A study of the seasonal dynamics of winter bud dormancy and flowering and starch contents in shoots of Shine Muscat grapevine grown in Guangzhou

Abstract:【Objective】Guangdong is a relatively new region for table grape production. With the use of rain shelter cultivation, the application of dormancy removing chemicals, and the introduction of high quality and disease resistant cultivars, such as Shine Muscat and Summer Black, table grape production is expanding rapidly in the province. The long growing season in Guangdong provides great flexibility in adjusting the harvesting period of table grape. Mastering the dynamics of dormancy status of winter buds, flowering process and carbohydrate reserve build-up provides important guidance for precise reg- ulation of bud dormancy and production season. This study was carried out to understand the effects of season, shoot maturity, illumination and trellis system on dormancy status and flowering process in the winter bud and carbon nutrition reserve build-up in Shine Muscat.【Methods】The study was carried out from June 2019 to January 2020 with four-year-old Shine Muscat vines under pergola or V shaped trel- lis systems. Starting from June 2019 till January 2020, current season shoots of different maturity (brown hardwood, greenish brown semisoftwood and green softwood) in vines with pergola trellis were pruned to observe bud break of the remaining top winter buds on tree. At the same time, hardwood, semisoftwood and softwood shoots were collected and separately cut into one-bud sections. 0.5 cm long internode segments were collected and frozen in -20 °C for starch determination. The one-bud stem sections were soaked in carbendazim for 5 min and placed in a tissue culture bottle filled with pure water by one fourth of the bottle volume with the cut end submerged in water. They were placed under a room temperature of 28 °C. Bud break was observed every 3 days. Cumulative budbreak rate within 30 days after pruning was calculated. In the late August, hardwood shoots of the vines under pergola system and under V trellis system were harvested for starch analysis, which was conducted using the iodine colori- metric method. In the experiment exploring shading effect on dormancy of winter bud, winter buds at the 4th or 5th node above the panicle were covered with aluminum foil to completely shade the winter buds in June. In the late August, the shoots were pruned so that the shaded bud became the top bud remaining on tree. With the non-shaded winter buds as the control, bud break was observed and total bud break rate within 30 days after pruning was calculated. The experiments were carried out with a randomized single-tree based block design and four biological replicates (trees).【Results】Bud break rate of the on-tree winter buds in the hardwood and semi-softwood decreased dramatically after September, while bud break in softwood was observed even in November. In off-tree experiment, bud break in hardwood dropped to zero in October, while bud break in softwood and semisoft wood continued to occur even in November. During the growing season in June, July and August, bud break rate was higher in off-tree hardwood than in on-tree hardwood, but the opposite was found in semi-softwood and softwood. However, from October, bud break rate in off-tree semi-softwood and softwood became higher than in on- tree ones. Shading the winter buds did not significantly influence bud break. Among the broken winter buds in the hardwood and semi-softwood, around half were with flower clusters in June, July and Au- gust. Winter buds in the softwood had the lowest flowering rate. In June and July, 11% and 30% of the winter buds in softwood produced flower cluster after bud break, suggesting that flowering in winter bud initiates early in the summer season. However, the flowering rate in the semi-softwood and soft- wood dropped to zero in September. Starch content in shoot was always the highest in the hardwood, and there was no significant difference between the semi-softwood and the softwood. There was a systematic increase in shoot starch in November. In August, shoot starch content was higher in Shine Muscat under pergola trellis than under V trellis.【Conclusion】For the observation of dormancy status of winter buds, off-tree method seems better than on-tree method. Winter buds in the hardwood enter deep dormancy after October in Guangzhou while those of the semi-softwood and softwood do not enter deep dormancy even in late autumn. Flower differentiation occurs very early in winter bud during the summer growing season in June and July. However, the flowering process in winter buds ceases in autumn. Therefore, flowering might be initiated by a seasonal signal in grapevine. Starch accumulates as the shoot becomes mature, and systematic starch accumulation occurs in response to short-day photope- riod in late autumn. Pergola trellis with horizontal cane growth is more favorable for the buildup of carbohydrate reserve than V trellis with shoot growing upwards. Therefore, pergola trellis is more suitable for production of a second crop in winter. Winter bud seems not the organ to sense light conditions as directly shading the winter did not significantly affect bud break.