Bud dormancy is one of the important issues in planting and cultivation of fruit trees that needs to be addressed in many trees such as orange (Citrus sinensis). Bud dormancy involves cessation of horizontal and vertical growth, lack of budbreak, and reduction in plant activity during cold weather. One of the commercial orange cultivars is Tarocco blood orange which compared to older blood cultivars, is larger in size and with lower alternate bearing has higher marketability. The nursery trees of this cultivar are grafted on sour orange, citrange, and vigorous rootstock of citrumelo (a hybrid between trifoliate orange and grapefruit) (Talon et al., 2020). One of the major problems of nurserymen in spring grafting of Tarocco cultivar on vigorous citrumelo rootstock is the failure of about 50 percent of buds to break compared to other cultivars on the same rootstock and other similar rootstocks. This unwanted dormancy leads to a one-year delay in the nursery tree production process and unnecessary occupation of nursery space. Given the commercial importance of blood orange and the adverse effects of bud dormancy on yield and fruit lifetime, solutions are used to control and overcome this problem. One of the effective solutions is the application of cytokinins which can stimulate the growth of graft buds (Yadav & Saini, 2018).

This research was conducted in a citrus nursery at the University of Agricultural Sciences and Natural Resources of Sari, in late May 2022. Citrumelo seedlings were grown in 5.3 liter pots containing a loamy-silt soil mix in the nursery location. Tarocco cultivar buds were prepared from a seven-year-old mother orchard and T-budding was performed in June (during rootstock bark slipping). All hormone treatments were applied after graft union and before bending the branch using a soft brush on the graft buds. The applied treatments included hormone treatment (control, 5000 mg.l-1 benzyladenine, 1000 mg.l-1 kinetin, and 50 mg.l-1 thidiazuron) and treatment time (13, 15, and 17 days after grafting). After two months, some traits related to budbreak and growth of the grafted buds were evaluated.

The results showed that thidiazuron and benzyladenine treatments had better effects compared to kinetin treatment on spring budbreak and initial growth of Tarocco grafted buds. In a way that 50 mg.l-1 thidiazuron treatment had the highest number of sprouted buds (67.91), largest leaf area (118.04 cm2), highest number of leaves (16.50), especially when applied 13 days after grafting. Also, in leaf size related traits, leaf area indices, graft growth rate as well as chlorophyll and carotenoid content of Tarocco graft leaves were significantly affected by different hormonal treatments and application times, with 50 mg.L-1 thidiazuron being more effective than other treatments. Cytokinins can promote division and expansion of leaf cells and thereby result in increased cell numbers and improvement of different leaf parameters. Also, cytokinins regulate important physiological processes like photosynthesis. Application of these materials provides cell division especially in areas like buds and growth points and also possibly more buds may form on the spring graft by using these treatments during the grafting process which can lead to increased bud break and faster plant growth (Cook & Bahar, 2017). Increasing cytokinin levels can stimulate the photosynthesis process which results in increased food production, leaf growth and ultimately increased leaf area. On the other hand, cytokinins affect plant metabolism and can regulate production and accumulation of different growth factors. This may lead to a better balance in nutrient distribution and metabolic activities which in turn aids leaf area increase (Hodchek et al., 2023). Finally, according to the obtained results, it can be recommended to nurserymen of this cultivar to use 50 mg.L-1 thidiazuron 13 days after grafting as a practical and effective strategy for increasing spring budbreak, growth and development of Tarocco buds grafted on citrumelo.

In general, based on the results, the application of 50 mg.l-1 of thidiazuron 13 days after grafting can be considered a practical and effective strategy to enhance bud awakening, as well as the growth and development of Tarocco spring shoots on citronmelo. This practice is recommended for producers of seedlings of this cultivar.