Wood formation during ex vitro acclimatisation in micropropagated true service tree (Sorbus domestica L.)

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 secondary xylem growth during ex vitro acclimatisation. In vitro rooted plantlets already initiated lignification of secondary 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 followed. 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 acclimatisation provides brittle plantlets with a mechanical support to cope better with deformations and mechanical injuries during handling at the subsequent transplantations.