FromChamber to Orchard: Limited Efficacy of Exogenous Salicylic Acid in Field-Grown Sweet Cherry Across Three Seasons

Salicylic acid (SA) is an endogenous phytohormone involved in stress signaling, yet its efficacy under commercial orchard conditions remains insufficiently tested. This multi-season study evaluated foliar SA (72.4 µM) on gas exchange, midday xylem water potential (Ψx), leaf temperature, water-use efficiency (WUE), and fruit quality in two sweet cherry cultivars (Prunus avium L. cv. Lapins and cv. Santina) across three consecutive seasons (2019/20–2021/22) in Pencahue, Maule Region, Chile. Generalized Linear Mixed Models applied to all 109 physiological date × variable comparisons, with joint correction for multiple testing (Benjamini–Hochberg FDR and Bonferroni), identified a single robust treatment effect: a less negative (higher) midday Ψx under SA in cv. Santina on 16 January 2021 (−0.65 vs. −0.80 MPa in Control; adjusted p = 0.020), i.e., a more favorable plant water status. No comparison involving net photosynthesis, stomatal conductance, leaf temperature, or WUE remained significant after correction, and fruit quality was unaffected in both cultivars. Principal Component Analysis separated a gas-exchange axis from a WUE axis, with the largest Control–SA centroid displacement in the same season as the confirmed Ψx effect. Because SA is expected to act as a stress-conditioned elicitor—that is, to be effective mainly under high atmospheric demand (VPD) or a pronounced water deficit—the season in which the single confirmed effect appeared constitutes a direct test of that expectation. Contrary to this, the effect occurred in the mildest rather than in the most demanding season, arguing against a simple high-VPD timing rule and supporting cultivar- and season-specific rather than universal expectations for SA in orchard water management. These conclusions apply to the single concentration tested (72.4 µM) under fully irrigated commercial management in one orchard and should not be extrapolated to higher application rates or to water-limited conditions.