Phenology and thermal accumulation in grapevines in the Fronteira Oeste region of Rio Grande do Sul, Brazil

The objective of this work was to evaluate the thermal requirement, obtained by different thermal time calculation methods, to characterize the grapevine (Vitis vinifera) phenology of 'Cabernet Sauvignon', 'Tannat', 'Ruby Cabernet', and 'Merlot' grown in the Fronteira Oeste region, of the state of Rio Grande do Sul, Brazil. The phenological development was followed for five seasons - 2005/2006 to 2009/2010. Minimum and maximum air temperatures were recorded daily, and the following eight thermal time methods were tested: M1.1, M1.2, and M1.3, which use only the down threshold temperature (10 degrees C); M2.1 and M2.2, which also consider 25 degrees C as the optimum temperature for development; and M3.1, M3.2, and M3.3, which, besides using the prior temperatures, consider 35 degrees C as the upper threshold temperature of development. These methods were compared using the standard error (SE) estimates of accumulated heat. The SNK test was used to compare the thermal requirement between cultivars. M3.3 was the method that best simulated 'Tannat' and 'Merlot' development (1,823.1 and 1,780.8 degree-day respectively). However, the least deviation was obtained in 'Cabernet Sauvignon' and 'Ruby Cabernet' using the M3.1 method (1,958.9 and 1,944.8 degree-day respectively). Methods employing the three cardinal temperatures showed greater accuracy. 'Tannat' and 'Merlot' are the cultivars that show the lowest thermal requirement to complete the cycle.