RESEARCH NOTE Wood formation during ex vitro acclimatisation in micropropagated true service tree (Sorbus domestica L.) Jaroslav D ˇ urkovic ˇ Æ Andrea Mis ˇalova ´ Received: 6 October 2008 / Accepted: 15 December 2008 / Published online: 31 December 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Micropropagated plantlets derived from a superior 90?-year-old slow-growing true service tree (Sorbus domestica L.) have been successfully acclimatised to the ex vitro environment. The temporal pattern of developmental changes was examined in relation to sec- ondary xylem growth during ex vitro acclimatisation. In vitro rooted plantlets already initiated lignification of sec- ondary xylem cells. During early days after transfer to ex vitro conditions, the growth of woody tissue was slow. The most prominent increase in woody tissue development occurred between days 7 and 35. From days 35 to 63, sizes of vessel lumen areas significantly increased. In developing woody tissue, a characteristic diffuse-porous pattern of roughly even vessel area distribution throughout the growing season, typical for mature wood, was not fol- lowed. The proportion of woody area occupied by vessels was significantly higher in stems of fully acclimatised plantlets than in stems sampled 35 days after transfer. On day 63 after transfer, a proportion of woody tissue area in fully acclimatised plantlets represented up to 14.8% of the stem area. Early formation of wood during ex vitro accli- matisation provides brittle plantlets with a mechanical support to cope better with deformations and mechanical injuries during handling at the subsequent transplantations. Keywords Ex vitro acclimatisation Á Secondary xylem Á True service tree Á Vessel Abbreviations MS Murashige and Skoog medium NAA 1-Naphthaleneacetic acid PPFD Photosynthetic photon flux density SH Schenk and Hildebrandt medium WPM Woody plant medium True service tree (Sorbus domestica L.) is a noble hard- wood, highly prized for timber and for horticultural qualities. Wood is quite dense, fine-grained, and has good bending strength. Although the growth rates are slow and the regeneration rate rather low, the species is well adapted to withstand moisture stress and has the potential to be used for reforestation of degraded areas (Arrillaga et al. 1991). In most countries in central Europe, true service tree is very rare and threatened, and is considered a valuable biological resource worthy of conservation. Micropropagation has become an important technique for mass production of many plant species. In forestry, however, the use of micropropagated noble hardwoods is still limited by its relatively high production costs and low survival rates of regenerants during ex vitro transplanting. Several investigations have linked low ex vitro survival directly or indirectly to the use of heterotrophic or photo- mixotrophic in vitro culture conditions (Seon et al. 2000; Mosaleeyanon et al. 2004; Valero-Aracama et al. 2007). Due to these stressful conditions, in vitro plantlets often show reduced amounts of epicutilar waxes, poor cuticle development and improperly functioning stomata. These abnormalities lead to an inability of the plantlet to control excessive epidermal transpiration, which is considered to be the main cause of the mortality of regenerants following the transfer to soil conditions (Ziv 1995). To improve the rate of survival and physiological functioning of micro- propagated plantlets, the procedures such as gradually J. D ˇ urkovic ˇ(&) Á A. Mis ˇalova ´ Department of Phytology, Technical University, Masarykova 24, 96053 Zvolen, Slovakia e-mail: durkovic@vsld.tuzvo.sk 123 Plant Cell Tiss Organ Cult (2009) 96:343–348 DOI 10.1007/s11240-008-9492-8