Calculation of Budbreak and Flowering Base Temperatures forVitis viniferacv. Touriga Francesa in the Douro Region of Portugal

Abstract

Plant growth and development is proportional to biological time, or thermal time, which can be defined as the integral of the product of time and temperature above a certain threshold. This constitutes the concept of units of growing degree days (GDD), calculated as the sum of the differences between the temperature of a certain time period and a threshold temperature (base temperature) for each period after a given starting date. The concept is simple to use and accurate in predicting phenological stages and has been used to forecast the main stages of plant development. For grapevines (<i>Vitis vinifera</i>), it is widely accepted as 10°C base temperature. Several methods were used to determine statistically the base temperatures for <i>Vitis vinifera</i> cv. Touriga Francesa in Douro Region (Portugal): <i>(a)</i> the least standard deviation in GDD; <i>(b)</i> the regression coefficient <i>(c)</i> the coefficient of variation in days; and <i>(d)</i> the least standard deviation in days. Three different procedures were followed to determine thermal time: <i>(a)</i> the average temperature; <i>(b)</i> the maximum temperature; and <i>(c)</i> the maximum temperature with a ceiling of 30°C if the maximum exceeded it. The procedure to calculate GDD based on the average temperature proved the more reliable. The least standard deviation in GDD was the most appropriate to calculate statistically the base temperature. We determined 8.7°C and 10.7°C as base temperatures, respectively, for budbreak and flowering.