Chilling requirement and budburst uniformity of cultivar 'Maxi Gala' grafted on different rootstocks

In marginal regions to grow apples, where warm winters result in insufficient winter chill, it is important to know how many chill hours accumulate seasonally. These data allow matching a cultivar chilling requirement to growing region winter chilling. Inadequate chill hours have a negative effect on budburst uniformity in plants. Chill hour requirements for the main cultivars grown in southern Brazil are known, but it is not known how rootstocks modify the chilling requirement of the scion. The aim of this trial was to evaluate scion chilling requirement and budburst uniformity on rootstocks G.213, G.814, G.202 and M.9, and the behavior of the scion grafted on them. The study was conducted at the Experiment Station of Temperate Clime Fruit of Embrapa Grapes and Wine, at Vacaria-RS, Brazil. Evaluation of four different winter chill levels was studied: 400, 600, 800 and 1000 hours under 7°C. The plants were stored in a cold room with temperatures between 2 and 4°C until the number of hours stipulated in each treatment were met. Afterwards, trees were moved to a controlled environment with a temperature between 20 and 22°C, 12 h of photoperiod and 70% relative humidity. Trees were evaluated for bud break weekly until bud break was complete. In the field, we evaluated the budburst uniformity of 5-year-old ‘Maxi Gala’ apple trees grafted on G.213, M.9 and Marubakaido with two different interstem lengths: 20 and 30 cm. ‘Maxi Gala’ grafted on different rootstocks and the freely growing rootstocks followed the same budburst trend, showing an influence of the rootstock on grafted cultivar. The rootstock G.213 induced greater budbreak on ‘Maxi Gala’ than M.9. With more than 800 chill hours the CG series rootstocks stimulated more budbreak of ‘Maxi Gala’ than M.9. The rootstocks G.814 and G213 got better budbreak than G.202 and M.9 for ‘Maxi Gala’ when exposed to 600 winter chill hours. In the field, G.213 rootstock had the best budbreak uniformity to ‘Maxi Gala’ than M.9 and Marubakaido with 20 and 30 cm long M.9 interstem.