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Pear rootstock OHF 333 (Pyrus communis L.), which is included in the US ‘Old Home’ × ‘Farmingdale’ series, is characterized by good compatibility with most other varieties, high yields, and a moderate degree of resistance to fire blight. Micropropagation in vitro has shown promise for rapid, large-scale cloning of disease-free plant material throughout the year. However, pear shoots are often recalcitrant to rooting, and this process is highly genotype-dependent. This study aimed to improve the micropropagation protocol by ex vitro rooting of pear rootstock OHF 333 (Pyrus communis L. ‘Old Home’ × ‘Farmingdale’). Charkor, a new plant growth regulator of natural origin was used, which contains metabolism products of symbiotic fungus-endophytes of ginseng roots, as an alternative to synthetically produced plant growth regulators (PGRs). Microcuttings were obtained from in vitro cultured shoots and subjected to four different treatments for ex vitro rooting: 1 g L−1 1-naphthaleneacetic acid (NAA) (as a powder), 0.5 mL L−1 Charkor for 3 hr or 6 hr, or the same concentration of Charkor prepared as a powder. Microshoots dipped in sterile distilled water with no additional hormonal treatments served as controls. Cultures were kept in a growth chamber under a 16-hr photoperiod, with air humidity maintained close to 100% (above 96%) for 2 weeks and then gradually reduced to 60%. Data on final acclimatization rate (survival rate; %), mean number of roots per plant, stem and root length, mean number of leaves per plant, and final acclimatization rate were collected 90 days after transplanting to ex vitro conditions. All treatments induced a successful acclimatization rate of more than 31%. The highest survival rate (86%) and longest stems were achieved by treatment with 0.5 mL L−1 Charkor for 6 hr. The greatest mean number of roots per plant, root length, and number of leaves was achieved in the variant treated with 1 g L−1 powdered NAA. [ABSTRACT FROM AUTHOR]

Rooting in vitro is considered one of the main problems in the micropropagation of woody species. The aim of the present study was to optimise the rooting and acclimatization of micropropagated pear plants through applying a new plant-growth biostimulator of natural origin, Charkor (Agrobiotech, Ukraine). Charkor contains metabolism products of symbiotic fungus-endophyte of ginseng roots, cultivated in vitro. Microshoots of pear rootstock OHF 333 (Pyrus communis L. 'Old Home' x 'Farmingdale') were rooted on DKW (Driver and Kuniyuki, 1984) media supplemented with Charkor (0.1 ml l-1, 0.3 ml l-1 or 0.5 ml l-1) and 10 µM indolyl-acetic acid (IAA; alone or in combinations). Plantlets cultivated on a medium free of growth regulators served as a control. After three weeks on rooting media, plants were transferred for acclimatization in pots (peat: perlite 2:1). Data on fresh weight of leaves, roots, stems and dry matter, leaf area, mean number of leaves per plant, stem and root length, chlorophyll fluorescence and net photosynthetic rate were recorded 20 days after acclimatization. The best result was obtained with plants treated with10 µM IAA (alone) and in combination with 0.5 ml l-1 Charkor. [ABSTRACT FROM AUTHOR]