Chiara, Miele (2026) From Microalgal Cultivation to Plant Stress Resilience: Emerging Biostimulant Extracts to Enhance Tomato Salt Tolerance. [Tesi di dottorato]

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Abstract

Salinity is a major constraint for irrigated cropping systems and markedly limits tomato performance through coupled osmotic, ionic and oxidative stresses. Microalgae-derived biostimulants are promising sustainable inputs, but their wider use is hindered by variable efficacy and by weak links between product composition and plant response. This thesis addresses these limitations by integrating a critical synthesis of current knowledge with an experimental framework that connects microalgal production regime, biochemical identity of aqueous extracts and tomato responses under salinity. Five microalgal accessions (Chlamydomonas sp., Scenedesmus sp., Chlorella sp., Chromochloris sp. e Klebsormidium sp.) were produced under two contrasting metabolic regimes (nitrate-based vs acetate-based), converted into aqueous extracts (0; 0,25; 0,5; 1 g L⁻¹) and assessed in tomato (Solanum lycopersicum cv. ‘Moneymaker’) from seed assays to foliar application in hydroponics under 0 or 80 mM NaCl. ATR–FTIR fingerprinting and targeted biochemical assays showed that cultivation regime is a primary determinant of extract identity and bioactivity: nitrate based production more consistently yielded non-phytotoxic, active extracts that improved germination and supported more favourable growth trajectories under salinity, whereas acetate-based production generally produced weaker or neutral effects. A nitrate-derived Chlorella extract (1 g L⁻¹) emerged as the most robust candidate. The selected extract was then tested in a dedicated hydroponic experiment (0 or 80 mM NaCl) via foliar application, integrating growth and photophysiological measurements with time-resolved targeted LC MS/MS quantification of phytohormones and polyamines in both roots and leaves. Salinity reconfigured endogenous regulation in a strongly organ dependent manner (ACC-dominated in roots; SA-driven in leaves; jasmonates negligible). Under prolonged salinity, the extract exerted its clearest effect by attenuating root ACC accumulation and reshaping root putrescine–spermidine dynamics during acclimation, consistent with modulation of ethylene-related stress signalling and metabolic cost control rather than broad hormone elevation. Physiological readouts supported this interpretation: chlophyll levels increased only in non-saline conditions, while Fv/Fm declined in salt-stressed treated plants, indicating persistent PSII constraints. Collectively, this thesis defines an actionable route to improve reproducibility in microalgae-based biostimulants by treating cultivation and extraction as core product descriptors supported by biochemical documentation, and by identifying organ- and time-dependent regulatory markers in plants that can guide product selection and optimisation of dose and timing. These outcomes strengthen the evidence base for Chlorella based, and more broadly microalgae-based, biostimulants for salinity management in tomato and set priorities for validation under field-relevant variability.

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