Ex vitro acclimatization of Lavandula angustifolia Mill. plants

The success of large scale micropropagation depends on the ability of the in vitro regenerated plants to acclimatize in the greenhouse and in the open field. The objective of this study was to determine the complex of structural and functional changes in regenerated plants of valuable Lavandula angustifolia cultivars under their acclimatization ex vitro. Rhizogenesis of lavender cultivars, ‘Belyanka’, ‘Record’, ‘Prima’, and ‘Sineva’ was induced on MS medium with 0.5-1.0 mg L^‑1 IBA or NAA. By histological analysis, it was found that the root regeneration started with the formation of a tiny morphogenic zone by division of cambium cells in the shoot. For plantlet adaptation, three types of relative air humidity regulation, such as culture vessel without an insulator, with a perforated insulator, and with an entire insulator were used. After 15 days, the insulators were removed in the second and the third types. Changes in morphological, anatomical, and physiological parameters of the vegetative organs after 7, 15, 30, 60, and 90 days of ex vitro culture were noted. In the leaf structure, an enhancement of mesophyll densening, leaf thickness increase, epidermis and cuticle thickness increase, and the formation of a number of non-glandular trichomes were noted. Significant increase in the water-holding capacity of the leaf blades subjected to almost equal water content of in vitro and ex vitro plants was shown. The maximum photochemical quantum yield and photosynthetic activity in the acclimatized plants after 30 days were the same as in the open field plants. Maximum intensity of transpiration was under in vitro conditions and it had negative correlation with the water-holding capacity of leaf tissues. The effective quantum yield was increased. After 90 days of plant growing in the greenhouse, their morphology and organ anatomy corresponded to those of the mother plants. The most favorable method for the control of the relative air humidity at the initial stages of the adaptation was the usage of perforated insulators for the first 15 days of ex vitro acclimatization.