Stage-Dependent Effects of Abscisic Acid and Seaweed Extract on Cold Hardiness and Protective Metabolism in Grapevine (Vitis vinifera L. cv)

Late winter and early spring frost events have become an increasing threat to grapevine production under changing climatic conditions, highlighting the need for practical strategies to improve bud freezing tolerance during dormancy. Although exogenous abscisic acid (ABA) and seaweed (SW) extracts have been reported to enhance plant tolerance to abiotic stresses, their comparative effectiveness throughout different dormancy stages and among grapevine cultivars remains insufficiently understood. This study evaluated the effects of foliar ABA and SW applications on bud cold hardiness and associated biochemical responses in three Vitis vinifera cultivars over two consecutive growing seasons. Field-grown vines were treated with ABA or SW, and bud freezing tolerance was determined by low-temperature exotherm (LTE) analysis during cold acclimation, mid-winter dormancy, and deacclimation. To investigate the physiological basis of treatment responses, proline, reducing sugars (RS), and malondialdehyde (MDA) contents were quantified, and multivariate relationships among variables were examined using principal component analysis (PCA). Both ABA and SW significantly improved bud freezing tolerance, although the magnitude of the response depended on cultivar, dormancy stage, and seasonal conditions. The strongest improvements were generally observed during the deacclimation period, when ABA increased freezing tolerance by up to 2.72°C and SW by up to 3.96°C. The treatments also induced distinct biochemical responses. ABA consistently promoted proline accumulation in responsive cultivars, whereas both treatments reduced lipid peroxidation in specific cultivar–stage combinations but were ineffective or even increased oxidative stress in others, particularly in the cold-sensitive cultivar Red Globe. PCA further demonstrated clear associations between freezing tolerance and biochemical traits and confirmed that treatment responses were strongly genotype and stage dependent.The results provide physiological evidence supporting the use of these treatments as practical tools for improving winter resilience in grapevine while emphasizing that their effectiveness is highly dependent on genotype and dormancy stage.