Hydrochar from garden waste enhances drought tolerance in sunflower via soil-plant-gene interactions

Enhancing crop drought resilience is essential for sustainable agriculture in a changing climate. Among potential strategies, hydrochar (HC), a carbon-rich product of hydrothermal carbonization, represents a promising soil amendment. We hypothesized that HC enhances drought tolerance in Helianthus annuus L. through coordinated soil–plant–gene interactions that modulate water retention, plant hydraulics, and drought-responsive gene networks. Using a multiscale approach combining soil physicochemical analyses, plant physiology, and transcriptomics, we assessed the effects of garden waste-derived HC on H. annuus under well-watered and water- limited conditions. HC application improved soil water retention without altering key chemical parameters and enhanced plant hydraulics. Across irrigation regimes, HC-treated plants showed greater biomass accumulation and photosynthetic capacity. During drought, HC mitigated stress by maintaining leaf water potential and membrane integrity, and by promoting a more negative turgor loss point via osmotic adjustment and increased cell wall stiffness. Transcriptomic analysis revealed that HC modulated drought-responsive genes, including transcription factors (e.g., WRKY51, bZIP11) and genes involved in osmotic regulation, antioxidant defense, and hormonal signaling. The distinct molecular signature in HC-treated plants under drought suggests a priming effect that sustains physiological function under stress. This study provides novel evidence linking HC-induced soil enhancement to molecular drought responses in crops, highlighting HC’s potential as a circular input for improving adaptation and productivity in climate-resilient agroecosystems.