Morphological and Anatomical Juvenility Markers in Silver Birch (Betula pendula Roth) In Vitro Shoots

During plant development, woody plants progress from a juvenile phase to the reproductive mature phase. This process is marked by morphological, physiological, and biochemical changes, including loss of competence to vegetative reproduction. During in vitro stabilisation, mature tissues undergo rejuvenation, however, the exact degree of juvenility attained is not known and differing regeneration capabilities are observed between genotypes. Markers indicating juvenility state would assist the in vitro rejuvenation process and increase the efficiency of successful micropropagation. Several leaf and stem morphological and anatomical traits were analysed as possible juvenility markers in mature and rejuvenated silver birch in vitro shoots. The results showed that juvenility state of birch in vitro shoots varies between different genotypes and between plants from the same genotype. Mature birch in vitro shoots had approximately two times larger and thicker leaves compared to rejuvenated and juvenile shoots. Stem radius, including cortex, phloem, xylem and pith width as well as phloem width : stem radius and phloem width : xylem width ratio was significantly larger in mature shoots. Periderm development and loss of rooting ability were observed in mature shoots. These anatomical and morphological traits could be used as possible juvenility/maturation markers.