/g DW; Mannitol: 13% mg/g DW; Total carbohydrates: 36% DW; Protein: 8% DW; Ash/Mineral Content: 17% DW; Phlorotannins (polyphenols): 6% DW).
• Treatment B: Laminaria digitata APP054.1 (Iceland) (composition: Alginic acid mg/g 26%; Laminarin mg/g 12%; Fucoidan mg/g 7%; Mannitol mg/g 11%; Total carbohydrates 48% (incl. polysaccharides); Polyphenols (phlorotannins) 6%; Proteins (total nitrogen) 7%; Ash (mineral content) 23%; Pigments (fucoxanthin) <1%; Vitamins (A, C, E, K) trace amounts).
• Treatment C: Ascophyllum nodosum APP055.1 (Greenland) (composition: Alginic acid 32%; Laminarin 11%; Fucoidan 6%; Mannitol 12%; Fucoxanthin 660 mg/Kg DW).
• Treatment D: Sargassum muticum APP031.10 (Italy, Venice) (composition: Proteins 8.3% mg/g DW; Total sugars 28 mg/g DW; Crude fiber (cell wall polysaccharides, mainly alginate): 32% mg/g DW; Macro-minerals (Na + K + Ca + Mg): 135 mg/g DW; Micro-elements (Se, Fe, Zn, Mn, Cu, Ni, Cr): 2 mg/g DW).
• Treatment E (Commercial Control 1): Seaweed-based commercial biostimulant (Ecklonia maxima-derived, Kelpak^®, composition: Cytokinins (natural): 0.03–0.06 mg/L; Auxins (natural): 11–14 mg/L; Organic matter: >60%; Carbohydrates: Including alginates and laminarins; Minerals: Potassium, calcium, magnesium, and trace elements).
• Treatment F (Commercial Control 2): Amino acid-based commercial biostimulant (Trainer^®, Enzymatic hydrolysate of plant proteins, composition: Total nitrogen (N): 6%; Organic nitrogen: 5.5%; Free amino acids: ≥24% w/w; Carbon from biological origin: >20%).
• Treatment G (Negative Control): Untreated control—plants received only standard basal fertilization, with no foliar or soil-applied biostimulants.

Kelpak^® and Trainer^® were used as commercial controls due to the following factors:

• Relevance: Both products are well-established in the European biostimulant market and commonly used in high-value crops including ornamentals, herbs, and medicinal plants.
• Contrast in biostimulant categories:

Kelpak^® represents the seaweed extract category, but based on a different species (Ecklonia maxima) and refined formulation.

Trainer^® represents the amino acid-based biostimulant category, which operates through a different mechanism than marine algae extracts.

3.

Benchmarking: Including these controls ensures that the efficacy of experimental macroalgae treatments (A–D) can be benchmarked against real-world, commercially available alternatives. This allows researchers and growers to assess whether novel algae sources (like Ascophyllum nodosum from Greenland or Sargassum muticum) offer competitive or superior performance.

Each treatment was applied to both unstressed and abiotic stress-exposed subgroups (drought and salinity), as described previously.

2.4. Fertilization Protocol

All plants across treatments, including the control group (Treatment G), received a uniform baseline fertilization consisting of the following:

• A slow-release organic fertilizer (NPK 6-5-10) incorporated at transplant (5 g/pot);
• Supplementary watering with a balanced liquid fertilizer (NPK 20-20-20 at 1 g/L) once every 2 weeks to avoid nutrient deficiencies under greenhouse conditions.

This fertilization strategy ensured that any differences observed in plant growth, physiology, or oil content could be attributed to the biostimulant treatments rather than nutrient availability.

2.5. Biostimulant Preparation and Application

All macroalgae materials (Treatments A–D) were provided in dry powdered form and stored in sealed containers at 4 °C until use. For each application, 50 g of dry algae biomass was extracted with 1 L of distilled water at 70 °C for 3 h. The solution was cooled, filtered through muslin cloth and then diluted to a 10% (v/v) working concentration. Commercial biostimulants (Treatments E and F) were applied at the manufacturer’s recommended concentrations: 2 mL/L for the seaweed product (E) and 3 mL/L for the amino acid product (F). All treatments were applied weekly, beginning 2 weeks post-transplant and continuing for 10 weeks.

Each application consisted of two parts:

• Foliar spray: A quantity of 100 mL of solution was sprayed uniformly over the plant canopy using a handheld mist sprayer, ensuring full leaf coverage.
• Soil drench: An amount of 150 mL of solution was applied directly to the substrate to reach the rhizosphere.

Applications were performed early in the morning to avoid evaporation and photooxidative stress.

2.6. Abiotic Stress Application

To assess stress mitigation effects, each treatment group was divided into three subgroups (n = 80 plants per stress type), subjected to the following conditions:

• Control (non-stressed): Plants were watered regularly (every 2–3 days) to maintain substrate moisture at 75% field capacity.
• Drought stress: Irrigation was withheld for 10 days during the flowering stage (week 8), after which plants were re-watered and monitored.
• Salinity stress: Starting from week 6, plants were irrigated with 100 mM NaCl solution twice a week for 4 weeks, simulating gradual salt buildup.

Stress was confirmed by monitoring substrate electrical conductivity and visible stress symptoms (leaf wilting, chlorosis, stunted growth).

2.7. Measured Parameters

2.7.1. Morphological Growth Parameters

• Plant height (cm) was measured weekly using a digital meter from the base of the stem to the apical meristem.
• Leaf area (cm²) was measured from three representative leaves per plant using a portable digital leaf area meter (Model LI-3000, LI-COR) [41].

2.7.2. Biomass Evaluation

At the end of the experiment, five plants per replicate were destructively sampled to measure biomass:

• Vegetative biomass (g) was recorded after drying the aerial part of the plant at 65 °C for 72 h.
• Root biomass (g) was similarly obtained by washing roots free of soil, drying, and weighing.

2.7.3. Chlorophyll Content

Relative chlorophyll content was estimated weekly using a SPAD-502 chlorophyll meter (Konica Minolta, Tokyo, Japan) [42]. Three readings per plant were taken from the midpoint of fully expanded leaves and averaged per replicate.

2.7.4. Microbial Activity in Substrate

To assess the influence of treatments on substrate microbiology [43], the following steps were carried out:

• Substrate samples (5 g) were collected at week 10 from the root zone of each replicate.
• Samples were diluted (1:10) in sterile water and serially plated on nutrient agar and PDA (potato dextrose agar) for bacterial and fungal counts, respectively.
• Colony-forming units (CFU) were counted after 48 h at 28 °C.

2.7.5. Essential Oil Yield Analysis

Essential oils were extracted by steam distillation using 100 g of dried flowering tops per replicate. The distillation lasted 2 h in a Clevenger-type apparatus [44].

• Oil yield was expressed as a percentage of dry biomass.
• Oils were stored in amber vials at 4 °C for future chemical profiling.

2.7.6. Antioxidant Enzyme Assays

• At the end of the stress period (week 10), fresh leaf tissue (0.5 g) was collected, frozen in liquid nitrogen, and stored at −80 °C. Enzyme extracts were prepared using phosphate buffer (pH 7.0), and the following assays were performed [45]:
• SOD activity: Based on the photoreduction of NBT (nitroblue tetrazolium);
• CAT activity: Measured by the decomposition rate of H₂O₂ at 240 nm;
• POD activity: Quantified via guaiacol oxidation at 470 nm.

2.7.7. Osmoprotectant Quantification

To determine osmotic stress response [46], the following were measured:

• Free proline content: Measured via the Bates method (1973), using sulfosalicylicacid extraction and ninhydrin reaction (absorbance at 520 nm);
• Soluble sugar content: Determined using the phenol-sulfuric acid method (absorbance at 490 nm).

2.8. Statistical Analysis

All data were subjected to analysis of variance (ANOVA) using R software (version 4.3.1). Data were checked for normality (Shapiro–Wilk test) and homogeneity of variances (Levene’s test) before ANOVA. Where significant differences were detected (p < 0.05), treatment means were compared using Tukey’s HSD post hoc test. Separate analyses were conducted for each parameter under control, drought, and salinity conditions.

Multivariate Principal Component Analysis (PCA) was also performed on standardized datasets (Z-scores) to visualize treatment clustering and relationships among growth, physiological, and biochemical variables [47].

3. Results

3.1. Plant Height

The data on plant height indicated that all biostimulant treatments significantly increased vertical growth in comparison to the untreated control (G). The tallest plants were observed in Treatment C (Ascophyllum nodosum from Greenland), with a mean height of 38.7 cm, followed closely by Treatment D (Sargassum muticum, Venice) at 36.8 cm and the commercial seaweed-based product (E) at 34.5 cm. Plants in the control group exhibited the lowest average height of 25.6 cm (Figure 1 and Figure 2).

Treatments A and B showed moderate height gains, with values of 30 cm and 32 cm respectively, while the amino acid-based commercial product (F) recorded a modest improvement at 33.2 cm. Statistical analysis revealed significant differences among treatments, with C and D categorized in the highest statistical group (“a”) and the control in the lowest (“d”). These results underscore the superior efficacy of algae extracts, especially those sourced from extreme environments like Greenland, in promoting elongation growth in lavender plants.

3.2. Leaf Area

The application of biostimulants influenced foliar development as reflected by the leaf area measurements. Plants treated with extract C displayed the highest average leaf area (11.4 cm²), followed by those in Treatment D (10.7 cm²) and commercial Treatment E (9.9 cm²). In contrast, the untreated group produced significantly smaller leaves, averaging only 6.3 cm² (Figure 3).

Treatments A, B, and F showed intermediate effects, ranging from 7.2 to 8.8 cm². Statistical groupings revealed that extract C was significantly superior to most treatments (group “a”), while the control was again grouped as “d”. These results suggest that certain macroalgae biostimulants are capable of stimulating foliar expansion, possibly by enhancing cellular turgor and division under semi-controlled conditions.

3.3. Chlorophyll Content (SPAD Index)

SPAD readings were used to estimate relative chlorophyll concentration. Consistent with the trends in growth parameters, Treatment C achieved the highest chlorophyll content (SPAD 54.2), with D (52.1) and E (50.5) closely following. The control group exhibited the lowest value (36.8), suggesting diminished chlorophyll synthesis or accelerated degradation in untreated plants (Figure 4).

Moderate improvements were seen in Treatments A (40.5), B (43.3), and F (47.8). The statistical grouping placed Treatment C again in the top rank (“a”), while the untreated plants were classified distinctly lower (“d”). Enhanced chlorophyll content is often associated with greater photosynthetic activity and energy production, which aligns with the observed biomass differences.

3.4. Microbial Colony Counts in the Rhizosphere

At the end of the experimental period, microbial activity in the substrate was assessed through colony-forming unit (CFU) counts on nutrient agar (bacteria) and PDA (fungi). Results revealed notable differences among treatments in both bacterial and fungal populations.

• The highest bacterial CFU count was observed in Treatment D (Sargassum muticum), averaging 9.1 × 10⁶ CFU g⁻¹, followed closely by Treatment C (A. nodosum—Greenland) at 8.6 × 10⁶ CFU g⁻¹.
• The commercial seaweed product (E) yielded 7.8 × 10⁶ CFU g⁻¹, while the amino acid biostimulant (F) produced 6.9 × 10⁶ CFU g⁻¹.
• The untreated control (G) had the lowest bacterial count at 5.3 × 10⁶ CFU g⁻¹.

Fungal populations followed a similar pattern:

• Treatment D recorded the highest fungal CFU value at 4.2 × 10⁶ CFU g⁻¹, with Treatment C close behind at 3.9 × 10⁶ CFU g⁻¹.
• Commercial Treatments E and F showed moderate fungal stimulation (3.4 and 3.1 × 10⁶ CFU g⁻¹, respectively).
• The control group again had the lowest fungal activity (2.2 × 10⁶ CFU g⁻¹).

3.5. Vegetative Biomass

Dry weight measurements of the above-ground biomass revealed substantial treatment effects. The highest biomass was found in Treatment C (44.8 g), followed by D (42.3 g), and E (39.2 g). All algae-treated groups exhibited greater biomass accumulation compared to the commercial amino acid-based Treatment (F, 33.6 g) and the control (21.4 g) (Figure 5).

Treatments A and B generated biomass values of 25.1 g and 28.7 g respectively, demonstrating an intermediate performance. Treatments C and D were statistically indistinguishable and belonged to the highest performing group (“a”), significantly outperforming the untreated control (“d”). These findings confirm that select macroalgal biostimulants strongly enhance vegetative growth, likely by activating metabolic and hormonal pathways related to cell division and elongation.

3.6. Improvement Root Biomass

Root development was also significantly influenced by the treatments. The most vigorous root systems were recorded in Treatment D (11.5 g), slightly surpassing Treatment C (10.9 g). The amino acid-based commercial product (F) also induced relatively high root mass (9.6 g), followed by Treatment E (9.2 g). The control group exhibited the least root development (5.4 g) (Figure 6).

Treatments A and B demonstrated limited stimulation of root biomass, yielding values of 6.1 g and 7.0 g, respectively. Statistical analysis showed Treatments C and D formed the top statistical group (“a”), confirming their superior ability to promote below-ground biomass. Root development is crucial for nutrient and water uptake, especially in container-based cultivation, and the algae treatments showed strong potential for enhancing this critical function.

3.7. Essential Oil Yield

Essential oil yield, a key quality trait for lavender, was significantly affected by treatment. The highest yield was achieved in Treatment C (1.49% of dry biomass), followed by D (1.42%) and E (1.33%). The control plants yielded the lowest oil content at only 0.81%, representing a nearly 50% reduction compared to the top-performing treatment (Figure 7).

Intermediate values were recorded in Treatments B (1.03%), F (1.21%), and A (0.92%). The statistical ranking placed C and D in the most favorable group (“a”), with clear superiority over the untreated group (“d”). These results suggest a strong link between improved physiological status and oil biosynthesis in lavender when treated with high-quality biostimulants.

3.8. Improvement Antioxidant Enzyme Activity

To assess the role of biostimulants in oxidative stress mitigation, the activities of three key antioxidant enzymes—superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)—were measured at the end of the stress period (week 10) in both drought and salinity-treated plants.

Across both stress conditions, enzyme activity was significantly elevated in all algae-treated plants compared to the untreated control (G), with the most pronounced effects observed in Treatment C (Ascophyllum nodosum, Greenland) and Treatment D (Sargassum muticum, Venice).

Under drought stress, the following was observed:

• SOD activity peaked in Treatment C at 85.2 U/mg protein, followed closely by D (80.6 U/mg) and E (75.1 U/mg), while the untreated control recorded only 47.3 U/mg (Figure 8).
• CAT activity in Treatment C reached 61.4 U/mg, significantly higher than all other treatments; the control group recorded only 29.7 U/mg (Figure 8).
• POD activity was similarly enhanced in C and D (102.5 and 98.3 U/mg, respectively), compared to 58.9 U/mg in the control (Figure 8).

Under salinity stress, the following observations were made:

• Trends were comparable, with SOD activity again highest in Treatment C (88.7 U/mg), followed by D (84.2 U/mg), while the control remained significantly lower (50.1 U/mg).
• CAT and POD levels followed similar patterns, with Treatments C and D consistently forming the highest statistical groupings (p < 0.05).

The commercial amino acid product (F) showed moderate activation of antioxidant enzymes but was significantly less effective than algae-based treatments. Treatments A and B produced intermediate results.

3.9. Proline and Soluble Sugar Accumulation

Under drought stress conditions, significant differences were observed in the accumulation of key osmoprotectants—proline and soluble sugars—across the biostimulant treatments (Figure 9A,B). These metabolites play vital roles in osmotic adjustment, membrane stabilization, and protection against oxidative damage during water deficit.

Proline Content:

The highest proline concentration was recorded in Treatment C (Ascophyllum nodosum—Greenland) at 6.12 µmol g⁻¹ FW, statistically grouped as “a,” indicating a strong enhancement in osmotic protection. This was followed by Treatment D (Sargassum muticum—Venice) at 5.58 µmol g⁻¹ FW and Treatment E (Kelpak^®) at 4.95 µmol g⁻¹ FW, which were classified as “ab” and “abc,” respectively. The lowest accumulation occurred in the untreated control (G) at 2.02 µmol g⁻¹ FW, significantly lower than all algae-based treatments and grouped as “d.”

Intermediate values were observed in Treatments F (4.42 µmol g⁻¹ FW), B (3.78 µmol g⁻¹ FW), and A (3.15 µmol g⁻¹ FW), with varying statistical distinctions. These results highlight the superior ability of high-latitude (A. nodosum) and invasive (S. muticum) algae to stimulate proline synthesis under drought conditions, likely through hormonal priming or stress-related signaling pathways.

Soluble Sugar Content:

A similar trend was seen for total soluble sugars. Treatment C again showed the highest accumulation (19.7 mg g⁻¹ FW, group “a”), followed by D (18.9 mg g⁻¹ FW) and E (15.6 mg g⁻¹ FW), forming a distinct statistical cluster. The control group had the lowest sugar content (8.7 mg g⁻¹ FW, group “d”), nearly half the concentration found in the top-performing treatments.

Biostimulant-treated plants showed significantly elevated sugar levels, with Treatment F (14.3 mg g⁻¹ FW), B (11.8 mg g⁻¹ FW), and A (10.4 mg g⁻¹ FW) achieving moderate improvements compared to the untreated group. These findings suggest that algae-based biostimulants may activate carbohydrate metabolism or reduce sugar degradation under stress.

3.10. Comparative Efficacy of Macroalgal and Commercial Biostimulants

To better evaluate the practical benefits of marine macroalgal biostimulants relative to available commercial formulations, a percentage-based comparison was performed for key physiological and biochemical traits. The untreated control (G) was used as the baseline for all relative calculations.

3.10.1. Plant Height

• Treatment C increased plant height by +51.2% over the control, while Treatment D showed a +43.7% gain.
• The commercial seaweed product E (Kelpak^®) led to a +34.8% increase, whereas the amino acid-based F (Trainer^®) improved height by +29.7%.
• Thus, Treatment C was 16.4% more effective than Kelpak^® and 21.5% better than Trainer^® in promoting vertical growth.

3.10.2. Leaf Area

• C-treated plants exhibited a +81.0% increase in leaf area compared to control; Treatments D and E achieved +69.8% and +57.1%, respectively.
• The amino acid product (F) provided only a +39.7% improvement.
• Treatment C outperformed the best commercial comparator (E) by 15.2%, confirming its superior capacity for foliar expansion.

3.10.3. Chlorophyll Content (SPAD Index)

• SPAD values rose by +47.3% with Treatment C and +41.6% with D, while commercial Treatments E and F showed increases of +37.2% and +29.9%, respectively.
• Algal extracts from Greenland and Venice were 10–18% more effective than commercial formulations in enhancing chlorophyll concentration, reflecting improved photosynthetic efficiency.

3.10.4. Vegetative Biomass

• Treatment C led to a +109.3% increase in shoot biomass relative to the control, D followed with +97.7%, and E achieved +83.2%.
• The amino acid biostimulant (F) contributed to a +57.0% increase.
• Compared to F, Treatment C nearly doubled biomass accumulation (+92.3% over F), highlighting the superior growth-promoting effect of cold-climate algae.

3.10.5. Root Biomass

• The most effective root biomass stimulation was recorded in D (+113.0%) and C (+101.9%) versus control.
• Commercial products showed +70.4% (F) and +70.3% (E) increases.
• These results indicate that whole-algae extracts foster better root architecture than either amino acids or standard seaweed formulations.

3.10.6. Essential Oil Yield

• Oil-yield improvements reached +83.9% in Treatment C and +75.3% in D, compared to +64.2% in E and +49.4% in F.
• In practical terms, Treatment C produced 12.0% more oil than Kelpak^®, and 34.5% more than Trainer^®, which is a substantial commercial advantage in aromatic crop production.

3.10.7. Antioxidant Enzyme Activity

• Under drought stress, SOD activity in Treatment C was +80.1% higher than in control, compared to +58.8% in E and +38.1% in F.
• Similarly, CAT and POD activities followed this trend, reinforcing the superior biochemical resilience of algae-based treatments from cold or dynamic environments.

3.10.8. Osmoprotectant Accumulation

• Proline levels in C-treated plants were +203% higher than the control, while Kelpak^® and Trainer^® improved proline by +145% and +119%, respectively.
• Soluble sugar content increased by +126.4% in Treatment C vs. +79.3% in Kelpak^® and +64.3% in Trainer^®.
• These results suggest a clear biochemical advantage of the experimental macroalgal treatments in promoting osmotic stress tolerance.

3.11. Improvement Plant Responses to Abiotic Stress Conditions

3.11.1. Effect of Drought and Salinity on Agronomic Characters of Lavender

Drought and salinity stress had pronounced negative impacts on the agronomic performance of Lavandula angustifolia, with significant reductions observed in plant height, leaf area and biomass production. The extent of these reductions varied depending on the treatment applied.

In the untreated control group (G), drought stress resulted in a substantial decrease in plant height and leaf area, with reductions of approximately 25–30% compared to non-stressed conditions. Vegetative biomass declined by over 40%, and the number of flowering spikes was reduced by nearly half. These changes reflect typical physiological constraints under water-deficit conditions, including reduced cell expansion, stomatal closure, and impaired carbon assimilation.

Salinity stress (100 mM NaCl) produced similar, though slightly less severe, effects. Treated plants showed chlorosis and reduced turgor, and biomass was decreased by 30–35% in the control. Root biomass was particularly affected under salinity, indicating that ionic toxicity may have interfered with root growth and function.

In contrast, macroalgae-based biostimulants—especially Treatment C (Ascophyllum nodosum—Greenland) and Treatment D (Sargassum muticum—Venice)—significantly mitigated these stress effects. Under drought, plants treated with C and D maintained over 90% of their control-level plant height and leaf area, and biomass reductions were limited to 10–15%. Similar resilience was observed under salinity, where treated plants maintained higher root biomass and fewer visible stress symptoms than the control and commercial biostimulant treatments.

3.11.2. Essential Oil Yield Preservation

The application of drought and salinity stress had significant adverse effects on essential oil yield in Lavandula angustifolia, yet biostimulant treatments notably mitigated these impacts. Figure 10 illustrates the essential oil yield across control, drought, and salinity conditions for all treatments.

Under non-stressed control conditions, all biostimulant treatments produced higher oil yields than the untreated control (G), with the greatest increase observed in Treatment C (Ascophyllum nodosum, Greenland) at 1.49% oil content. Treatment D (Sargassum muticum, Venice) also achieved a high yield of 1.42%, followed by the commercial product Kelpak^® (E) at 1.33%. The untreated control produced the lowest yield (0.81%).

Under drought stress, essential oil production declined across all treatments, but the degree of reduction varied. The control group showed a substantial decrease to 0.45%, indicating strong sensitivity to water limitation. In contrast, Treatments C and D retained high oil content under drought, measuring 1.31% and 1.27%, respectively. These values were only modestly lower than their non-stressed counterparts, suggesting effective drought stress mitigation. The commercial products E and F showed intermediate resilience, while Treatment G exhibited the most severe decline.

A similar trend was observed under salinity stress (100 mM NaCl). Oil yield in the untreated control dropped to 0.51%, whereas Treatments C and D maintained values of 1.20% and 1.18%, respectively. This indicates a reduced impact of ionic and osmotic stress in algae-treated plants. Commercial biostimulants again performed moderately, and Treatments A and B showed limited protective effects.

Statistical analysis confirmed that Treatments C and D formed the highest significance group (“a”) across all stress conditions, while the control was consistently ranked the lowest. The centered significance letters in Figure 11 highlight these differences. Standard deviations were generally low, indicating consistent treatment responses.

Overall, these results demonstrate that select macroalgal biostimulants, particularly those derived from cold or stress-adapted environments, significantly buffer the negative effects of abiotic stress on essential oil yield in lavender. Their superior performance under drought and salinity conditions underscores their potential for sustainable cultivation in suboptimal environments.

3.12. Principal Component Analysis (PCA)

To assess the overall multivariate response of Lavandula angustifolia to different biostimulant treatments, a Principal Component Analysis (PCA) was performed using standardized data from six major parameters: plant height, leaf area, chlorophyll content, shoot biomass, root biomass, and essential oil yield. The first two principal components (PC1 and PC2) explained approximately 78.3% of the total variance in the dataset, with PC1 accounting for 52.4% and PC2 for 25.9%.

Figure 11 illustrates the spatial distribution of treatments in the PCA biplot. Treatments C (Ascophyllum nodosum, Greenland) and D (Sargassum muticum, Venice) clustered closely and were positioned in the quadrant associated with positive loadings for all growth and productivity parameters. This reflects their consistently superior performance across all measured traits.

In contrast, the untreated control (G) was isolated in the opposite quadrant, clearly separated from all other treatments. This highlights its negative association with key growth indicators and essential oil yield, further reinforcing its status as the lowest-performing group.

Commercial biostimulants (E: Kelpak^® and F: Trainer^®) occupied intermediate positions in the PCA space. While they showed some improvement compared to the control, their separation from Treatments C and D indicates a more limited influence on plant performance.

The PCA results thus confirm and visually reinforce the univariate findings. Treatments C and D not only performed best in individual parameters but also showed a coordinated enhancement across multiple plant traits. The clustering pattern provides robust evidence of the holistic effect of specific macroalgal biostimulants, suggesting they support overall plant vigor rather than selectively improving isolated traits.

4. Discussion

4.1. Growth and Morphological Improvements

The promotion of plant height, leaf area, and overall vegetative biomass observed in algae-treated lavender aligns with previous findings in various horticultural crops. Studies on lettuce, tomatoes, and cucumbers have shown that seaweed extracts stimulate vegetative growth, likely due to their content of natural growth regulators such as cytokinins, auxins, and gibberellins [48,49,50]. These hormones are known to regulate cell division and elongation, which could explain the superior performance of Treatments C and D in this experiment. Increased leaf area in algae-treated plants may contribute to improved light capture and photosynthetic efficiency. Prior work in peppers and spinach treated with algal biostimulants reported similar trends, linking leaf expansion to the action of signaling molecules such as laminarins and oligosaccharides [51,52]. These compounds can stimulate metabolic activity and boost internal resource allocation toward foliar development [53].

4.2. Chlorophyll Content and Photosynthetic Capacity

SPAD index values, indicative of chlorophyll content, were significantly higher in algae-treated plants. This finding is consistent with studies showing that seaweed extracts enhance chlorophyll biosynthesis and stability under both optimal and stress conditions [54]. In eggplant and maize, for example, biostimulant application resulted in increased pigment retention and delayed senescence [55,56]. The likely mechanism involves the upregulation of chlorophyll biosynthetic enzymes and the antioxidant properties of seaweed compounds such as betaines and phenolics [57,58]. These substances may reduce oxidative damage to chloroplasts and maintain pigment functionality over time [59].

4.3. Biomass Accumulation and Nutrient Efficiency

Dry weight measurements of shoots and roots revealed clear improvements following biostimulant application, particularly in Treatments C and D. These results mirror findings in arugula and barley, where macroalgae enhanced biomass accumulation through improved nitrogen uptake and metabolic efficiency [60,61]. Enzymatic activities such as nitrate reductase and glutamine synthetase are often stimulated by biostimulant exposure, leading to enhanced nitrogen assimilation and amino acid synthesis [62]. Additionally, increased root mass in algae-treated plants may reflect the auxin-like activity of macroalgal extracts, promoting lateral root initiation and elongation [63]. While the commercial amino acid product (Treatment F) provided some improvement in root biomass, it fell short of the algae treatments. This may be due to the broader spectrum of bioactive compounds present in whole-algae extracts compared to targeted commercial formulations [64].

4.4. Essential Oil Production

Essential oil yield is a critical economic trait for lavender. The observed increases in oil content in Treatments C and D support earlier research indicating that biostimulants can upregulate secondary metabolite pathways [65]. In other aromatic plants like basil and thyme, similar responses were linked to increased activity in the methylerythritol phosphate (MEP) and mevalonate (MVA) pathways [66,67]. Algae extracts may influence the expression of terpene synthase genes and boost precursor availability for monoterpene and sesquiterpene synthesis [61]. Enhanced photosynthesis and nutrient assimilation in treated plants also likely support higher carbon flux toward secondary metabolite production [62].

4.5. Rhizosphere Effects and Microbial Stimulation

The observed increase in microbial activity in algae-treated substrates suggests a prebiotic effect of macroalgal biostimulants on soil microbial communities [63,64]. Treatments C and D, in particular, significantly enhanced both bacterial and fungal CFU counts compared to the control and even outperformed commercial products.

These effects may be attributed to the bioavailable organic compounds in macroalgal extracts—such as polysaccharides (alginates, laminarins), phenolics, and organic acids—which can act as carbon sources and microbial stimulants [65]. Enhanced microbial populations, especially in confined systems like pots, can contribute to the following:

• Improved nutrient cycling;
• Increased enzyme activity;
• Suppression of pathogenic organisms.

The greater rhizospheric stimulation by Sargassum muticum (Treatment D) may reflect its unique polysaccharide profile and regional adaptation to brackish, nutrient-rich environments [66]. Similarly, the strong performance of A. nodosum (Greenland) suggests that environmental stressors in the algae’s origin habitat contribute to the accumulation of compounds that indirectly benefit root microbiota [67].

4.6. Abiotic Stress Tolerance

Although this study was conducted under optimal growing conditions, prior research offers insights into how macroalgae may support stress tolerance. In grapes and tomatoes, seaweed biostimulants reduced the physiological effects of salinity and drought, preserving growth and yield [68,69]. Mechanisms behind this include the accumulation of osmoprotectants (e.g., mannitol, proline) and activation of antioxidant defenses such as catalase and peroxidase [70,71]. Treatments C and D may have similar potential, a hypothesis supported by their strong performance across all traits measured.

4.7. Comparison with Commercial Products

Both commercial biostimulants tested (E and F) showed improved performance compared to the control, but were less effective than algae treatments. This suggests that the complexity and diversity of bioactive molecules in whole-algae extracts offer superior outcomes compared to formulations based on singular active ingredients [72]. Amino acid biostimulants, for example, support protein synthesis and osmotic adjustment, but lack the hormonal and polysaccharide components present in marine algae [73]. Seaweed-based commercial products may lose efficacy through standardization and processing, reducing bioavailability of key molecules [74].

4.8. Origin and Composition of Algae Matter

The origin of the algae used in Treatments C and D may partly explain their superior performance. Algae from cold or variable climates tend to accumulate higher concentrations of antioxidants and protective compounds, which may translate into stronger biostimulant effects [75]. This phenomenon has been documented in Fucus and Ascophyllum species collected from different latitudes [76]. Invasive species like Sargassum muticum (Treatment D) also show promise, offering dual environmental and agronomic benefits by reducing biomass pressure in coastal areas while contributing to sustainable agriculture [77].

4.9. Antioxidant Defense and Osmotic Adjustment Under Drought Stress

The application of marine macroalgae-based biostimulants significantly enhanced the antioxidant and osmoprotective responses in Lavandula angustifolia subjected to drought stress, as reflected in increased enzyme activities (SOD, CAT, POD) and elevated levels of osmoprotectants (proline, soluble sugars) [78]. These findings provide strong biochemical evidence for the stress-mitigating role of select macroalgae species [79], particularly Ascophyllum nodosum (Greenland) and Sargassum muticum (Venice).

4.9.1. Enhanced Antioxidant Enzyme Activity

Plants exposed to drought stress typically experience an overproduction of reactive oxygen species (ROS), which can damage cellular membranes, proteins, and DNA [22]. In this study, the pronounced upregulation of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in algae-treated plants—especially those in Treatments C and D—suggests that these biostimulants prime the enzymatic antioxidant defense system.

This priming effect aligns with previous research in crops such as tomato and wheat, where Ascophyllum and Sargassum extracts were shown to enhance enzymatic activity, reduce lipid peroxidation, and maintain chloroplast integrity under abiotic stress [80]. The coordinated increase in SOD (which dismutates superoxide radicals), CAT (which decomposes hydrogen peroxide), and POD (involved in detoxifying peroxides) suggests a systemic activation of ROS scavenging pathways, contributing to improved cellular homeostasis and photosynthetic resilience [81].

The differential performance of algae species also points to a relationship between environmental origin and biostimulant potency. For instance, A. nodosum from Greenland is adapted to highly variable and stressful climates, possibly leading to the accumulation of unique protective metabolites such as polyphenols and betaines, which may act synergistically with plant defense mechanisms [82].

4.9.2. Osmoprotectant Accumulation and Osmotic Regulation

Parallel to the antioxidant response, biostimulant application also stimulated the accumulation of key osmoprotectants, including proline and soluble sugars. These molecules serve multiple protective roles under water-deficit conditions: osmotic adjustment, membrane stabilization, and protection of proteins and enzymes from denaturation [83].

The elevated proline concentrations in Treatments C and D—more than double those in the control—are particularly noteworthy. Proline is not only an osmolyte but also functions as a ROS scavenger, molecular chaperone, and signaling molecule, modulating gene expression related to drought tolerance [84]. Its synthesis is often triggered by hormonal signals, which may be upregulated by the natural phytohormones or elicitor compounds found in macroalgae extracts [85].

Soluble sugars, similarly elevated in algae-treated plants, contribute to carbon storage, osmotic buffering, and ROS detoxification [86]. Their accumulation may also be indicative of enhanced photosynthetic performance, supported by higher SPAD values and chlorophyll content observed in previous results. Together, the data from Figure 8 and Figure 9 strongly suggest that macroalgal biostimulants facilitate an integrated stress response, involving both enzymatic and metabolic adaptations.

4.10. Plant Responses to Abiotic Stress Conditions

The results of this study clearly indicate that both drought and salinity exert strong negative effects on the agronomic performance of Lavandula angustifolia. Significant reductions in plant height, leaf area, and biomass under these stress conditions are in line with established physiological responses to limited water availability and salt-induced osmotic imbalance. These include reduced cell expansion, stomatal closure, and impaired nutrient and water uptake, all of which compromise growth and productivity [87].

However, the application of macroalgal biostimulants, particularly those derived from Ascophyllum nodosum (Greenland) and Sargassum muticum (Venice), effectively mitigated these adverse impacts. Plants treated with these extracts exhibited significantly less reduction in vegetative growth and root biomass compared to untreated controls. This suggests that the biostimulants supported physiological stability under stress, likely through improved water-use efficiency, osmotic regulation, and maintenance of turgor pressure [88]. The observed increase in root biomass under stress conditions further indicates enhanced capacity for water and nutrient acquisition, which is critical in mitigating drought and salinity effects.

Essential oil yield, a central economic trait in lavender cultivation, was also notably preserved in biostimulant-treated plants. While all treatments experienced some decline under stress, plants receiving A. nodosum and S. muticum extracts maintained oil content levels close to those observed under non-stressed conditions. This outcome reflects the importance of maintaining overall plant vitality and metabolic function, as essential oil synthesis is closely linked to the plant’s physiological condition. The improved chlorophyll content and reduced biomass losses in these treatments likely contributed to sustained oil production [89].

Commercial biostimulants (Kelpak^® and Trainer^®) provided moderate protective effects but were less effective than the experimental macroalgal extracts. The untreated control consistently showed the greatest reductions across all parameters, reaffirming the plant’s vulnerability in the absence of supportive inputs.

These findings reinforce the value of macroalgal biostimulants as practical tools for improving lavender resilience and productivity under abiotic stress. Their ability to support growth, preserve quality traits, and reduce stress symptoms highlights their potential in greenhouse and field applications, especially in regions facing water scarcity or soil salinization. Their integration into sustainable management practices can thus enhance crop performance while reducing reliance on synthetic inputs [90].

4.11. Principal Component Analysis (PCA)—Interpretation

Principal Component Analysis (PCA) was employed in this study as an integrative statistical tool to explore patterns of association among multiple plant traits and treatments. Unlike univariate analysis, which isolates each parameter, PCA provides a multidimensional view of the data, highlighting how different treatments perform across a suite of physiological and agronomic indicators. The PCA revealed strong internal consistency among parameters such as plant height, chlorophyll content, shoot and root biomass, leaf area, and essential oil yield. These variables were tightly correlated and contributed significantly to the first principal component (PC1), which effectively summarized general plant vigor and productivity. Treatments that performed well across all or most parameters aligned strongly along this component. The second principal component (PC2) added orthogonal contrast, enabling separation based on more nuanced differences, such as relative emphasis on root development or secondary metabolite accumulation. What emerged from the PCA was a clear grouping of treatments based on their overall performance profile. Treatments based on marine macroalgae, particularly Ascophyllum nodosum from Greenland and Sargassum muticum from the Mediterranean, formed a distinct cluster, characterized by strong contributions from all growth-related and oil-yield variables. Their proximity in the PCA space suggests not only high performance but also consistency across replicates and robustness under varying conditions. This supports the hypothesis that biostimulants derived from stress-adapted algae can promote uniform physiological benefits, likely due to their complex mix of organic compounds, including phytohormones, antioxidants, and osmoprotectants [91]. In contrast, the untreated control stood apart from the main treatment clusters, reflecting its weak performance across most parameters. This separation underscores the stress susceptibility of lavender in the absence of supportive inputs, and reaffirms the value of biostimulants in buffered systems such as greenhouses or pots, where environmental stress can be experimentally introduced. Interestingly, commercial products—though somewhat effective—occupied more central or intermediate positions in the PCA plot. This spatial neutrality suggests partial benefit, but not to the same extent or consistency as the high-performing macroalgal treatments. These findings raise important questions about formulation efficacy and the role of extract source and processing in determining biostimulant performance. Beyond treatment ranking, the PCA offers broader agronomic insights. Treatments aligning with the upper-right quadrant—where traits like chlorophyll content, oil yield, and biomass converge—can be interpreted as offering a full-spectrum benefit, supporting not only primary growth but also commercial quality traits. This visualization can aid growers in selecting inputs that optimize plant development while safeguarding essential oil production, even under suboptimal conditions. In summary, PCA effectively confirmed the multivariate superiority of certain algae-based treatments, capturing treatment performance through the simultaneous lens of productivity, quality, and stress resilience. It serves as a compelling visual and statistical endorsement for their integration into lavender cultivation programs aimed at sustainability and yield stability [92].

4.12. Agronomic Relevance and Applications

The results support the use of macroalgal biostimulants in commercial lavender production. Beyond yield gains, these treatments offer environmental advantages such as reduced chemical input and improved soil health [93,94,95]. This aligns with sustainable intensification strategies promoted in modern agriculture [96]. These findings are particularly relevant for organic and low-input systems, where natural inputs are prioritized [97,98,99]. The positive effects on both primary growth and essential oil production position algae extracts as viable alternatives to synthetic enhancers [100,101].

4.13. Future Directions

While this study provides strong evidence of efficacy under greenhouse conditions, field trials are necessary to assess performance under variable environmental conditions. Further biochemical and molecular analyses would help elucidate the exact pathways modulated by algae treatments. Additionally, integration with microbial inoculants or compost extracts could reveal synergistic effects and promote development of multifunctional bioformulations.

5. Conclusions

This study evaluated the effectiveness of marine macroalgal biostimulants in enhancing the growth, physiology, essential oil production, and drought stress resilience of Lavandula angustifolia under semi-controlled greenhouse conditions. Seven treatments were tested, including four experimental macroalgal extracts, two commercial biostimulants, and an untreated control. Results demonstrated that biostimulant application significantly influenced multiple agronomic and biochemical parameters. The most effective treatments were derived from Ascophyllum nodosum (Greenland) and Sargassum muticum (Venice), both of which consistently outperformed commercial products and the control in promoting vegetative growth, leaf development, and chlorophyll content. These improvements translated into higher biomass accumulation and greater flowering potential, indicating enhanced plant productivity. Essential oil yield, a critical quality attribute for lavender, was also significantly improved by the same two treatments. This suggests a positive link between plant physiological status and secondary metabolite biosynthesis, likely mediated by enhanced nutrient assimilation and metabolic activity stimulated by the biostimulants. Under drought stress, plants treated with macroalgal extracts exhibited elevated antioxidant enzyme activities (SOD, CAT, POD) and greater accumulation of osmoprotectants (proline and soluble sugars). These responses are indicative of improved oxidative stress management and osmotic adjustment, both essential for maintaining physiological function under water-limited conditions. Additionally, microbial colony counts in the substrate revealed that macroalgal treatments enhanced rhizospheric microbial populations, particularly bacterial and fungal CFUs. This microbial stimulation may contribute to improved nutrient availability and plant–microbe interactions, especially in pot-based systems with limited soil biological diversity. In comparison to commercial biostimulants, the selected macroalgal extracts showed equal or superior efficacy across most parameters measured, suggesting their potential as effective alternatives. Their performance may be attributed to the presence of diverse bioactive compounds inherent to macroalgae from environmentally challenging habitats. In conclusion, the results support the targeted use of specific macroalgal biostimulants as multifunctional inputs in lavender cultivation. Their demonstrated capacity to enhance growth, stress tolerance, and product quality under controlled conditions provides a strong basis for further field-level validation and integration into sustainable aromatic crop production systems.