The temperature-based Grapevine Sugar Ripeness (GSR) model for adapting a wide range of Vitis vinifera L. cultivars in a changing climate

Content Partner: Lincoln University. Context and purpose of the study ‐ Temperatures are increasing due to climate change leading to advances in grapevine phenology and sugar accumulation in grape berries. This study aims (i) to develop a temperature‐based model that can predict a range of target sugar concentrations for various cultivars of Vitis vinifera L and (ii) develop extensive classifications for the sugar ripeness of cultivars using the model. Material and methods ‐ Time series of sugar concentrations were collected from research institutes, extension services and private companies from various European countries. The Day of the Year (DOY) to reach the specified target sugar concentration (170, 180, 190, 200, 210, and 220 g/l) was determined and a range of models tested using these DOYs to develop the best fit model for Vitis vinifera L. Results ‐ The best fit linear model– Growing Degree Days (parameters: base temperature (t0) = 0°C, start date (Tb) = 91 or 1 April), Northern Hemisphere) – represented the model that required the least parameters and therefore the simplest in application. The model was used to characterise and classify a wide range of cultivars for DOY to reach target sugar concentrations. The model is referred to as the Grapevine Sugar Ripeness Model (GSR). It is viticulturist‐ friendly as it's simple in form (linear) and its growing degree day units are easily calculated by adding average th temperatures (base temperature was optimized at 0°C) derived from weather stations from the 91 day of the year (Northern Hemisphere). The classifications based on this model can inform cultivar choice as an alternative adaptation strategy to climate change, where changing cultivars may prevent the harvesting of grapes at high sugar concentrations which leads to higher alcohol wines.