Effect of Meristem Size and Scion Origin on Micrografting of some Sweet Cherry Cultivars in vitro

 Sweet Cherry (Prunus avium) belongs to the Rosaceae family, which due to vegetative propagation problems, in vitro propagation is recommended to increase mass and disease-free production. Micropropagation has many advantages over other vegetative methods. Although the most suitable organ that preserves the genetic characteristics of the cultivar is bud meristem, plant regeneration from meristem culture is difficult in many species of woody plants, so micro-grafting is a suitable technique to overcome these problems. The aim of this study was to investigate the effect of scion size and origin of commercial sweet cherry cultivars interact with micrografting on the vegetative rootstocks.

 In this study, factorial experiment was used as a test unit in a completely randomized design (CRD) with two factors in five replications and ten seedlings per replication. The first factor was cultivar in seven levels (B: Bing, D: Dovomres, H: Haj Yousefi, P: Pishres, S: Siah- Mashhad, T: Takdaneh, Z:  Zard) and the second factor was scion type in four levels (M1R1: 2 mm in vivo explant, M2R1: 5 mm in vivo explant , M1R2: 2 mm in vitro explant and  M2R2: 5 mm in vitro explant). To prepare the scion, 1.5 to 2 cm long explants were isolated from shoot tips and then disinfected with 75% ethanol and 20% Sodium hypochlorite. After rinsing with distilled water, the shoot tips with 2 and 5 mm length were extracted for in vivo explants. In vitro explants were obtained from shoo tips that was previously established in MS culture medium with supplement of 1 mg.l-1 of BAP. The meristems were prepared in 2 and 5 mm and used as in vitro explant.  5 cm length in vitro shoots of sweet cherry ‘Gisella 6’ was used as rootstock. Micro-grafting was performed according to the standard method for sweet cherries. Micro-grafted plantlets were transferred to MS medium supplemented with 1 mg.l- l BAP, and kept under low light (100 lux) condition for one week, then  transferred to growth chamber at 27.1 °C photoperiods 16/8 hrs light/darkness (1500 lux). In order to induce root, grafted plantlets were transferred to Perlite: MS medium supplement with 1 mg.l- l IBA. After rooting, plants were placed in polyethylene pots containing perlite: peatmoss (1:1) for acclimation. Micro-grafting success indices were recorded in each of the micro-grafted plantlets. The data were analyzed by SAS statistical software (9.1) and the means were compared by Duncan's multiple range test (1 and 5 % of probability levels).

 The results showed that in all indices there was a significant difference between scion types and cultivar scion type interactions except grafting time, but there was no difference between cultivars (except longitudinal growth of scion). Among the scion types, the 5 mm in vitro scion (M2R2) had the highest micro-grafting success rate (42%), number of leaves (3.7), longitudinal growth (6.3 cm) and taken grafting time (two days). Finally, in successful micro-grafted plants, ‘Pishres’ cultivar had better results in rooting (32.8%) and ‘Zard’ cultivar in acclimation (3.4%) traits. Probably the presence of leaves led to better nutrient supply and surface contact, so it mostly improved the success of micrografting technique. In this study, micro-grafting success indices were lower than previous reports using seedling rootstocks. This might be due to difficult grafting operations, poor rootstock-scion communication, low physiological activity, and high in vitro oxidase activity. In the type of scion, micro-grafting success rate of 5 mm in vitro scions (include leaf primordia), was better than 2 mm scions (without leaf primordia). These results were consistent with most reports in sweet cherries and other stone fruit that were more successful in micro-grafting using larger in vitro explant.

 Based on our results, it can be concluded that the micro-grafting method in sweet cherry micro-propagation is a fast practical method with high potential for production and regeneration of healthy orchards, which is also possible for other cultivars. In micro-grafting success, in vitro explants are preferable to explants taken directly from in vivo mother trees, and the use of larger explants for scion is recommended due to the presence of leaf primordia in micro-grafting success. However, smaller-size explants are more likely to produce healthy plants.