One Step Forward in Understanding the Mechanism of Action of Wood Vinegar: Gas Exchange Analysis Reveals New Information

Wood vinegar (<b>WV</b>), a by-product of woody biomass pyrolysis, is increasingly used in agriculture as a sustainable biostimulant, although its effects on plant stress resistance and underlying mechanisms remain poorly understood. Recent studies propose that <b>WV</b> may act through a <i>eustress</i>-based mechanism, defined as a mild and controlled stress that activates adaptive physiological responses and enhances plant performance without causing structural or metabolic damage. This study investigated the physiological and biochemical effects of <b>WV</b> on strawberry plants grown under three water-deficit stress levels [no stress (<b>NS</b>), moderate stress (<b>MS</b>), and high stress (<b>HS</b>)] and treated with <b>WV</b> either via fertigation (0.5% <i>v</i>/<i>v</i>, <b>WV₁</b>) or foliar spray (0.2% <i>v</i>/<i>v</i>, <b>WV₂</b>). Gas exchange parameters (<b>A</b>, <b>g_sw</b>, <b>E</b>, <b>Ci</b>, <b>WUE</b>), total chlorophyll content, and nutrient balance ratios (<b>Fe/Mn</b> and <b>K/Ca</b>) were measured after a three-month growth period. PERMANOVA revealed significant effects of both <b>WV</b> and water-deficit stress, as well as their interaction, on most parameters. Under <b>NS</b> and <b>MS</b> conditions, <b>WV</b> reduced <b>A</b>, <b>gsw</b>, <b>E</b>, and <b>Ci</b> while increasing <b>WUE</b>, indicating enhanced water-use efficiency and improved physiological adjustment to water limitation. Chlorophyll content remained stable, demonstrating preserved photosynthetic integrity. Nutrient ratios further supported a controlled ion rebalancing associated with adaptive stress responses under <b>NS</b> and <b>MS</b>, whereas <b>HS</b> conditions indicated the onset of distress. Overall, the data demonstrate that <b>WV</b> enhances plant stress resistance primarily by inducing eustress-mediated physiological regulation rather than by directly stimulating growth.