Abstract

The Colt rootstock (Prunus avium & times; Prunus pseudocerasus) is widely used in Chilean cherry orchards due to its high compatibility and vigor; however, its low drought tolerance limits its performance in water-scarce environments. Although biostimulants derived from seaweed extracts combined with plant growth-promoting bacteria (PGPB) have shown positive effects in herbaceous crops, their impact on woody species remains largely unexplored. This study evaluated the physiological and morphological responses of young Colt plants to applications of Macrocystis pyrifera extracts obtained by alkaline hydrolysis (A1) and mixed hydrolysis (alkaline + enzymatic; A2), applied alone or in combination with Pseudomonas koreensis AG 97 (B), including the treatments A1B, A2B, and B alone, under well-watered (WET) and deficit irrigation (DRY) conditions. Under WET conditions, which induced periods of substrate saturation conducive to transient root-zone hypoxia, the A1B treatment increased stomatal conductance by 90% and reduced leaf temperature by approximately 2 degrees C, mitigating excess-moisture stress and enhancing photochemical efficiency (Fv/Fm) under low-light conditions, as well as shoot elongation (+23 cm versus 10 cm in the control). Under DRY conditions, A1B reduced stomatal conductance by approximately 70% when plant water potential dropped below -1.1 MPa, limiting dehydration and reducing defoliation to less than 15% compared to 40% in plants treated with B alone, while treatments A1 and A1B also showed higher carbon assimilation rates (>41%). Overall, these results confirm a synergistic effect between Macrocystis pyrifera seaweed extracts and PGPB, improving stomatal regulation, physiological resilience, and growth of the Colt rootstock under contrasting water regimes.