Effect of exogenous salicylic acid on photosynthetic efficiency and yield of wheat under heat stress

Introduction

What happens to wheat productivity when temperatures climb beyond the crop's tolerance window? Wheat feeds roughly 2.5 billion people, yet it remains sensitive to temperatures exceeding 30 °C during anthesis [1] . Heat events of three to five days above 35 °C can reduce grain number per spike by 30-40% [2] . PSII reaction centres are particularly vulnerable, and Fv/Fm drops sharply above 35 °C [3] . Rubisco activase denaturation compounds carbon fixation losses [4,5] . Salicylic acid (SA) is a phenolic signalling molecule that improves thermotolerance through membrane stabilisation and antioxidant upregulation [6,7,8] . In wheat, SA pre-treatment reduced malondialdehyde under high temperatures [9] , but field evidence linking SA to photosynthetic gains and grain yield is limited. Open-top chambers allow controlled field warming while retaining natural conditions [10,11] . This research tested whether foliar SA at booting would protect PSII function and sustain grain yield in wheat exposed to a 13 °C temperature increase above ambient.

Materials and Methods

Research area and Environmental conditions

The experiment was conducted at the Dışkapı Agricultural Research Station (39°49′N, 32°52′E, 891 m a.s.l.) of Ankara University. The soil is a Typic Haploxerept (clay-loam, pH 7.8, organic matter 1.46%). Mean annual rainfall is 387 mm. During the 2021-2022 season, April-June mean temperature was 16.3 °C (max 34.8 °C); the 2022-2023 season was warmer (17.1 °C mean, 36.2 °C max). Humidity during heat-stress imposition ranged 28-42%.

Field experimental design

A split-plot design in randomised complete blocks (three replications) was used. Main plots had two temperature regimes: ambient and heat stress via polycarbonate open-top chambers (2.5 m diameter, 2.0 m height). Sub-plots received SA at 0, 0.5, 1.0, or 1.5 mM with 0.05% Tween-20 in 500 L ha⁻¹, applied at booting (Zadoks GS 45) and five days later. Each sub-plot was 3 m × 2 m. Chambers were maintained 15 days from booting.

Materials

Spring wheat cv. Bezostaya was sown at 450 seeds m⁻² on 15 October 2021 and 12 October 2022. Basal fertiliser: 60 kg N ha⁻¹ + 40 kg P₂O₅ ha⁻¹ at sowing, 40 kg N ha⁻¹ top-dress at tillering. SA (≥99% purity, Sigma-Aldrich) was prepared fresh; controls received water plus surfactant.

Methods

Gas exchange (Pn, gs, E, Ci) was measured with a LI-6400XT (LI-COR) on flag leaves at 0, 3, 6, 9, 12, and 15 days after SA application. Chlorophyll fluorescence (Fv/Fm) was recorded using a PAM-2500 (Walz); chlorophyll content was estimated with a SPAD-502 meter. At maturity, grain yield, 1000-grain weight, and harvest index were determined from 1 m² harvests. Split-plot ANOVA was performed in SAS 9.4 with Tukey's HSD at p≤0.05. 1). Application of 1.0 mM SA maintained Pn at 16.9 µmol CO₂ m⁻² s⁻¹, not statistically different from the unstressed control. The 0.5 mM dose gave intermediate protection while 1.5 mM was less effective than 1.0 mM. Grain yield averaged 4,870 kg ha⁻¹ under ambient conditions versus 3,210 kg ha⁻¹ under heat alone (Table 2). The 1.0 mM SA treatment produced 3,970 kg ha⁻¹ under heat, recovering 57% of the yield gap. The 1000-grain weight was most responsive to SA, increasing from 31.4 g (heat, no SA) to 37.8 g (heat, 1.0 mM SA). Harvest index dropped from 0.41 to 0.36 under heat regardless of SA dose.

Results

Discussion

The SA-mediated Fv/Fm recovery of 34.0% exceeds the 18-22% reported for controlled-environment wheat [9] , likely because field chambers allowed gradual warming rather than abrupt transitions, giving SA-primed defence pathways more response time. Similar protective effects have been reported for maize [12] and rice [13] . The bell-shaped dose-response where 1.5 mM was less effective than 1.0 mM matches observations across multiple stress contexts [6] , attributed to excessive H₂O₂ accumulation at higher SA concentrations [14] .

Stomatal conductance was partially maintained by SA under heat, consistent with evidence that SA modulates aquaporin expression in wheat [15] . The harvest index showed limited response, indicating SA protected source activity without altering assimilate partitioning. On-farm validation under natural heatwaves is a logical next step [16] .

Conclusion

Foliar SA at 1.0 mM during booting protected field-grown wheat photosynthesis under 38 °C heat stress, recovering Fv/Fm by 34.0% and maintaining Pn near unstressed levels. Grain yield improved by 23.7%, driven by higher 1000-grain weight. The effective dose (69 g SA ha⁻¹) is inexpensive and field-applicable. These two-season results support SA sprays as a heat-stress management tool for wheat in semi-arid regions.

References

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