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Rhododendron dauricum L., a semi-evergreen shrub growing in the south of Western Siberia, Mongolia, and China, is a valuable genetic resource for breeding of new rhododendron cultivars with winter-hardiness, polymorphism of corolla color, and the terms of flowering. To create efficient systems of micropropagation, floral explants consisting of isolated pistil with pedicel were used. Contrary to the other types of vegetative explant, floral explants have a number of advantages, such as a low level of contamination. While micropropagation protocols based on floral explants have been developed for evergreen rhododendrons, they were found not to initiate the regeneration from floral explants of semi-evergreen R. dauricum. As a result, a new optimized two-step procedure for direct shoot regeneration from R. dauricum floral explants was presented: (i) preculture of floral explants on hormone-free Anderson medium for 4 d; (ii) induction of shoot regeneration under 1.0 µM thidiazuron (TDZ) and 2.5 µM zeatin. The detailed histological analysis of morphogenesis pathways revealed that regeneration from floral explants induced by TDZ occurs along the direct pathway of morphogenesis based on two parallel processes: dedifferentiation of epidermal cells with subsequent acquisition of competence to morphogenesis, and activation and development of already existing meristems. According to the present study, this is the first histological study of floral reversion in semi-evergreen R. dauricum. [ABSTRACT FROM AUTHOR]

For the first time, the effect of mechanocomposite (MC) based on biogenic silica and green-tea catechins on axillary shoot formation, physiological and biochemical characteristics of two strawberry cultivars ('Alpha' and 'Solnechnaya polyanka') was studied. The shoots were cultured on Gamborg-Eveleg's medium (B5) supplemented with 0.75 mg l−1 6-benzylaminopurine (BA) (control) and 0.0, 2.5, 5.0 or 10.0 mg l−1 MC for 60 days. The highest regeneration responses (100%) and axillary shoot proliferation (up to 15.06 ± 0.81) correlating with increasing fresh weight (FW), dry weight (DW) on the media supplemented with 5.0 mg l−1 MC for both genotypes were recorded. The improvement in shoot regeneration accompanied with enhancing some antioxidant enzymes activity (superoxide dismutase and peroxidase) on the media supplemented with 5.0 mg l−1 MC for both genotypes. The increase in endogenous H2O2 concentration displaying a high morphogenetic potential in the presence of 2.5 and 5.0 mg l−1 MC was only observed in cv. 'Alpha' shoots. Overall, a positive effect of MC on the quality of in vitro shoots was established. Concentrations of photosynthetic pigments (chlorophylls a and b, and carotenoids), and their ratios (chlorophyll a/b, chlorophyll (a + b)/carotenoids) indicated a high physiological state of the plantlets grown with 5.0 mg l−1 MC. The accumulation of endogenous cytokinin (iP) and indole-3-acetic acid (IAA) is associated with the effect of MC in various concentrations on the organogenic response of both cultivars. Thus, the outcomes of this study can be utilized for the development of new systems for a healthy planting material production using "green chemistry" approaches and recommended for commercial strawberry micropropagation. Key message: Silicon chelates from plant waste are the useful component of plant tissue culture media for optimizing commercial strawberry micropropagation and preventing the occurrence of in vitro-derived disorders in plantlets. [ABSTRACT FROM AUTHOR]

In order to induce in vitro axillary shoot proliferation from single-node explants of Rhododendron mucronulatum Turcz., two techniques of thidiazuron (TDZ) application were tested: (i) two-step procedure including cultivation on Anderson medium (AM) supplemented with varying TDZ concentrations (0.1 μM; 0.25 μM; 0.5 μM; 1.0 μM; 2.5 μM) for 8 wk followed by cultivation on hormone-free medium (AM0) for 6 wk and (ii) 4-h liquid-pulse treatment with different TDZ concentrations (7.5 μM, 15.0 μM, or 30.0 μM) followed by cultivation on AM0 for 8 wk. The highest number of axillary shoots per explant was achieved with 0.1-μM TDZ after the two-step procedure. The best response in terms of percent regeneration (87%), shoot length (13 mm), absence of structure anomalies, and the shortest shoot production cycle (8 wk) was obtained with 30.0-μM TDZ liquid-pulse treatment for 4 h. The clonal fidelity of regenerated shoots was evaluated by random amplified polymorphic DNA (RAPD) and inter-simple sequence repeats (ISSR) markers and flow cytometry. Genetic similarity of all regenerants between themselves and with the mother seedlings was 99%. Flow cytometric analysis revealed that all samples studied were diploid. The nuclear DNA content of microshoots obtained under the TDZ treatments varied from 1.26 to 1.32 pg per 2C. There were no significant differences in DNA content among mother seedlings and in vitro developed shoots triggered by 0.1- and 2.5-μM TDZ nor by those triggered by the 30.0-μM TDZ pulse treatment. [ABSTRACT FROM AUTHOR]

Thidiazuron (TDZ)-induced shoot organogenesis from in vitro-derived leaf explants of winter-hardy Rhododendron sichotense Pojark. and Rhododendron catawbiense cv. Grandiflorum was studied using two TDZ application methods: adding various concentrations to Anderson nutrient medium and a liquid pulse treatment. The highest frequency of shoot regeneration (93%) and maximum number of shoots per explant (24.6) from R. sichotense was obtained after a two-stage culture procedure that included exposure of leaf explants to 1.0 μМ TDZ followed by TDZ-free cultivation. TDZ at concentrations of 0.5 and 1.0 μМ was effective for leaf explants of R. catawbiense cv. Grandiflorum, inducing 60 and 85% regeneration frequencies, and 36.0 and 25.0 shoots per explant, respectively. The liquid-pulse treatment of leaf explants with 30 μM TDZ for 4 h was effective only for R. catawbiense cv. Grandiflorum. Histological analysis revealed that epidermal cells of the adaxial side at the leaf-blade base were involved in de novo shoot regeneration in both genotypes. However, the adventitious shoot buds of R. sichotense developed from protuberances, whereas shoot organogenesis of R. catawbiense cv. Grandiflorum occurred through the formation of embryo-like structures. Optimum root formation for both genotypes was achieved using indole-3-butyric acid liquid-pulse treatment of microshoots followed by ex vitro rooting. An efficient system was developed for multiplication of R. sichotense and R. catawbiense cv. Grandiflorum, which are cold-hardy genotypes promising for future genetic transformation. [ABSTRACT FROM AUTHOR]