| In Ohio, the grape and wine industry is increasing rapidly in acreage and production and currently contributes more than half a billion dollars to Ohio's economy. However, the industry is limited by climatic constraints, primarily seasonal freezing temperatures. The goal of this research is to improve the freezing tolerance (FT) of important and premium wine grape cultivars using exogenous abscisic acid (ABA). Initially, a greenhouse study was conducted: 1) to determine the optimum foliar concentration of ABA and 2) to evaluate the morphological and physiological changes of greenhouse-grown 'Cabernet franc' and 'Chambourcin' grapevines. Subsequently, a field study was conducted with the same cultivars: 1) to determine optimum ABA concentration and timing of application in field-grown grapevines, and 2) to evaluate morphological and physiological changes including shoot growth, senescence, periderm formation, dormancy, and FT. In order to further our understanding of the physiological responses, biochemical changes of soluble sugars were determined and their association with bud desiccation and FT were evaluated.;In the greenhouse study, 'Cabernet franc' and 'Chambourcin' grapevines were treated with foliar ABA at different concentrations at different leaf ages and cold treatment. Concentrations of 800 mg/L or higher were phytotoxic and the optimum concentrations were between 400 and 600 mg/L. Optimum concentrations of ABA inhibited shoot growth, and advanced growth cessation, periderm formation, and leaf senescence, which led to advanced dormancy in both cultivars. Furthermore, grapevine response to ABA was enhanced by increasing leaf age and the cold treatment.;In the field study, 'Cabernet franc' and 'Chambourcin' grapevines grown at two locations were treated with 0, 400, and 600 mg/L of ABA at different stages of development. ABA did not affect yield components or the basic fruit chemical composition. However, it enhanced anthocyanin concentration, advanced dormancy, decreased bud water content, and ultimately increased FT.;To characterize the relationship between bud desiccation, soluble sugars, and FT of buds, greenhouse- and field-grown 'Cabernet franc' and 'Chambourcin' were used. Fructose, glucose, and sucrose were the main soluble sugars and positively correlated with FT of grape buds. Raffinose concentration also increased with bud desiccation, but correlated strongly during the early acclimating stages and not in mid-winter (when FT is at its maximum). Our results suggest that bud desiccation coincides with soluble sugars accumulation especially raffinose and these changes precede the acquisition of FT.;In summary, the optimum concentration of ABA was 600 mg/L for 'Cabernet franc' and 400 mg/L for 'Chambourcin'. ABA decreased shoot growth, increased periderm formation, and anthocyanins in berries leading to deeper dormancy and increased FT. The increased FT was preceded with bud desiccation and accumulation of soluble sugars, particularly raffinose. Raffinose concentration increased linearly with bud desiccation and peaked prior to the occurrence of maximum FT. Unlike previous reports, our study suggests that raffinose may play a more important role in desiccation tolerance than FT. It is concluded that ABA application has the capacity to increase FT and can be utilized as a prophylactic tool to protect against winter injury of grape cultivars grown in cold regions. |
