Fluctuations in winter chilling availability impact bud dormancy and budburst. This study aimed to quantify the thermal requirements during dormancy for ‘Italia’ grape, under different thermal regimes. Cuttings of grapevines ‘Itália’ were collected in Veranópolis-RS, on April/2017, with zero chilling hours (CH). The cuttings were exposed to constant (7.2°C) or alternating (7.2 and 18°C for 12/12h, 12/12h or 18/6h) temperatures, or yet, a constant temperature (7.2°C) or alternating (7.2 and 18°C for 12/12h), combined with one or two days a week at 25°C. Periodically, part of the cuttings was transferred to 25°C for daily budburst evaluation. The induction of the endodormancy (dormancy induced by cold) occurred with 200 CH, independent of the thermal regime, and the overcoming with 300 HF, at 7.2°C. The alternating heat of 18°C in the middle of the cold did not affect the process of overcoming endodormancy. Heat waves during endodormancy resulted in an increased CH to overcome the bud dormancy. The negative effect of high temperature depended on the exposure time. Chilling was partly cancelled during dormancy when the heat wave lasted 36 continuous hours or more. These evidences serve as basis for new model adjustments for budburst prediction, especially for regions with mild and irregular winters, such as those of Southern Brazil.
Testing for the effects of pre-season temperature and winter-chilling on land-surface phenology of coniferous and broadleaved forests in Central Europe
