Does the application of biostimulants trigger grapevine responses to soil water availability differently depending on vineyard exposure and microclimate?

Abstract

Climate projections highlight substantial risks for viticulture (Sgubin et al., 2023), with heat and drought being major threats to grape quality and yield. Grapevine tolerance thresholds remain uncertain and highly cultivar-dependent (Van Leeuwen et al., 2024). Phenylpropanoid metabolism is particularly sensitive to temperature, with high temperature reducing anthocyanins and flavonols (Wilson et al., 2024) and diurnal heat reducing the concentration and the galloylation rate of seed tannins (Gouot et al., 2019). Water stress also affects flavour development (Koundouras et al., 2006). Biostimulants offer a promising means to enhance vine resilience by modulating defence and polyphenolic pathways, although their mechanisms are frequently not fully elucidated, particularly in field conditions.

During the 2025 growing season, field trials were carried out in three Vitis vinifera L. vineyards of Merlot and Nebbiolo. Two Merlot vineyards were selected for their contrasting water availability despite similar altitude: Diano d’Alba (south-facing, steep slope) represented a water-stressed (WS) site, whereas Serralunga d’Alba (north-facing) was characterized by higher soil moisture and considered well-watered (WW). A randomized block design with eight replicates compared NT and biostimulant-treated vines (T), which received five applications at 1.6–6.0 L ha⁻¹ across key phenological stages. Gas exchange measurements, performed with CIRAS-4 on fully expanded, sun-exposed leaves, revealed significant increases in gs and related parameters (Ci, A, E) under WS after two biostimulant applications. The observed trends suggest that the biostimulant promotes a conservative photosynthetic strategy under limited water availability, while enhancing photosynthetic performance when water is not limiting. The greater bunch weight in treated vines at WW supports this hypothesis, indicating improved carbon assimilation under favourable conditions. No significant treatment effects were observed on water use efficiency, ΦPSII, or stem water potential. Technological analyses indicated similar °Brix evolution, while titratable acidity and pH varied according to site and sampling date. After Bonferroni-adjusted non-parametric testing, the results suggest a possible influence of the biostimulant on the maintenance of must acidity under heat and water stress conditions. Bunch weight did not differ significantly in WS, whereas in WW treated vines produced markedly heavier clusters (p<0.001), indicating a site/water status-dependent physiological response. Ongoing chemical analyses will further clarify these preliminary trends: berries were sampled for the determination of organic acids, total polyphenols (Folin–Ciocalteu), anthocyanins, flavonols and hydroxycinnamates (HPLC-DAD). Preliminary results indicate that the biostimulant may modulate vine physiology in a water-dependent manner, promoting conservative metabolism under stress and enhancing productivity when water is not limiting.

References

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