Foliar silicon enhances drought resilience and productivity in buckwheat by stabilizing photosynthetic performance

Abstract

Drought stress is a major constraint to buckwheat (Fagopyrum esculentum Moench) productivity, primarily by impairing photosynthesis, growth, and reproductive output. This study investigated foliar-applied silicon (Si; 0.5, 1.0, and 2.0 mM) in mitigating drought stress. Drought reduced growth traits and seed yield, with decreased CO₂ assimilation, electron transport rate (ETR), Photosystem II efficiency, and relative water content (RWC), while electrolyte leakage (EL) increased. The OJIP transient showed reduced maximum fluorescence (F_m), indicating photoinhibition. Additionally, the difference kinetics (ΔW) analysis revealed over-reduction of the plastoquinone (PQ) pool, restricted Q_A-Q_B electron transport, and partial PSI acceptor limitations under drought. Foliar Si, particularly at 1.0 mM, alleviated these effects by maintaining PSII function, stabilizing ETR, reducing PQ over-reduction, and improving light use. Si enhanced water relations by lowering EL and sustaining RWC, supporting cellular stability. Consequently, 1.0 mM Si restored biomass and seed yield to levels comparable to well-watered plants, whereas 0.5 and 2.0 mM Si provided partial mitigation. Following re-irrigation, most parameters partially or fully recovered, highlighting buckwheat resilience. The results indicate that Si acts as a physiological modulator integrating photochemical protection, water retention, and growth stabilization, with optimal concentration crucial for maximizing drought resilience and productivity.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-53286-w.

Introduction

Climate change, characterized by rising temperatures, altered precipitation patterns, and an increasing frequency of extreme weather events, has emerged as a major constraint on global agricultural productivity, causing substantial direct and indirect economic losses^1–4. Sudden drought events affect crop yields for approximately 55 million people globally, causing annual losses of roughly US$5.4 billion⁵. Recent climatic extremes highlight the severity of this challenge, with multiple instances of unprecedented droughts in Europe, including the exceptionally dry summer of 2022⁶. The combination of prolonged heatwaves and exceptional water scarcity resulted in widespread agricultural losses, including an estimated 10% reduction in cereal production across the European Union during both 2022 and 2023⁷.

Buckwheat (Fagopyrum esculentum Moench), a fast-growing annual species of the Polygonaceae family, is cultivated worldwide for its nutritionally rich pseudocereal grains and its multifunctional role as a summer cover crop. Owing to its short growth cycle (10-12 weeks), rapid establishment, and efficient utilization of limited soil nutrients, buckwheat is well suited for cultivation on marginal lands where conventional cereals often fail^8–10. In addition, buckwheat flowers are highly attractive to pollinators, which contributes positively to seed setting and yield stability¹¹. In many regions of Eastern Europe and Asia, buckwheat remains an economically and ecologically important crop supporting rural livelihoods and diversified farming systems, serving as a bridge between agricultural production and environmental sustainability¹². Studies have shown that buckwheat grown as a full-season crop develops a higher leaf area index, produces more flower clusters, forms better-developed seeds, and achieves up to 42% higher yield compared to buckwheat cultivated as a stubble crop¹³.

The buckwheat genotype “Smuga” is a short-season, fast-maturing Polish cultivar of common buckwheat, registered in 2019, developed for temperate regions with limited growing periods, and characterized by high yield potential, enhanced lodging resistance, and greater drought tolerance compared to other Polish varieties such as Kora and Panda¹⁴. Its reported responsiveness to foliar nutrient inputs makes “buckwheat” a suitable model for evaluating strategies aimed at improving stress resilience through external applications¹⁵.

Despite these advantages, drought stress remains a major constraint to buckwheat productivity^16,17, as it induces osmotic imbalance and disrupts membrane stability, increasing metabolite leakage and potentially causing cell death¹⁸. Additionally, water deficits reduce soil moisture availability, lowering leaf water potential and relative water content (RWC)^19,20.

Drought induces abscisic acid (ABA) production, causes stomatal closure to conserve water, thereby restricting CO₂ diffusion into the leaf and, together with non-stomatal limitations, suppressing net photosynthetic rate, and ultimately reducing carbon availability for growth²¹. Declines in chlorophyll fluorescence (OJIP transient curve), photosystem II efficiency (ΦPSII), electron transport rate (ETR), and chlorophyll stability are commonly observed under drought, reflecting damage to the photosynthetic machinery and increased susceptibility to oxidative stress¹⁴. At the cellular level, dehydration disrupts the metabolic homeostasis, ultimately reducing biomass accumulation and reproductive development²². Buckwheat is particularly sensitive to water deficits during the flowering, a stage that accounts for most of its total water demand, when moisture limitation leads to poor pollination, reduced seed setting, and sharp yield declines²³. Drought stress compromise stomatal regulation, enhance oxidative damage, accelerate leaf senescence, and weaken the plant’s capacity to maintain physiological stability, resulting in lower grain quality, and significant economic losses^21,24,25.

Silicon (Si), although not classified as an essential plant nutrient, is the second most abundant element in the Earth’s crust and has been widely recognized for its beneficial role in enhancing plant tolerance to abiotic stresses^26,27. Si accumulation strengthens cell walls, enhances root development, and improves water uptake, while its rapid absorption through the leaf cuticle, stomata, and trichomes allows Si to quickly support physiological mechanisms that mitigate stress²⁸. Under drought stress, Si enhances plant water regulation by stimulating ABA accumulation and controlled stomatal closure to limit transpirational water loss, while Si application further improve leaf pigment content, photosynthetic performance, stomatal function, hydraulic conductance, transpiration stability, and leaf turgor, thereby strengthening overall drought tolerance^29–32. In addition, recent studies suggest that auxin plays a role in coordinating growth and stress adaptation under drought, highlighting the importance of hormonal crosstalk in regulating plant responses. In this context, the beneficial effects of Si may be associated with modulation of hormone-mediated pathways, including ABA and auxin signaling³³. Recent studies indicate that silicon-mediated drought tolerance is closely associated with hormonal crosstalk, particularly involving ABA and auxin signaling pathways that regulate stomatal behavior and stress adaptation^34,35. Moreover, Si modulates antioxidant defence responses by reducing oxidative damage to cellular membranes and chloroplasts, thereby protects the photosynthetic machinery³⁶. In addition, Si has been reported in tomato to maintain the integrity of thylakoid membranes, protect photosystem II activity, and sustain higher photochemical efficiency by stabilizing chloroplast ultrastructure and reducing reactive oxygen species (ROS) accumulation³⁷. These coordinated effects contribute to improved biomass production, physiological resilience, and yield performance under drought stress³⁸.

Despite growing evidence for the beneficial role of Si in enhancing drought tolerance, the concentration-dependent effectiveness of foliar-applied Si and its influence on photosynthetic performance, biomass accumulation, and yield recovery during progressive drought and post-stress rehydration remain insufficiently understood. This knowledge gap is particularly relevant for buckwheat, a short-season crop often cultivated under rainfed and marginal conditions where water availability is highly variable. Therefore, the present study is aimed to evaluate the effects of different foliar Si concentrations on plant physiology, growth, and yield parameters of buckwheat under controlled drought stress and subsequent recovery. By integrating physiological, morphological, and yield-related responses, this study tests the following hypotheses: (H1) foliar Si improves drought tolerance in a non-linear, concentration-dependent manner; (H2) Si-mediated protection operates through combined effects on photochemistry (PSII stability) and water relations and (H3) an optimal Si concentration (1.0 mM) enables physiological and yield recovery comparable to well-watered conditions while minimizing potential negative effects under non-stress conditions.

Material and method

Plant material and growth conditions

Seeds of Fagopyrum esculentum Moench (cv. ‘Smuga’; supplier: MHR Kraków, Poland; www.mhr.com.pl, Logistic Sp. z o.o.) were rinsed under tap water and surface-sterilized in 1% (v/v) sodium hypochlorite (NaClO) for 5 min, followed by three rinses with deionized water (3 × 50 mL). Seeds were placed on Petri dishes lined with sterile paper towels moistened with 5 mL sterile water and incubated in darkness at 25 ± 1 °C for 72 h until radicle emergence (seed soaking and germination: 27-30 June 2025). Germinated seedlings were transplanted into pots of approximately 1 L volume (height 150 mm, diameter 125 mm) filled with a peat-based substrate (200 g weight), which, due to its low bulk density, fully occupied the container. After initial establishment seedlings were transferred to semi-controlled glasshouse conditions under natural daylight, with an average day/night temperature of 30/18 °C and relative humidity of 65-75%. Seedlings were thinned to one uniform plant per pot at 10 Days after Sowing (10 DAS).

Experimental design and treatments

Uniform plants were selected 15 DAS and assigned to eight treatment groups, each consisting of five biological replicates (n = 5), where each replicate represented an individual plant. Plants were spatially randomized to minimize positional effects. Two irrigation regimes were established (i) a well-watered control, maintained at 80-90% field capacity (FC), and (ii) a drought treatment, maintained at 30% FC by daily gravimetric monitoring and rewatering to a predetermined target weight. FC (W_wet) was determined by saturating the substrate and allowing excess water to drain freely, while W_dry represents the dry weight of the substrate. Target pot weights (W_target) corresponding to 80–90% (well-watered) and 30% (drought) substrate moisture were calculated as: W_target = W_dry + f × (W_wet − W_dry), where f = 0.80–0.90 for well-watered conditions and f = 0.30 for drought. Pots were weighed daily and water was added to maintain the desired moisture level.

Drought stress was initiated at 23 days after sowing by withholding irrigation and maintained until 40 DAS, resulting in a total drought duration of 17 days, after which drought-stressed plants were re-irrigated to field capacity for recovery.

Si was supplied as sodium silicate (Na₂SiO₃·9H₂O; Merck KGaA, Darmstadt, Germany) and applied as a foliar spray at three concentrations (0.5, 1.0, and 2.0 mM) for both irrigation regimes. This resulted in the following treatments: well-watered (WW), drought-stressed (DS), WW + 0.5 mM Si, WW + 1.0 mM Si, WW + 2.0 mM Si, DS + 0.5 mM Si, DS + 1.0 mM Si, and DS + 2.0 mM Si. Si solutions were freshly prepared and applied twice during early vegetative growth, with the first application coinciding with the initiation of drought stress (21 DAS) and the second application administered one week later (28 DAS). Each plant received approximately 10 mL of spray solution using a hand-held sprayer, applied between 09:00 and 10:00 h until uniform wetting of the leaf surface. No surfactant was added. Plants in the non-Si control treatments received distilled water sprays.

All plants were grown under semi-controlled glasshouse conditions throughout the experiment. Measurements were taken at four time points: 25 DAS during early drought stress, 32 DAS (29 July 2025) during progressive drought stress, 39 DAS at peak or prolong drought stress, and 48 DAS during the recovery phase, nine days after re-irrigation. After recovery, plants were maintained under normal irrigation conditions until maturity and were harvested at 112 DAS (17 October 2025) for biomass and yield determination (Fig. 1). For seed weight determination, a standardized subsample of 50 seeds was used to ensure uniformity and comparability across treatments, as the minimum seed number per treatment (under drought) exceeded this threshold (~60 seeds).

Fig. 1.

Experimental timeline and measurement scheme. The diagram illustrates seed soaking, germination, vegetative growth, silicon application, progressive drought stress (initial, progressive, and prolonged phases), and post-drought re-irrigation. DAS indicate days after sowing and, respectively, with measurement points indicated throughout the experiment.

Measurement of plant growth and leaf morphological traits

Plant height, stem diameter, and leaf traits were measured weekly at four time points for all plants under both control and drought treatments. Plant height was recorded from the soil surface to the apex of the main stem using a ruler or measuring tape, while stem diameter was measured 1 cm above the soil surface with a digital Vernier caliper. Leaf area was assessed from the top, third fully expanded leaf using images captured against a white background with a scale and analyzed with ImageJ³⁹, and leaf number per plant was recorded at each sampling. Phenotypic changes were documented weekly using standardized digital photographs taken at a fixed distance and angle under uniform lighting.

Measurement of electrolyte leakage and relative water content

Electrolyte leakage (EL) and relative water content (RWC) were assessed after two weeks of growth under stress conditions. For EL determination, 10-mm leaf discs were excised from plants in both treatments, immersed in 10 mL deionised water, and gently shaken at room temperature before measuring the initial electrical conductivity (C₁); samples were then heated at 100 °C for 1 h to release all electrolytes, cooled, and the final conductivity (C₂) was recorded, with EL (%) calculated as EL (%) = ⁴⁰.

RWC was measured using the fourth fully expanded leaf by recording fresh weight (FW), rehydrating leaves overnight to obtain turgid weight (TW), drying them at 55 °C for 72 h to determine dry weight (DW), and calculating RWC using the standard formula described by⁴¹: .

Gas exchange and chlorophyll fluorescence measurements

Gas-exchange parameters were measured using a GFS-3000 portable photosynthesis system (WALZ GmbH, Germany). Measurements were conducted on fully expanded leaves between 11:00 AM to 14:00 PM to avoid diurnal variability. The chamber conditions were standardized using a photosynthetic photon flux density (PPFD) of 1000 µmol m⁻² s⁻¹, supplied by the integrated LED light source (3057-FL, Walz, Germany). The reference CO₂ concentration was maintained at 400 ppm, relative humidity was controlled using a water vapor concentration of 17,000 ppm, and the flow rate was set to 750 µmol s^-1. Leaf temperature was allowed to equilibrate to ambient conditions and remained stable throughout each measurement. For each plant, gas-exchange parameters, including net photosynthetic rate (A), stomatal conductance (gs), intercellular CO₂ concentration (Ci), and electron transport rate (ETR), were recorded after the leaf reached steady state. Immediately following gas-exchange measurement, pulse-amplitude-modulated chlorophyll fluorescence was recorded from the same leaf area using the integrated fluorescence module. All measurements were completed within 3 h to minimize environmental variation.

Chlorophyll a fluorescence is a widely used technique for evaluating plant responses to abiotic stress, based on the measurement of light re-emitted by chlorophyll-a during photosynthesis. In this study, fluorescence was measured using a FluorPen (PSI, Czech Republic) following the manufacturer’s standard (OJIP) protocols. Measurements were taken four times at weekly intervals; fluorescence was recorded between 11:00 AM and 12:00 PM to maintain consistent diurnal conditions. For the dark-adapted protocol (OJIP), leaves were dark-adapted for 20 minutes using a dark clip before measurement. For each plant, fluorescence was measured on two leaves, and the mean value was used for analysis. Chlorophyll fluorescence parameters, including F₀, and F_m, were measured, and fast fluorescence kinetics (OJIP transients) were analyzed based on the O–J–I–P phases. Normalized fluorescence was calculated as (F_t/F₀), and relative variable fluorescence as V_t = (F_t – F₀) / (F_m – F₀) for construction of relative OJIP curves. OJIP phase increments (ΔW_OJ, ΔW_JI, and ΔW_IP) were derived according to standard PSI analysis procedures^42,43. Phase-specific analysis was performed using normalized variable fluorescence transients, where ΔW_OJ = (F_t − F₀)/(F_j − F₀), ΔW_JI = (F_t – F_j)/(F_i – F_j), ΔW_IP = (F_t – F_i)/(F_p – F_i), describing the kinetics of the O–J, J–I, and I–P phases, respectively. The parameter ΔW was calculated as the difference between initial and final normalized variable fluorescence values for each OJIP phase, expressed as ΔW = (W _initial - W _final) (Supplementary Table 1)^44.

Statistical analysis

Data were tested for normality and homogeneity of variance prior to analysis. The effects of irrigation regime (well-watered vs. drought), silicon concentration (Si- 0, 0.5, 1.0, and 2.0 mM), and their interaction were evaluated using two-way analysis of variance (ANOVA). In addition, for each variable, differences among individual treatment combinations were assessed using one-way ANOVA, in which each irrigation × Si combination was treated as an independent level. This approach allowed direct comparison among all experimental variants within a single model. When significant effects were detected (p < 0.05), post hoc multiple comparisons were performed using Tukey’s honestly significant difference (HSD) test. Results are presented as mean ± standard error (SE). Superscript letters derived from Tukey’s test indicate statistically significant differences among treatment combinations. For each trait, F-values and corresponding p-values from the two-way ANOVA are reported in the figures.

Pearson correlation analysis was performed using data from the prolonged drought stage to evaluate relationships among physiological, growth, and yield parameters. Correlation coefficients (r) and their significance (p-values) were calculated using pairwise complete observations. Significance levels are discussed as *p < 0.05, **p < 0.01, and ***p < 0.001. The analysis was performed using pooled data across all treatments (n = 5).

All statistical analyses were performed using (RStudio version 4.5.2 and jamovi version 2.7.6). Data are reported as mean ± standard error (SE).

Results

The primary aim of this study was to determine an effective foliar Si doses for drought stress amelioration in buckwheat. Drought stress occurs when water availability is insufficient to meet the physiological demand in plant, resulting in impaired parameters of photosystem II (PSII) efficiency, altered gas exchange, and overall growth inhibition. Here we aimed to evaluate the potential protective role of Si foliar spray in enhancing plant resilience to prolonged drought conditions.

Effects of drought and silicon on growth traits (stem diameter, plant height, and leaf area)

Stem diameter and plant height responded differently to drought stress and Si application over time. At the first measurement, stem diameter and plant height were comparable across all treatments, indicating uniform growth and no early effects of drought or Si application (Fig. 2a, e). From the second measurement onward, drought stress significantly reduced stem diameter and progressively limited plant height compared with well-watered plants (Fig. 2b-c, f-g). Under well-watered conditions, Si application had no significant effect on either trait. Under drought stress, however, 1.0 mM Si significantly alleviated the reduction in stem diameter, whereas 0.5 mM Si showed limited effectiveness and 2.0 mM Si produced an intermediate response (Fig. 2b-c). Similarly, Si-treated drought-stressed plants maintained greater plant height than untreated drought-stressed plants during the stress period (Fig. 2f-g). During the recovery phase, stem diameter in drought-stressed plants remained significantly lower than in well-watered plants; however, 1.0 mM Si fully restored stem diameter to levels comparable with well-watered plants, while 0.5 and 2.0 mM Si resulted in partial recovery (Fig. 2d). In contrast, following re-irrigation, plant height differences were determined solely by prior water regime, with all drought-exposed plants remaining significantly shorter than well-watered plants regardless of Si treatment, indicating a limited role of Si in post-drought height recovery (Fig. 2h).

Fig. 2.

Effects of drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM) on plant morphological traits in buckwheat plants following drought stress: (a-d) stem diameter, (e-h) plant height and (i-k) leaf area. Plant height and stem diameter were measured during initial drought, drought progress, prolonged drought, and after recovery by re-irrigation. Leaf area was measured only during initial drought, drought progress, and prolonged drought, and not during the recovery phase. Data represent means ± SE (n = 5). Different letters indicate significant differences among treatment combinations at each time point (p < 0.05), as determined by Tukey’s HSD test. F-values from two-way ANOVA for irrigation, silicon, and their interaction are presented in each panel. Significance levels are indicated as: ***p < 0.001, **p < 0.01, *p < 0.05.

The initial measurement depicted no significant differences in leaf area among treatments, including well-watered and drought-stressed plants (Fig. 2i). This lack of variation reflects the short duration of water withdrawal (2 days), during which sufficient soil moisture was still available and visible leaf wilting had not yet developed. With the progression of drought stress, leaf started wilting and the leaf area was markedly reduced in drought-stressed plants, whereas well-watered plants maintained stable leaf expansion. Under drought conditions, application of 1.0 mM Si effectively mitigated leaf area reduction, while 0.5 and 2.0 mM Si had limited effect (Fig. 2j). Whereas, under prolonged drought conditions, leaf area remained consistently higher in well-watered plants, including those receiving Si, compared with drought-stressed plants. Drought-stressed plants sprayed with Si maintained higher leaf area than drought-stressed plants without Si, with increase of approximately 10%, 21%, and 19% for 0.5, 1.0, and 2.0 mM Si, respectively (Fig. 2k).

Effects of drought and silicon on electrolyte leakage and relative water content

Electrolyte leakage and relative water content were assessed at the end of the stress cycle under peak drought conditions. Drought stress significantly increased EL%, indicating impaired membrane stability, whereas well-watered plants maintained low EL and Si application under non-stress conditions had no significant effect (Fig. 3a). Under drought stress, 1.0 mM Si significantly reduced EL compared with untreated drought-stressed plants, while 0.5 and 2.0 mM Si provided weak protection.

Fig. 3.

Effects of drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM) on leaf water status and membrane integrity in buckwheat plants: (a) relative water content (RWC%) and (b) electrolyte leakage (EL%). Measurements were conducted at prolonged drought stress to assess tissue hydration and stress-induced membrane stability. Data represent means ± SE (n = 5). Different letters indicate significant differences among treatment combinations at each time point (p < 0.05), as determined by Tukey’s HSD test. F-values from two-way ANOVA for irrigation, silicon, and their interaction are presented in each panel. Significance levels are indicated as: ***p < 0.001, **p < 0.01, *p < 0.05.

Drought stress also caused a significant reduction in RWC relative to well-watered plants, and Si application under well-watered conditions did not significantly influence RWC (Fig. 3b). Under drought conditions, foliar application of 1.0 mM Si significantly increased RWC compared with untreated drought-stressed plants, whereas 0.5 and 2.0 mM Si showed limited mitigation.

Photosynthesis and chlorophyll fluorescence responses to drought and silicon application

Stomatal conductance was similar among treatments at 25 DAS (Fig. 4a). As soil moisture declined to FC 30%, drought stress caused a significant reduction in gs compared with well-watered plants. Si application under well-watered conditions did not significantly affect gs. Under drought stress, 1.0 mM Si maintained numerically higher gs. During the second measurement (32 DAS), gs in plants treated with 1.0 mM Si was approximately 200% higher than in untreated drought-stressed plants, while 0.5 and 2.0 mM Si increased gs by ~8% and ~59%, respectively (Fig. 4b). In the third measurement (39 DAS), gs further declined across all treatments; however, 1.0 mM Si maintained gs ~248% higher than drought alone (Fig. 4c). After re-irrigation, gs recovered across treatments, with no significant differences observed (Fig. 4d).

Fig. 4.

Effects of drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM) on photosynthetic parameters in buckwheat plants: (a-d) stomatal conductance (gs), (e-h) net CO₂ assimilation rate (A), (i-l) intercellular CO₂ concentration (Ci), and (m-p) electron transport rate (ETR). Measurements were conducted during initial drought, drought progress, prolonged drought, and after recovery by re-irrigation. Data represent means ± SE (n = 5). Different letters indicate significant differences among treatment combinations at each time point (p < 0.05), as determined by Tukey’s HSD test. F-values from two-way ANOVA for irrigation, silicon, and their interaction are presented in each panel. Significance levels are indicated as: ***p < 0.001, **p < 0.01, *p < 0.05.

Net CO₂ assimilation (A) did not differ significantly among treatments at 25 DAS (Fig. 4e). At the 30 DAS and 39 DAS, drought stress caused a significant decline in A compared with well-watered plants. Si spray in well-watered plants, caused minor, non-significant changes in A. Under drought, 1.0 mM Si significantly improved A relative to drought stress alone (Fig. 4f, g). Following recovery, A values were restored and no significant differences among treatments were detected (Fig. 4h).

Intercellular CO₂ concentration (Ci) did not show significant differences among treatments during the 25, 39, and 48 DAS measurements (Fig. 4i-k). However, at the second measurement, corresponding to the initial phase of drought, Ci depicted significant diminution in all drought-treated plants compared to well-watered control, irrespective of Si application. (Fig. 4l).

ETR did not vary significantly among treatments at the 25 DAS, indicating no early effect of drought or Si application on photosynthetic electron transport (Fig. 4m). At the 32 DAS measurement, drought stress significantly reduced ETR compared to well-watered control. Under drought stress, 1.0 mM Si maintained ETR was comparable to well-watered plants, whereas 0.5 mM Si showed a significantly lower ETR and 2.0 mM Si exhibited an intermediate response (Fig. 4n). In the course of 39 DAS measurement, drought-induced more pronounced reductions in ETR, and Si application under drought, particularly at 1.0 mM, slightly mitigated the decline alone (Fig. 4o). Following re-irrigation, ETR fully recovered, and no significant differences were observed among treatments (Fig. 4p).

The OJIP fluorescence transient (F_t/F₀) showed no major differences among treatments at 25 DAS, indicating that PSII functionality was not affected at the early stage of drought (Fig. 5a). At 32 DAS, drought stress markedly altered the OJIP curve, with the strongest reduction observed at the P step (F_m) compared to well-watered plants, reflecting impaired maximum PSII fluorescence. Foliar Si spray in well-watered plants caused only minor changes in the OJIP transient. 1.0 mM Si effectively alleviated drought stress induced reduction at the P step, maintaining higher Fₜ/F₀ values relative to untreated drought-stressed plants, whereas 0.5 and 2.0 mM Si provided limited protection (Fig. 5b). At the 39 DAS measurement, drought-induced reductions at the P step became more pronounced, while Si application, particularly at 1.0 mM, partially preserved the OJIP transient, indicating improved PSII stability under prolonged drought (Fig. 5c). During the recovery phase, following re-irrigation, WW and Si-treated drought plants showed substantial recovery of the P-step, whereas DS and low-Si treatments exhibited incomplete recovery, indicating persistent impairment of PSII function in the absence of adequate Si supplementation (Fig. 5d).

Fig. 5.

Effects of drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM) on chlorophyll a fluorescence and PSII performance in buckwheat plants: (a-d) normalized fluorescence (F_t/F₀), during drought stress (initial, developing, and prolonged drought) and after re-irrigation. Data represent mean values ± SE (n = 5), and all values are expressed relative to well-watered control plants.

Double-normalized OJIP transients () revealed marked differences in electron transport kinetics among treatments. Drought-stressed plants exhibited elevated values during the I-P phase (1-10 s), indicating increased reduction of the intersystem electron transport chain and a limitation at the PSI acceptor side. This response reflects reduced electron sink capacity when in drought, leading to downstream electron accumulation. In contrast, WW plants maintained stable I-P kinetics, while Si-treated drought plants showed a reduced I-P rise, indicating improved electron utilization and partial alleviation of acceptor-side limitation (Supplementary Fig. S2a-d).

The difference kinetics analysis further revealed treatment-specific patterns of electron transport regulation. Phase-specific normalization of the OJIP transient (W_OJ​, W_JI, W_IP​) enabled detection of subtle alterations in electron transport dynamics not apparent from conventional fluorescence parameters. Difference kinetics (ΔW) values are expressed relative to the well-watered reference, which is zero by definition (Fig. 6a-i).

Fig. 6.

Effects of drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM) on phase-specific difference kinetics (ΔW) of the OJIP fluorescence transient in buckwheat plants: (a-d) ΔW_OJ, (F_t − F₀)/(F_j − F₀), (e-h) ΔW_JI, (F_t – F_j)/(F_i – F_j), and (i-l) ΔW_IP, (F_t – F_i)/(F_p – F_i). The ΔW values were calculated as the difference between each treatment and the well-watered control, representing relative changes in PSII electron transport during the O-J, J-I, and I-P phases. Measurements were recorded during progressive drought stress (initial, developing, and prolonged drought) and after re-irrigation. Data represent mean values ± SE (n = 5), with all values expressed relative to well-watered controls.

In the O-J phase (ΔW_OJ), drought-stressed plants showed positive values during the initial measurement (25 DAS), shifting to negative values (-0.023) during developing drought and further declining under prolonged drought (-0.055), reflecting progressive downregulation of PSII excitation. Well-watered plants with Si remained near zero initially and became slightly negative under prolonged drought, while drought-stressed plants supplied with Si were mostly positive early and became negative during stress, though less pronounced than in untreated drought plants. Values tended to recover toward the reference following re-irrigation, indicating partial restoration of PSII excitation (Fig. 6a-d).

In the J-I phase (ΔW_JI), all treatments showed positive values, reflecting transient over-reduction of the plastoquinone pool due to restricted electron transport beyond Q_A, representing a bottleneck between PSII and PSI under drought. Drought-stressed plants exhibited peaks of ~0.08 at 15 ms during developing drought, rising to 0.3 under prolonged drought and declining to 0.03 during recovery. Well-watered plants and drought-stressed plants with Si had smaller peaks (generally <0.1), with minor negative peaks in some Si treatments (-0.1 to -0.06), indicating transient Q_A re-oxidation. Si application reduced the amplitude of ΔW_JI​, suggesting improved Q_A-Q_B re-oxidation and partial alleviation of plastoquinone over-reduction (Fig. 6e-h).

In the I-P phase (ΔW_IP), initial measurements were positive across treatments, indicating minor PSI acceptor-side limitation. During developing drought, ΔW_IP remained positive but very low (0-0.02) in drought-stressed plants, whereas well-watered and Si-treated plants showed higher peaks. Under prolonged drought, ΔW_IP initially became negative (~-0.1) before rising to ~0.05, reflecting pronounced limitation at the PSI acceptor side. During recovery, well-watered plants with Si remained positive throughout the I-P phase, whereas drought-stressed plants with Si showed negative values, indicating delayed restoration of PSI electron acceptors. Overall, Si partially alleviated drought-induced limitation at the PSI acceptor side (Fig. 6i-l).

OJIP parameters: Drought stress significantly altered chlorophyll fluorescence characteristics, indicating impairment of the photosynthetic apparatus. The maximum quantum efficiency of PSII (Fv/Fm) did not differ significantly among treatments at the initial measurement. However, during the development and prolongation of drought stress, Fv/Fm was significantly reduced in drought-stressed plants compared to well-watered controls, reflecting photoinhibition of PSII. Foliar application of Si under drought conditions significantly alleviated this decline, with 1.0 mM Si maintaining Fv/Fm values comparable to well-watered plants (Fig. 7a-d).

Fig. 7.

Effects of drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM) on key OJIP-derived photosynthetic parameters in buckwheat plants: (a-d) maximum quantum yield of PSII (Fv/Fm), (e-h) performance index on absorption basis (PI_abs), (i-l) quantum yield of electron transport (ΦEo), and (m-p) quantum yield of energy dissipation (ΦDo). Measurements were recorded during progressive drought stress (initial, developing, and prolonged drought) and after re-irrigation. Data represent means ± SE (n = 4). Different letters indicate significant differences among treatment combinations at each time point (p < 0.05), as determined by Tukey’s HSD test. F-values from two-way ANOVA for irrigation, silicon, and their interaction are presented in each panel. Significance levels are indicated as: ***p < 0.001, **p < 0.01, *p < 0.05.

The performance index on absorption basis (PI_abs) was particularly sensitive to drought stress. Drought caused a pronounced and significant reduction in PI_abs, indicating strong limitations in overall PSII functionality and energy conservation. Si application under drought conditions significantly improved PI_abs, with the greatest enhancement observed at 1.0 mM, while lower (0.5 mM) and higher (2.0 mM) concentrations resulted in partial recovery. Under well-watered conditions, Si treatments caused only minor, non-significant changes in PI_abs (Fig. 7e-h).

The quantum yield of electron transport (ΦEo) followed a trend similar to Fv/Fm and PI_abs. Drought stress significantly decreased ΦEo, whereas 1.0 mM Si significantly restored electron transport efficiency under drought, approaching values observed in well-watered plants (Fig. 7i-l). Conversely, the quantum yield of energy dissipation (ΦDo) increased significantly under drought stress, indicating enhanced non-photochemical energy loss and photoprotective dissipation. Si application under drought significantly reduced ΦDo, particularly at 1.0 mM, suggesting improved utilization of absorbed light energy for photochemistry. No significant differences in ΦDo were observed among Si treatments under well-watered conditions (Fig. 7m-p).

Biomass and yield responses to drought, silicon, and re-irrigation

Harvest biomass was measured after drought and subsequent re-irrigation. Drought stress reduced FW compared with well-watered plants. Well-watered plants with Si had FW comparable to the control, although 0.5 mM Si showed ~30% lower FW. Under drought, 1.0 mM Si improved FW to levels similar to well-watered plants (Fig. 8a).

Fig. 8.

Effects of drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM) on harvest and reproductive traits of buckwheat plants: (a) plant fresh weight (excluding roots), (b) plant dry weight, (c) total seed dry weight (yield), (d) total number of seeds, and (e) dry weight of 50 seeds. Measurements were taken after completion of the drought period and subsequent re-irrigation. Data represent means ± SE (n = 5). Different letters indicate significant differences among treatment combinations at each time point (p < 0.05), as determined by Tukey’s HSD test. F-values from two-way ANOVA for irrigation, silicon, and their interaction are presented in each panel. Significance levels are indicated as: ***p < 0.001, **p < 0.01, *p < 0.05.

A similar trend was observed for DW. Drought decreased DW relative to well-watered plants, while Si under well-watered conditions had little effect. Under drought, all Si treatments resulted in intermediate DW values, suggesting partial maintenance of biomass (Fig. 8b).

Seed yield and individual seed weight responded strongly to drought and Si application. Drought stress significantly reduced total seed yield, including both seed number and dry weight, compared with well-watered plants. Under well-watered conditions, seed yield was largely maintained across Si treatments, although 0.5 mM Si tended to increase seed number at the expense of individual seed weight, indicating a seed number-seed mass trade-off without an overall yield penalty. Under drought stress, Si influenced reproductive performance in a concentration-dependent manner. Foliar application of 1.0 mM Si restored total seed count and 50-seed weight to levels comparable with well-watered plants, whereas 0.5 and 2.0 mM Si provided only partial mitigation (Fig. 8c-e).

Correlation among physiological and yield traits

Pearson correlation analysis revealed strong and significant associations among growth, physiological, and yield-related traits under varying treatments. Plant biomass parameters, including plant dry weight (Plant DW) and fresh weight (Plant FW), showed a very strong positive correlation (r = 0.99, p < 0.001), and were positively associated with yield components such as total seed weight (TSW), fifty-seed weight, and total seed count (TSC) (r = 0.73–0.76, p < 0.01).

Photosynthetic traits, particularly A, gs, and ETR, were strongly and positively correlated with growth and yield attributes (r = 0.75–0.89, p < 0.01), indicating that improved photosynthetic performance contributed directly to biomass accumulation and reproductive output. Notably, A exhibited a strong positive relationship with RWC (r = 0.96, p < 0.001) and ETR (r = 0.89, p < 0.001), highlighting the close linkage between plant water status, photochemical efficiency, and carbon assimilation.

In contrast, EL, an indicator of membrane damage, showed strong negative correlations with most physiological and growth parameters, including RWC (r = –0.97, p < 0.001), plant height (r = –0.96, p < 0.001), and A (r = –0.96, p < 0.001), suggesting that increased cellular damage under stress severely impaired plant performance. Similarly, Ci was negatively correlated with key photosynthetic and yield traits, including TSW (r = –0.79, p < 0.001) and ETR (r = –0.65, p < 0.05), indicating reduced CO₂ utilization efficiency under stress conditions (Fig. 9).

Fig. 9.

Pearson correlation matrix showing relationships among growth, physiological, and yield-related traits in buckwheat under different irrigation and silicon (Si) treatments. Correlation coefficients (r) are indicated by colour intensity and direction, with positive correlations shown in blue and negative correlations in red. The strength of the correlation is proportional to colour intensity. Significance levels are indicated as *p < 0.05, **p < 0.01, and ***p < 0.001. The analysis was performed using pooled data across all treatments of prolong drought (n = 5).

Discussion

This study aims to evaluate the potential of foliar-Silicon application to mitigate drought stress in buckwheat, in the process deciphering key growth parameters, photosynthetic performance, and chlorophyll fluorescence dynamics. Drought stress, as expected, led to significant reductions in plant growth, photosynthetic efficiency, and membrane integrity (Fig. 10). The differential responses observed across Si concentrations suggest a dose-dependent effect. Our findings indicate that the lower concentration (0.5 mM) may have been insufficient to effectively trigger protective physiological and biochemical responses^45,46, whereas 1.0 mM appeared optimal for stress mitigation. In contrast, the reduced effectiveness observed at 2.0 mM indicates a potential inhibitory effect at higher concentrations. However, as silicon was supplied as Na₂SiO₃, the contribution of Na⁺ cannot be excluded, particularly at higher concentrations, and may have partially influenced the observed responses and should be considered in future studies using appropriate ionic controls.

Fig. 10.

Schematic representation of the response of key physiological, growth, and photosynthetic parameters in buckwheat plants to drought stress and foliar silicon (Si) application (0.5, 1.0, and 2.0 mM). Arrows indicate the direction of change relative to well-watered controls: ↑ = increase, ↓ = decrease. Parameters include plant growth traits (height, stem diameter, leaf area), water status (RWC, EL%), gas exchange (A, gs, Ci, ETR), PSII performance (Fv/Fm, PI_abs, ΦEo, ΦDo), chlorophyll fluorescence transients (, Vt, ΔW phases), and yield components (biomass, total seed number, 50-seed dry weight). The scheme highlights the mitigating effect of Si, particularly at 1.0 mM, on drought-induced reductions, illustrating the coordinated physiological and reproductive responses that contribute to improved drought tolerance.

Stem diameter is highly sensitive to plant water status, and its reduction under drought reflects impaired radial growth caused by loss of turgor and restricted cell wall extensibility⁴⁷. Application of 1.0 mM Si effectively mitigated this decline and supported full recovery after re-irrigation, indicating improved structural stability under stress (Fig. 2b–c). This response is consistent with the role of Si in reinforcing vascular tissues and cell walls through silica deposition, thereby maintaining membrane integrity and facilitating water and nutrient transport during dehydration⁴⁸. Similar increases in stem diameter and drought tolerance following Si application have been reported in coffee, where enhanced structural support reduced tissue collapse under water deficit⁴⁹. In contrast, following re-irrigation, stem diameter showed limited recovery in drought-stressed plants, indicating that radial growth is constrained after water deficit due to sustained effects on cambial activity and cell expansion⁵⁰. However, 1.0 mM Si effectively restored stem diameter to well-watered levels (Fig. 2d), suggesting that optimal Si supply enhances structural recovery by supporting tissue integrity and water transport, whereas lower or higher doses were less effective⁵¹.

Plant height was similarly reduced under drought, reflecting suppressed elongation growth commonly associated with reduced cell division and expansion⁵⁰. Si application partially alleviated this reduction during the stress period, likely through improved water status and sustaining carbon assimilation. However, the limited effect of Si on post-stress height recovery suggests that vertical growth is more sensitive to cumulative drought duration and less reversible than radial growth, indicating that Si primarily supports growth maintenance during active stress rather than recovery⁵². Following re-irrigation, plant height increased in all drought-stressed plants but remained significantly lower than in well-watered plants, indicating incomplete recovery (Fig. 2h). The similar responses across Si treatments suggest that height recovery was primarily driven by water availability, whereas Si mainly mitigated growth reduction during active drought rather than promoting post-stress elongation⁵⁰.

Leaf area was highly sensitive to drought, consistent with dehydration-induced growth inhibition and accelerated senescence⁵³. Si application, particularly at 1.0 mM, substantially preserved leaf area under drought, indicating delayed senescence and improved leaf hydration, as evidenced by reduced wilting compared with drought-stressed plants without Si (Supplementary Fig. S3-S5). This effect is likely mediated through Si-enhanced ABA-regulated stomatal regulation, which limits excessive transpiration and reduces leaf water loss, thereby maintaining leaf structural integrity under stress. The concentration-dependent response further indicates that optimal Si levels enhance drought tolerance without adversely affecting growth under well-watered conditions⁵⁴.

Electrolyte leakage and relative water content measured at peak drought stress provided clear evidence of drought-induced cellular damage and impaired plant water status. The marked increase in EL% under drought reflects loss of membrane integrity, a common consequence of dehydration-induced oxidative stress and lipid peroxidation³¹, while the concurrent decline in RWC indicates reduced tissue hydration and turgor, together constraining physiological function under water deficit. Foliar Si application, particularly at 1.0 mM, effectively mitigated these detrimental effects by reducing EL% and maintaining higher RWC, thereby preserving cellular integrity and water balance. This protective response is consistent with previous reports, suggesting that Si enhances membrane stability, reinforces cell wall structure, stabilizes membrane lipids, and mitigates oxidative damage, collectively improving cellular water retention under stress^32,52,]55. Si facilitates photoprotection and membrane stabilization in buckwheat under drought by modulating the antioxidant defense system to scavenge ROS, thereby mitigating lipid peroxidation and preserving cellular integrity⁵⁶. The partial protection observed at 2.0 mM and the weak response at 0.5 mM indicate a concentration-dependent effect, highlighting the importance of optimal Si levels for effective drought mitigation. Mechanistically, Si also enhances ABA-mediated stomatal regulation, promoting timely stomatal closure and reducing transpiration, which further contributes to the maintenance of tissue hydration and RWC^14,54,57. The close association between reduced EL% and improved RWC emphasise the critical role of Si in preserving cellular integrity, water status, and physiological function, thereby enhancing overall drought tolerance in plants (Fig. 3).

In the initial drought, gas exchange measurements showed no significant differences in CO₂ assimilation, gs, Ci, or ETR, indicating that early drought did not severely affect stomatal function (Fig. 3a-d). Drought stress substantially impaired photosynthetic performance in buckwheat, as evidenced by declines in CO₂ assimilation, gs, and ETR. As drought progressed into the second and third weeks, gs declined sharply in drought-stressed plants, reflecting stomatal closure to reduce water loss. This response aligns with reports in wheat, soybean, and maize, where increasing drought severity consistently reduces gs through stress-induced stomatal regulation^52,]52,58,59. Si application mitigated this decline in a concentration-dependent manner. During the second measurement, 1.0 mM Si maintained gs nearly 200% higher than drought-stressed plants, whereas 0.5 and 2.0 mM Si showed only modest increases (~8% and ~59% higher than drought, respectively). By the third measurement, gs further declined across all treatments; however, 1.0 mM Si consistently preserved substantially higher gs, remaining ~248% above drought, while 0.5 and 2.0 mM Si exhibited comparatively smaller improvements (~62% and ~25% above drought, respectively). Although gs in Si-treated plants remained below well-watered levels, these results indicate that Si partially sustains stomatal function under progressive drought, with 1.0 mM Si showing the strongest and most consistent protective effect (Supplementary Fig. S1). This response is likely mediated through ABA-regulated stomatal control and improved leaf hydration, contributing to enhanced water-use efficiency and photosynthetic potential⁵⁵.

In the second measurement, CO₂ assimilation declined markedly in drought-stressed plants and those treated with 0.5 mM Si, both exhibiting similarly low rates, while well-watered plants maintained higher assimilation. In contrast, plants treated with 1.0 mM Si showed a pronounced enhancement in A, coinciding with the substantially higher gs observed. This trend persisted into the third measurement, where 1.0 mM Si maintained gs higher than drought, supporting continued CO₂ diffusion into the leaf and sustained carbon fixation despite progressive water limitation. The enhanced stomatal opening under 1.0 mM Si treatment allowed partial CO₂ uptake under drought conditions, supporting photosynthetic activity^14,55,60.

Mechanistically, this effect may be associated with Si-induced improvements in stomatal regulation and is consistent with possible involvement of ABA-mediated pathways reported in previous studies, leading to optimal stomatal aperture under moderate drought stress⁶¹. Additionally, Si may improve leaf water retention and membrane stability, thereby preventing dehydration-induced damage to the photosynthetic apparatus and preserving photosynthetic efficiency^60,62. In contrast, the relatively lower gs observed in 0.5 and 2.0 mM Si treatments, despite slight improvements over drought alone, likely limited internal CO₂ availability and constrained assimilation rates. Overall, these findings underscore a clear concentration-dependent role of Si, with 1.0 mM Si being the most effective concentration for sustaining stomatal conductance and photosynthetic carbon fixation under progressive drought stress^63.

In the first measurement, internal CO₂ showed no significant differences, indicating minimal stress. By the second measurement, Ci decreased in all drought-stressed plants, reflecting restricted CO₂ diffusion due to stomatal closure. In the third measurement, Ci increased in drought plants while significantly lower values were observed in 1.0 mM Si treatments under drought, reflecting the inverse relationship between assimilation and Ci. Under severe drought, photosynthetic capacity, rather than stomatal opening, becomes limiting, causing CO₂ accumulation in the intercellular spaces. In contrast, 1.0 mM Si maintained lower Ci by partially sustaining assimilation through improved stomatal function and leaf hydration, enabling more efficient CO₂ fixation despite stress^14,55.

Drought reduced electron transport by restricting electron flow, increasing excitation pressure, and lowering PSII efficiency (Fv/Fm, PI_abs), reflecting impaired photochemistry and potential photoinhibition⁶⁴. Si-treated plants, particularly at 1.0 mM, maintained significantly higher ETR in the second measurement, indicating more efficient use of light energy for photochemistry. By the third measurement, ETR in Si-treated plants remained higher than drought-stressed plants, but differences were not statistically significant, suggesting that prolonged stress limits the protective effect of Si. Overall, Si likely stabilizes chloroplast structure, preserves PSII function, and reduces oxidative stress, supporting sustained photosynthetic performance under drought^37,65.

Following re-irrigation, A, gs, Ci, and ETR recovered across all treatments, reaching levels comparable to well-watered plants. This notable recovery indicates that photosynthetic and stomatal functions are highly reversible once water availability is reinstated. The recovery reflects effective rehydration, restoration of stomatal conductance, and repair of the photosynthetic apparatus, even in previously drought-stressed plants, highlighting the resilience of plant physiological processes under transient stress^66,67.

Insights from OJIP fluorescence, including Fv/Fm, PI_abs, ΦEo, and ΦDo, provide a mechanistic understanding of PSII energy flow. Drought markedly reduced the P-step (Fm), Fv/Fm, PI_abs, and ΦEo, indicating impaired excitation, restricted electron transport, and diminished energy conservation⁶⁰. These declines were accompanied by increased qN, F₀, and ΦDo, reflecting enhanced photoprotective heat dissipation and damage to PSII reaction centers. Double-normalized OJIP transients (Vt) further confirmed these acceptor-side limitations. Elevated Vt values during the I-P phase in drought-stressed plants indicate an over-reduced intersystem electron transport chain and limited PSI electron sink capacity. Conversely, Si-treated plants (1.0 mM) showed a reduced I-P rise and improved difference kinetics (ΔW_OJ​, ΔW_JI​, ΔW_IP​). By facilitating efficient Q_A^−​ reoxidation and coordinating electron flow from PSII to PSI, optimal Si supplementation (1.0 mM) preserves photosynthetic structural integrity and minimizes energy loss, whereas the partial recovery observed at 0.5 and 2.0 mM highlights the concentration-dependent nature of Si in enhancing drought tolerance¹⁴. Si stabilizes photosynthetic electron transport under water deficit by protecting the D1 protein repair cycle in Photosystem II and the PsaA/PsaB core subunits in Photosystem I, while maintaining the structural integrity of the Oxygen-Evolving Complex (OEC) and the fluidity of the thylakoid membrane^60,68,69.

However, despite re-irrigation, the photosynthetic apparatus of drought-stressed plants exhibited incomplete recovery. Normalized OJIP transients (F_t/F₀) revealed a persistently lower P-step (F_m) in untreated drought-stressed plants and those receiving 0.5 mM Si, indicating sustained impairment of PSII reaction centers. This suggests that although re-watering restores gas exchange and growth processes, structural or photochemical damage to PSII-likely induced by drought-associated oxidative stress-may be only partially reversible or require extended recovery periods. In contrast, plants supplied with 1.0 mM Si maintained higher F_m values, indicating improved preservation of PSII integrity during drought and enabling a more evident recovery of photochemical capacity upon re-irrigation^14,60.

Drought stress markedly reduced fresh and dry biomass and seed yield, consistent with the high sensitivity of vegetative growth and reproductive processes to water deficit⁷⁰. Restricted cell expansion, suppressed metabolic activity, and drought-induced stomatal closure limited CO₂ influx, reducing gs, A, and overall carbon assimilation, thereby constraining biomass production and yield formation³². Similar drought-driven yield losses have been widely attributed to impaired pollination, reduced grain filling, and disrupted assimilate partitioning, leading to decreases in both seed number and individual seed weight²³.

Foliar Si application mitigated these drought-induced effects, with 1.0 mM Si providing the improvement in shoot biomass and yield components. Under prolonged drought, Si-treated plants particularly at 1.0 mM maintained a greater number of leaves and flower pods, as visually illustrated in Supplementary Fig. S3-S5, which translated into higher shoot biomass, increased seed number, and enhanced yield¹¹. These responses indicate that Si enhances productivity under drought primarily by preserving physiological function rather than directly stimulating growth. Comparable yield recovery under drought has been reported in wheat and maize, where Si improved shoot carbon status, water-use efficiency, gas exchange, and antioxidant protection, resulting in higher seed weight and number^23,54.

The partial restoration of fresh and dry biomass at 1.0 mM Si further suggests improved water-use efficiency and sustained photosynthetically derived carbon supply during drought. This aligns with evidence that Si stabilizes photosynthetic membranes, preserves electron transport, and limits oxidative damage, thereby supporting continued biomass accumulation under water deficit. In contrast, the weaker responses at 0.5 and 2.0 mM Si highlight the importance of an optimal Si concentration for effective drought mitigation⁶².

Reproductive performance was particularly sensitive to drought, as reflected by reductions in seed number and seed dry weight, likely due to restricted carbohydrate availability during flowering and seed filling. Application of 1.0 mM Si restored total seed yield and individual seed weight to levels comparable with well-watered plants, indicating that Si-mediated protection extends beyond vegetative tissues to reproductive sinks by sustaining carbon assimilation and assimilate partitioning. Although this study does not address the balance between vegetative growth and reproductive allocation, the stress period encompassed early reproductive development, and previous reports in maize and rice suggest that Si application may confer greater ameliorative effects during reproductive phases than during vegetative growth.^71,72. Similar Si-driven improvements in reproductive traits have been linked to enhanced water status, reduced lipid peroxidation, and coordinated growth responses in stress sensitive genotypes, including buckwheat¹⁴.

Differences among Si concentrations further highlight trade-offs between seed number and seed size. At 0.5 mM Si, the tendency toward increased seed number coupled with reduced individual seed weight suggests incomplete compensation of source–sink balance, whereas 1.0 mM Si optimized both assimilate supply and allocation, supporting balanced seed development⁷³. The intermediate response at 2.0 mM may reflect diminishing returns or altered nutrient interactions at higher Si levels. The study on impact of Si in rainfed buckwheat system is limited, however Si application, including nano-Si, generally boosts plant growth and resilience by positively restructuring rhizospheric microbial communities and improving soil health⁷⁴. Correlation analysis revealed a strong linkage between plant water status, photosynthetic efficiency, and yield, with Si treatments reinforcing these relationships under drought. Positive associations of A, gs, and ETR with growth and yield highlight the central role of coordinated carbon assimilation and photochemistry. In contrast, electrolyte leakage showed strong negative relationships, indicating membrane damage as a key constraint, while the negative correlation of intercellular Ci suggests reduced carbon use efficiency. The negative correlation of ΦD₀ suggests increased energy dissipation as heat under stress conditions, reflecting reduced photochemical efficiency and enhanced photoprotective responses. Collectively, these findings indicate that Si-mediated maintenance of water balance and photochemical integrity is critical for sustaining productivity under drought¹⁴.

Collectively, these findings, together with previous reports showing that Si enhances physiological protection, carbon sequestration, and stress resilience, support the hypothesis that optimal Si supplementation improves yield under drought by integrating physiological, and biochemical adjustments rather than direct growth stimulation^62,75.

Conclusion

This study demonstrates that foliar silicon application enhances drought tolerance in buckwheat through coordinated physiological and photochemical regulation rather than direct growth stimulation. Drought-induced reductions in growth, photosynthesis, biomass accumulation, and seed yield were consistently mitigated by Si, with 1.0 mM emerging as the most effective concentration across both vegetative and reproductive stages.

Mechanistically, Si-mediated drought resilience was associated with improved plant water relations and cellular stability, as reflected by the preservation of stem diameter, plant height, and leaf area under water deficit. At the photosynthetic level, Si stabilized PSII function, improved Q_A-Q_B electron transfer, moderated plastoquinone pool redox status, and partially alleviated acceptor-side limitations at PSI. Together, these effects enhanced electron transport efficiency, optimized light energy utilization, and reduced the risk of photoinhibition during drought stress.

The maintenance of photosynthetic performance and CO₂ assimilation in Si-treated plants supported greater biomass accumulation and more efficient carbon allocation to reproductive sinks, resulting in higher seed number, seed weight, and overall yield under drought, particularly at 1.0 mM Si. The concentration-dependent responses observed across traits emphasize the importance of optimizing Si supply, as both lower and higher concentrations provided only partial protection. Following re-irrigation, clear recovery of gas exchange (A, gs, Ci), ETR, plant height, leaf area, and biomass was observed, especially in plants treated with 1.0 mM Si. In contrast, stem diameter and PSII maximum fluorescence (F_m) remained partially reduced in untreated drought and low-Si plants, indicating slower recovery of structural and photochemical components.

Overall, these findings reinforce the emerging view of silicon as an active physiological modulator that integrates physiological protection with dynamic regulation of photosynthetic electron transport, water-use efficiency, and yield formation under drought. Foliar Si application at an optimal concentration therefore represents a promising strategy to enhance drought resilience and productivity in buckwheat and potentially other drought-sensitive crops.

Author contributions

Md Intesaful Haque: Conceptualization, Methodology, Investigation, Formal analysis, Visualization, investigations, Writing – original draft, Writing – review & editing. Bedabrata Saha: Methodology, Formal analysis, Visualization, Writing – review & editing. Radosław Juszczak: Conceptualization, Supervision, Writing – review & editing. Anshu Rastogi: Conceptualization, Supervision, Funding acquisition, Writing – review & editing.

Funding

This research was supported by the National Science Centre, Poland (NCN) under grant number 2021/43/I/NZ9/01356. The publication was co-financed by the Polish Minister of Science and Higher Education as part of the Strategy of the Poznan University of Life Sciences for 2024-2026 in the field of improving scientific research and development work in priority research areas.

Data availability

The raw data related to article is avaliable in the Open Data Repository RepOD (https://repod.icm.edu.pl/) – the PULS institutional repository with DOI: 10.18150/VDGCAQ.

Competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.