Gas Exchange and Yield Responses of Sesamum indicum L. to Salt Stress Under Mineral and Organic Fertilization

Abstract

Sesame (Sesamum indicum L.) cv. BRS Anahí is a promising crop for agricultural diversification in the Brazilian semi-arid region, although saline irrigation can limit its development and productivity. The objective of this study was to evaluate the physiological and productive responses of sesame irrigated with brackish water under different fertilization treatments. The experiment was conducted in a greenhouse at the Federal University of Ceará, using a completely randomized design in a 4 × 2 factorial scheme, with four replications. These corresponded to four fertilization treatments (F1: HomeBiogas liquid biofertilizer from CAGECE; F2: HomeBiogas from food waste; F3: shrimp biofertilizer; and F4: mineral fertilization—NPK) and two irrigation water salinity levels (0.8 and 3.0 dS m⁻¹). Irrigation with 3.0 dS m⁻¹ significantly reduced the analyzed variables; however, the intensity of these effects varied according to the fertilization treatment. Treatment F1 promoted greater physiological activity under saline conditions, partially mitigating saline stress. At low salinity, treatments F1 and F3 showed productivity performance similar to F4, while at high salinity, F3 was less effective. The results highlight the potential of the selected biofertilizers to improve sesame performance under saline irrigation.

1. Introduction

Sesame (Sesamum indicum L.) is cultivated in approximately 70 countries, particularly across Asia and Africa, where the leading producers—Sudan, India, Myanmar, and Tanzania—together account for approximately 64% of the world’s planted area [1]. In Brazil, sesame shows high production potential due to its rusticity and adaptability, representing an important alternative for agricultural diversification, particularly in semi-arid regions [2]. During the 2025/26 harvest season, Brazilian sesame output reached approximately 1.13 million tonnes with cultivated area expanding modestly year-on-year to roughly 280 thousand hectares nationwide [3]. However, its cultivation is often concentrated in areas characterized by irregular rainfall and high temperatures, where water scarcity and quality constraints are major limiting factors.

In the Brazilian semi-arid region, the use of saline water for irrigation is common due to edaphoclimatic conditions and limitations in water resource management [4]. Salt stress affects plant development through two combined mechanisms: initially, the reduction in soil osmotic potential hinders water uptake—even when soil moisture is adequate—creating a condition of physiological drought; subsequently, the excessive accumulation of Na⁺ and Cl⁻ ions in plant tissues compromises ionic homeostasis and membrane integrity, intensifying damage to the photosynthetic apparatus [5,6]. In sesame, these responses have been reported as reductions in plant height and leaf area [7], as well as decreases in stomatal aperture and photosynthesis caused by salts present in irrigation water [8]. Thus, leaf gas exchange parameters serve as early indicators of salt stress, directly reflecting its effects on crop growth and productivity.

To mitigate the adverse effects of salinity, different fertilization strategies have been investigated. Mineral fertilization has shown potential to alleviate salt stress by improving plant nutritional status and physiological performance. For instance, potassium fertilization enhanced peanut tolerance to saline conditions by competing with Na⁺ for root uptake sites, thereby limiting cytosolic Na⁺ accumulation and supporting osmotic adjustment and stomatal regulation [9]. Similarly, phosphate fertilization can contribute to greater root system development, improving water uptake under saline conditions [10].

In addition to mineral inputs, the use of organic sources has gained attention as a sustainable alternative. Biofertilizers derived from organic residues can improve soil structure, increase nutrient availability, and enhance plant tolerance to abiotic stress [11]. Previous studies have demonstrated that organic fertilization—such as biofertilizers based on cattle and poultry manure—can mitigate salt stress and increase the productivity of crops irrigated with brackish water [6,12,13].

Despite these advances, few studies have directly compared how biofertilizers derived from different feedstocks such as biogas sludge from sanitation systems, food waste digestate, and crustacean (shrimp) processing waste differ in their physiological modes of action and their ability to maintain source–sink regulation under saline irrigation, particularly in oilseed crops. This information is relevant because these biofertilizers differ substantially in composition and bioactive fractions: biogas-derived digestates are characterized primarily by their nutrient and humic substance content [14], whereas shrimp-derived biofertilizers contain chitin/chitosan fractions with distinct elicitation and antioxidant priming properties [15]. The literature reports physiological and biochemical tolerance mechanisms across various sesame genotypes in response to stress conditions worldwide, and plant growth-promoting rhizobacteria have been shown to mitigate salt stress in sesame through osmolyte accumulation and antioxidant enzyme activation [16,17]; however, the comparative efficacy of biofertilizers derived from markedly different feedstocks remains underexplored for this crop.

Based on this, it is hypothesized that biofertilizers can mitigate the negative effects of salinity on gas exchange and yield components of sesame, with this mitigating capacity depending on the composition and raw material of each biofertilizer. Therefore, this study aimed to evaluate the physiological and productive responses of sesame irrigated with brackish water under mineral fertilization and biofertilizers of different origins. The originality of this study lies in the direct physiological comparison between biofertilizers of contrasting raw materials and mineral fertilization in sesame under saline irrigation. Practically, identifying biofertilizers able to sustain sesame’s physiological performance and productivity under salinity offers smallholder farmers in the Brazilian semi-arid region a low-cost, circular-economy alternative to mineral fertilizers, while adding value to regional agro-industrial and sanitation waste streams.

2. Materials and Methods

2.1. Experimental Conditions

The experiment was conducted in a greenhouse from August to November 2024 at the experimental area of the Agrometeorological Station of the Federal University of Ceará (UFC), Pici Campus, Fortaleza, Ceará, Brazil (3°44′44″ S, 38°34′55″ W; 25 m altitude) (Figure 1).

Daily maximum and minimum air temperatures and mean relative humidity were recorded throughout the experimental period (Figure 2) using a digital thermo-hygrometer (FEPRO-MUT50), Exbom, São Paulo, Brazil.

2.2. Experimental Design

A completely randomized design in a 4 × 2 factorial scheme was adopted, with 4 replications. It consisted of four fertilization strategies: F1 = 100% liquid biofertilizer derived from organic waste processed by the Water and Sewage Company of Ceará (CAGECE) using a HomeBiogas system; F2 = 100% liquid biofertilizer derived from food waste processed via HomeBiogas; F3 = 100% shrimp-based biofertilizer; and F4 = 100% mineral fertilization (NPK) and two levels of electrical conductivity of the irrigation water (0.8 and 3.0 dS m⁻¹).

2.3. Preparation and Experimental Procedures

The experiment was conducted in 20 L polyethylene pots filled with a substrate composed of arisco soil, sand, and organic compost in a 7:2:1 ratio (v/v). The soil was air-dried and sieved (2 mm mesh) prior to substrate preparation, following standard procedures described in the EMBRAPA Manual de Métodos de Análise de Solo [18]. The chemical properties of the substrate are presented in

Table 1. Chemical properties of the substrate used in the experiment.

OM	N	P	K	Ca	Mg	Na	pH	ESP	ECse
g kg−1							-	(%)	dS m−1
4.18	0.25	0.06	0.27	1.80	0.60	0.41	6	9	1.13

OM—organic matter; ESP—exchangeable sodium percentage; ECse—electrical conductivity of the substrate saturation extract.

.

Sesame seeds of the cultivar ‘BRS Anahí’ were sown, with five seeds per pot; thinning began seven days after sowing (DAS) and continued until one plant per pot remained. The biofertilizers from CAGECE and food waste were produced using a HomeBiogas™ Model 2.0 system (HomeBiogas Ltd., Beit Yanai, Israel), an intermediate-scale anaerobic biodigester with a 1200 L digestion chamber and 700 L gas storage capacity. The system includes an organic waste inlet, a biodigestion tank, a gas storage chamber with a filtration unit, a pressure system for gas distribution, and an effluent outlet. The shrimp biofertilizer was supplied by BioTech4Life Soluções Biotecnológicas, Fortaleza, Brazil. Shrimp residues were ground and subjected to fermentation in a bioreactor with the addition of a consortium of lactic acid bacteria. The material underwent hydrolysis for 48 h until complete degradation of the solid fraction.

All biofertilizers were chemically characterized by the Laboratory for Soil, Water, Plant Tissue, and Fertilizer Analysis of the Soil Science Department at UFC (

Table 2. Chemical composition of the biofertilizers.

Sources	Nutrients
	N	P	K	Ca	Mg	Fe	Cu	Zn	Mn
	g L−1					mg L−1
F1	0.6	0.55	0.71	0.13	0.06	7.3	0.21	29.32	3.22
F2	1.1	0.45	1.19	0.02	0.6	9.06	0.53	7.15	2.19
F3	1.6	2.16	0.69	0.49	0.12	12.72	12.6	5.80	15.3

F1—CAGECE’s HomeBiogas liquid biofertilizer; F2—HomeBiogas liquid biofertilizer from food waste; F3—shrimp biofertilizer.

).

Mineral fertilization was based on substrate analysis (Table 1) and crop recommendations [19], corresponding to 40 kg ha⁻¹ N, 80 kg ha⁻¹ P₂O₅, and 60 kg ha⁻¹ K₂O. Considering a density of 10,000 plants ha⁻¹, the equivalent per plant was 4 g N, 8 g P₂O₅, and 6 g K₂O, applied via urea (45% N), single superphosphate (18% P₂O₅), and potassium chloride (60% K₂O).

Organic fertilization was based on substrate properties and biofertilizer composition (Table 2). Because F1 and F2 had lower nutrient concentrations per unit volume than F3, they were applied at a higher weekly rate (2.0 L pot⁻¹ week⁻¹, split into 500 mL applications over the week), whereas the more concentrated F3 was applied at 500 mL pot⁻¹ week⁻¹. To determine nutrient supply from each source, a balance was calculated between the initial nutrients provided by the substrate, the input from the biofertilizer, and the crop’s nutritional requirements. Based on this estimate, the need for mineral and organic supplementation was determined to meet the N, P, and K nutritional demands per plant.

Table 3. Summary of the nutritional management adopted in the experiment.

Nutrient Supply Estimate
Chemical Characteristics	Nutrients
	N	P2O5	K2O	Ca2+	Mg2+
Substrate (g kg−1)	0.25	0.06	0.27	1.80	0.60
Crop recommendation (g plant−1)	4.0	8.0	6.0	-	-
Supplemental mineral nutrition (g plant−1)	3.75	7.94	5.73	-	-
Organic supplementation	N	P2O5	K2O	Ca2+	Mg2+
Quantity of biofertilizer applied	8 (L plant−1)
Nutrient content—F1 (g L−1)	0.6	1.25	0.86	0.13	0.06
Total nutrients supplied (g plant−1)	4.8	10	6.88	1.04	0.48
Nutrient content—F2 (g L−1)	1.1	1.03	1.43	0.02	0.60
Total nutrient supplied (g plant−1)	11.0	8.24	11.44	0.16	4.8
Quantity of biofertilizer applied	2.5 (L plant−1)
Nutrient content—F3 (g L−1)	1.6	4.94	0.86	0.49	0.12
Total nutrients supplied (g plant−1)	4.0	12.35	2.15	1.22	0.30

Ca and Mg values represent supply only, as no fixed crop demand target was established for these nutrients. Sodium content of the biofertilizers was not analyzed. F1—CAGECE’s HomeBiogas liquid biofertilizer; F2—HomeBiogas liquid biofertilizer from food waste; F3—shrimp biofertilizer.

summarizes the nutritional management adopted.

Saline irrigation water (3.0 dS m⁻¹) was prepared using NaCl, CaCl₂·2H₂O, and MgCl₂·6H₂O in a 7:2:1 ratio, following the relationship between electrical conductivity and ionic concentration (mmol L⁻¹ = EC × 10) [20]. Regarding the ionic composition of the irrigation waters, samples of the supply water (0.8 dS m⁻¹) and the saline solution (3.0 dS m⁻¹) were analyzed at the Soil, Water, Plant Tissue, and Fertilizer Analysis Laboratory of the Department of Soil Science at UFC (

Table 4. Chemical characteristics of the irrigation waters.

Water	pH	EC	Ca2+	Mg2+	K+	Na+	Cl−	HCO3−	SAR	Classification
-	-	dS m−1	mmolc L−1						(mmol L−1)−0.5	-
Supply	6.9	0.8	1.2	1.6	0.1	0.7	3.4	0.6	0.42	C2S1
Brackish	6.5	3.0	8.1	2.5	0.1	25.8	33.8	2.4	11.21	C4S3

Water—water source (Supply: irrigation water supply, 0.8 dS m⁻¹; Brackish: prepared saline solution, 3.0 dS m⁻¹); EC—electrical conductivity; SAR—sodium adsorption ratio; classification according to the USSL/Richards system for irrigation water salinity (C) and sodicity (S) hazard.

).

Irrigation with saline water started 15 days after sowing (DAS) and continued until harvest (80 DAS). Irrigation was applied daily, maintaining soil moisture near field capacity, with a leaching fraction of 0.15 [21], determined according to the drainage lysimeter method [22]. The irrigation volume was calculated as:

$$VI={\displaystyle \frac{\left(Vp -Vd\right)}{\left(1-LF\right)}}$$

(1)

where:

VI—volume of water to be applied in the irrigation event (mL);

Vp—volume of water applied in the previous irrigation event (mL);

Vd—volume of water drained (mL);

LF—leaching fraction of 0.15;

2.4. Measured Variables

At 45 DAS, physiological variables were measured: CO₂ assimilation rate (A, µmol CO₂ m⁻² s⁻¹), stomatal conductance (gs, mol H₂O m⁻² s⁻¹), transpiration (E, mmol m⁻² s⁻¹), internal CO₂ concentration (Ci, µmol mol⁻¹), and leaf temperature (LT, °C). Leaf gas exchange was measured between 08:00 and 10:00 h on fully expanded leaves using a portable open-system infrared gas analyzer (ADC BioScientific LCi Analyser, ADC BioScientific Ltd., Hoddesdon, UK). Measurements were taken under ambient greenhouse conditions, with photosynthetic photon flux density ranging from 79 to 482 µmol m⁻² s⁻¹ (mean 229.1 ± 139.8), reference CO₂ concentration ranging from 393 to 404 µmol mol⁻¹ (mean 396.5 ± 2.2), and leaf chamber temperature ranging from 33.5 to 36.1 °C (mean 34.7 ± 0.7).

At 80 DAS, yield components were evaluated: number of capsules per plant (NC), determined by counting all capsules per plant; capsule diameter (CD, mm) and capsule length (CL, mm), measured with a digital caliper (Davely, 150 mm) on five capsules randomly selected per plant and averaged; mean capsule mass (MCM, g), determined by weighing all capsules per plant on an analytical balance (Mylabor, São Paulo, Brazil, precision 0.0001 g) and dividing by the total number of capsules; and yield (Y, g plant⁻¹), determined by weighing seeds from all capsules per plant.

2.5. Statistical Analysis

Data normality was assessed using the Kolmogorov–Smirnov test (p ≤ 0.05). Data were subjected to a two-way analysis of variance (ANOVA) based on a fixed-effects model, with fertilization strategy, salinity level, and their interaction treated as fixed factors, using the F-test (p ≤ 0.05). When a significant effect was detected, means were compared using Tukey’s HSD test (p ≤ 0.05): lowercase letters were used to compare fertilization strategies within the same salinity level, and uppercase letters were used to compare salinity levels within the same fertilization strategy. Pearson’s correlation analysis was performed to evaluate relationships among variables, using the corr_coef function of the “metan” package (version 1.19.0) [23]. All analyses were conducted using R software (version 4.1.1) [24].

3. Results

3.1. Physiological Variables

The analysis of variance revealed a significant interaction between fertilization strategies and salinity for CO₂ assimilation (p ≤ 0.05), stomatal conductance, and transpiration (p ≤ 0.01), indicating that the physiological response of sesame to salinity depends on the fertilization approach. No significant effects were observed for internal CO₂ concentration and leaf temperature (

Table 5. Summary of the analysis of variance for gas exchange in sesame plants irrigated with brackish water under different fertilization methods.

SV	DF	Mean Squares
		A	gs	E	Ci	LT
		(µmol CO2 m−2 s−1)	(mol H2O m−2 s−1)	(mmol m−2 s−1)	(µmol mol−1)	(°C)
Fertilization methods (F)	3	118.27 **	0.045 **	2.35 **	1792.10 ns	0.521 ns
Salinity (S)	1	14.33 ns	0.001 ns	0.01 ns	338.00 ns	0.551 ns
F × S	3	20.10 *	0.014 **	1.14 **	1719.25 ns	0.141 ns
Residue	24	6.53	0.001	0.232	2652.92	0.537
CV (%)	-	14.95	10.43	13.96	23.74	2.12

SV—source of variation; DF—degrees of freedom; CV—coefficient of variation; A—CO₂ assimilation rate; gs—stomatal conductance; E—transpiration; Ci—intercellular CO₂ concentration; LT—leaf temperature; **, * and ns—significant at p ≤ 0.01, p ≤ 0.05, and not significant, respectively (F test).

).

CO₂ assimilation was not affected by salinity under F1, F2, and F3, whereas a reduction was observed under F4 when irrigated with saline water (Figure 3A), indicating greater sensitivity of mineral fertilization to salt stress.

For stomatal conductance, F1 and F2 showed increases of 18% and 52%, respectively, under higher salinity, while F4 exhibited a 32% reduction (Figure 3B). These results indicate that biofertilizers favored the maintenance of stomatal activity under saline conditions, whereas mineral fertilization was negatively affected. No significant differences were observed between salinity levels under F3.

Transpiration did not differ between salinity levels under F1 and F3, whereas F2 showed higher values under saline conditions and F4 showed a reduction (Figure 3C), reinforcing the differential effect of fertilization strategies on plant water regulation.

3.2. Yield Variables

The analysis of variance indicated a significant interaction between fertilization strategies and salinity for number of capsules, mean capsule mass (p ≤ 0.01), and capsule diameter (p ≤ 0.05), demonstrating that these yield components are influenced by the combination of both factors. Capsule length and yield showed isolated effects of fertilization and salinity (

Table 6. Summary of the analysis of variance for yield components in sesame plants subjected to brackish water irrigation under different fertilization methods.

SV	DF	Mean Squares
		NC	MCM	CD	CL	Y
		-	(g)	(mm)	(mm)	(g plant−1)
Fertilization methods (F)	3	529.25 **	914.65 **	3.98 **	12.62 *	10.43 **
Salinity (S)	1	4371.12 **	5511.71 **	0.04 ns	23.22 *	140.49 **
F × S	3	199.70 **	444.05 **	1.20 *	6.32 ns	1.47 ns
Residual	24	13.08	9.883	0.33	2.97	0.76
CV (%)	-	12.69	11.21	6.75	6.26	7.04

SV—source of variation; DF—degrees of freedom; CV—coefficient of variation; NC—number of capsules; MCM—mean capsule mass; CD—capsule diameter; CL—capsule length; y—Yield; **, * and ns—significant at p ≤ 0.01, p ≤ 0.05, and not significant, respectively (F test).

).

The number of capsules decreased under higher salinity in all treatments, with reductions of 60, 40, 77, and 48% for F1, F2, F3, and F4, respectively (Figure 4A), indicating high sensitivity of this variable to salt stress. Mean capsule mass was higher under lower salinity across all fertilization strategies (Figure 4B), confirming the consistent negative effect of salinity on capsule filling.

Capsule diameter was affected by salinity only under F2, with a 10% reduction (Figure 4C), suggesting lower sensitivity of this trait compared to other yield variables.

Capsule length was higher under F1 and F4 (Figure 5A) and reduced by 6% under higher salinity (Figure 5B), indicating the moderate effect of salt stress on this parameter.

Yield was lower under F2 compared to the other treatments (Figure 6A), while salinity reduced overall productivity by 28% (Figure 6B), confirming the negative impact of saline irrigation on sesame performance.

3.3. Pearson Correlation

The correlation analysis revealed strong positive associations among yield variables. Mean capsule mass was strongly correlated with the number of capsules (r = 0.92 ***), consistent with the significant fertilization × salinity interaction detected for both traits by the two-way ANOVA (Table 6). Yield was positively correlated with both number of capsules (r = 0.89 ***) and mean capsule mass (r = 0.83 ***); although yield itself responded to fertilization and salinity as isolated main effects rather than through their interaction (Table 6), its strong association with these interaction-driven components indicates that yield formation is ultimately shaped by traits that are jointly regulated by fertilization strategy and salinity level.

Among physiological variables, CO₂ assimilation was strongly and positively correlated with both stomatal conductance (r = 0.82 ***) and transpiration (r = 0.85 ***), while stomatal conductance and transpiration were also strongly correlated with each other (r = 0.81 ***), reflecting the tight physiological coupling that governs gas exchange under stomatal control: as stomata open or close in response to salinity and fertilization treatments, CO₂ diffusion into the leaf and water vapor loss respond proportionally, since both processes share the same diffusive pathway through the stomatal pore.

This physiological coordination extended to yield formation: the number of capsules was positively correlated with stomatal conductance (r = 0.41 *), CO₂ assimilation (r = 0.49 **), and transpiration (r = 0.37 *), indicating that treatments capable of sustaining gas exchange under saline conditions also translated into more favorable yield outcomes (Figure 7).

4. Discussion

Salinity negatively affected gas exchange and productivity in sesame (Sesamum indicum L.), although the magnitude of these effects depended on the fertilization strategy, as evidenced by the significant interaction between salinity and fertilization for physiological variables. The reduction in CO₂ assimilation under saline conditions, particularly under mineral fertilization (F4), indicates impaired photosynthetic performance, mainly associated with stomatal limitation—an adaptive response that minimizes water loss but consequently restricts CO₂ diffusion into the substomatal cavity [25]. Similar responses have been widely reported in sesame and other crops, confirming the sensitivity of photosynthetic processes to osmotic stress [26,27].

In contrast, the maintenance of CO₂ assimilation and stomatal conductance under biofertilizer treatments (F1 and F2) suggests a mitigating effect of organic inputs on salt-induced physiological constraints. This response is likely associated with improvements in soil physicochemical properties, enhanced nutrient availability, and activation of plant defense mechanisms [28,29]. Additionally, the presence of humic substances may promote root development and water uptake efficiency, supporting physiological activity under stress conditions [30]. This pattern is consistent with findings in sesame under non-saline conditions, where the slower nutrient release of an organomineral biofertilizer favored net CO₂ assimilation during the flowering stage compared to mineral fertilization, despite not surpassing it in yield components, reinforcing that the physiological benefits of organic sources may be more closely tied to the timing and gradual availability of nutrients than to nutrient content alone [31].

The higher stomatal conductance observed under salinity in F1 and F2 indicates a distinct physiological adjustment compared to mineral fertilization, suggesting improved stomatal functionality and sustained gas exchange capacity. Similar responses have been reported in peanut under saline irrigation with organic fertilization [10,13], reinforcing the role of organic inputs in enhancing tolerance to abiotic stress. Likewise, smaller reductions in stomatal conductance have been observed in sesame genotypes under salt stress, indicating a potential tolerance mechanism to brackish water use [8].

The divergent physiological responses among biofertilizers can be more directly linked to their contrasting nutrient supply profiles (Table 3) than to organic amendment per se. F2 delivered potassium far in excess of crop demand (191% of the estimated requirement), F1 supplied potassium moderately above demand (115%), whereas F3 supplied only 36% of the potassium demand—despite supplying nitrogen at the exact crop requirement (100%) and phosphorus in the largest excess among all treatments (154%). Given the central role of K⁺ in stomatal guard cell osmoregulation and in competing with Na⁺ for root uptake sites [32], this graded potassium availability—highest in F2, moderate in F1, and markedly restricted in F3—is the most plausible driver of the differential stomatal conductance sustained under salinity (Figure 3B). Notably, the fact that F3 received adequate-to-excess nitrogen and phosphorus yet still showed the weakest stomatal response reinforces potassium, rather than overall nutrient sufficiency, as the key limiting factor. Sodium content of the biofertilizers was not determined in this study; therefore, biofertilizer-derived sodium cannot be entirely ruled out as a contributing factor to these differential responses, alongside the potassium-driven mechanism proposed above.

These results reinforce a broader principle of organic fertilization: the benefits attributed to biofertilizers are not a uniform or guaranteed outcome of using organic inputs but depend heavily on the specific raw material, nutrient profile, and mode of action of each source. This heterogeneity has also been documented outside the context of salinity. In an evaluation involving multiple crops and the use of insect waste as organic fertilizer, the magnitude and consistency of the response varied considerably across crops, application rates, and soil fertility conditions, with no simple dose-dependency pattern observed; this highlights that organic fertilizer performance is highly source- and context-specific, and not intrinsically superior or inferior to mineral fertilization [33]. In this regard, the present findings indicate that the choice of biofertilizer raw material—rather than the generic use of organic fertilization—is the decisive factor in determining whether physiological and productivity benefits will be achieved under saline conditions.

These findings align with international evidence on biofertilizer-mediated salt tolerance across oilseed crops. In sesame itself, biofertilization with the phosphate-solubilizing bacterium Rhodopseudomonas spp. improved nitrogen and phosphorus uptake, growth, and yield under salt-affected soil conditions, supporting the broader role of microbial biofertilizers in sustaining nutrient acquisition under salinity [34]. In peanut, halotolerant plant growth-promoting rhizobacteria enhanced nitrogen content and maintained ionic homeostasis under saline conditions, reducing reactive oxygen species accumulation [35]. In sunflower, the co-application of Bacillus subtilis and biogas slurry improved the potassium-to-sodium ratio and physiological performance under salinity [36], a mechanism consistent with the potassium-driven stomatal responses observed for F1 and, more markedly, F2 in the present study. However, contrasting results have also been reported: microbial inoculation effects on salinity tolerance vary considerably with feedstock composition, plant species, and salinity intensity, which may explain why F3 (shrimp-derived biofertilizer, rich in phosphorus but markedly low in potassium relative to the other sources) did not replicate the stomatal protection observed in F1 and F2.

Regarding transpiration, the stability observed in F1 and F3 further supports the buffering effect of organic fertilization on plant water relations, whereas the reduction observed under mineral fertilization (F4) indicates greater sensitivity to osmotic stress, limiting water flux and gas exchange, in agreement with previous findings [37,38]. This physiological stability may also be related to gradual nutrient release and improved osmotic adjustment capacity under organic inputs [30].

Salinity significantly reduced both the number and mass of capsules, confirming its strong impact on the reproductive development of sesame. The number of capsules was the most affected variable, indicating high sensitivity of the reproductive phase to saline stress, as previously reported [39,40]. This effect may be associated with the reduction in soil water potential caused by salt accumulation, which increases the energetic cost required to maintain metabolic activities, ultimately leading to the formation of smaller reproductive structures [41]. Additionally, these reductions are linked to impaired flower formation and increased reproductive abortion under stress conditions.

A similar reduction in the number of capsules under increased salinity was reported in sesame cultivation [39]. Furthermore, similar effects were also observed in peanut plants under saline stress, in which a reduction in capsule length was reported [42]. Although biofertilizers showed comparable performance to mineral fertilization at low salinity—particularly F1 and F3—which reinforces their potential as substitutes under favorable conditions, the lower efficiency of F3 at high salinity highlights the importance of selecting appropriate organic sources, since differences in composition directly influence plant responses [43]. The similarity between biofertilizers is possibly associated with the presence of microorganisms in their composition, which can increase the availability of nutrients through biological processes such as nutrient solubilization and the production of metabolites that promote plant growth and fruit development [44,45].

Strong positive correlations among yield components reinforce their interdependence, with the number of capsules emerging as a key determinant of capsule mass and overall productivity [46]. Similar relationships have been reported in sesame, including strong correlations between yield and number of capsules (0.90 ***) and between capsule traits and seed formation [47]. Salinity also negatively affected capsule mass, reflecting limitations in assimilate allocation and reproductive sink strength [48]. Reductions in yield components under saline conditions have been widely documented not only in sesame [39,40] but also in other oilseed crops such as peanut [5], sunflower [49], and cotton [50], indicating a generalized sensitivity of reproductive processes to salt stress. The reduction in capsule diameter under salinity, particularly in F2, further reflects impaired water and nutrient uptake due to osmotic constraints [48]. Although plants may attempt to reallocate assimilates to reproductive structures under stress, this compensatory mechanism becomes insufficient under higher salinity levels [51].

The positive correlations between physiological variables (gs, A, and E) and the number of capsules indicate that the maintenance of gas exchange under saline conditions is closely linked to reproductive performance. Higher photosynthetic rates likely enhance assimilate supply to reproductive organs, supporting capsule formation and development [52]. These relationships are also governed by source–sink dynamics, in which salinity-induced limitations in assimilate transport can impair reproductive development [53]. Stomatal regulation plays a central role in this process, as stomatal opening directly controls both CO₂ assimilation and transpiration [54]. While positive correlations between stomatal conductance and CO₂ assimilation have been reported in Cucumis anguria [55], contrasting negative correlations among gas exchange variables have also been observed in sesame under stress conditions [56], indicating that environmental factors can alter the coordination of physiological processes.

The strong positive correlations between gas exchange variables and the number of capsules (Section 3.3) indicate that salinity constrains reproductive development largely through its impact on source activity. Under salt stress, stomatal closure limits CO₂ diffusion and reduces net photosynthesis, decreasing the pool of photoassimilates available for translocation to reproductive sinks; concurrently, salinity can impair phloem loading and long-distance sucrose transport, further restricting assimilate delivery to developing capsules [53]. This source limitation is compounded by increased maintenance respiration costs under osmotic and ionic stress, which divert carbon away from reproductive structures [41]. Biofertilizers such as F1 and F2, by sustaining stomatal conductance and CO₂ assimilation under salinity, likely preserved a larger photoassimilate pool and more stable source-to-sink flux, consistent with their comparatively higher capsule numbers and mean capsule mass under high salinity. In contrast, the limited source activity under F4 (mineral fertilization) and F3 under high salinity likely constrained sink filling capacity, reinforcing that biofertilizer-driven physiological maintenance operates primarily by preserving source strength rather than by directly enhancing sink development. Interestingly, this decoupling between physiological performance and yield has not always been observed: in sesame grown without saline stress, an intermediate biofertilizer dose achieved the highest CO₂ assimilation rate without a corresponding improvement in yield components [31], contrasting with the direct translation of physiological maintenance into yield benefits observed for F1 and F2 in the present study. This discrepancy may indicate that source–sink coupling becomes more physiologically decisive specifically under stress conditions, when assimilate supply is a limiting factor for reproductive development, rather than under favorable growing conditions where sink capacity itself may be the primary constraint.

Overall, these findings demonstrate that biofertilizers represent a promising strategy to improve sesame performance under both non-saline and saline conditions, contributing to more resilient agricultural systems in semi-arid regions. The use of organic residues—including food waste, shrimp processing waste, and sanitation-derived inputs—reinforces the role of nutrient recycling in sustainable agriculture [32,51,57]. In particular, the biofertilizer derived from CAGECE HomeBiogas highlights the potential integration of sanitation systems into circular production models, reducing dependence on mineral fertilizers while enhancing crop resilience under abiotic stress conditions.

5. Conclusions

A salinity level of 3.0 dS m⁻¹ impaired gas exchange and sesame productivity, confirming the sensitivity of physiological and reproductive processes to salt stress. However, the magnitude of these effects depended on the fertilization strategy; the efficacy of each biofertilizer was linked primarily to its nutrient supply, particularly potassium availability. Fertilization treatments F1 and F2, which supplied potassium at levels meeting or exceeding crop demand, demonstrated a greater capacity to maintain CO₂ assimilation and stomatal conductance under saline conditions; conversely, treatment F3—characterized by limited potassium supply despite adequate phosphorus levels—showed a more restricted physiological response.

Under low-salinity conditions, biofertilizers derived from CAGECE waste (F1) and shrimp waste (F3) yielded productivity comparable to mineral fertilization (F4), highlighting their potential as sustainable fertilization alternatives. However, under higher salinity, treatment F3 resulted in significant reductions in capsule number and mass, indicating lower efficiency in mitigating salt stress under more severe conditions.

Overall, these results demonstrate that biofertilizer efficacy under saline irrigation depends on the raw material used—meaning success is not guaranteed simply by organic fertilization—and that the proper selection of biofertilizer sources can enhance sesame performance, contributing to more sustainable production systems in semi-arid regions.