Phase change-related variations of dome shape in Eucalyptus urophylla c Eucalyptus grandis shoot apical meristems

摘要

Shoot apical meristem (SAM) domes derived from five different outdoor and in vitro sources of juvenile and mature Eucalyptus urophylla c Eucalyptus grandis akin genotypes were compared. Overall measurements of SAM dome height H and diameter D ranged from 2 to 35 om and 20 to 80 om, with significant differences according to the various physiological origins of plant material investigated. SAM domes from the mature trees Mat were taller than those from the rejuvenated ministock plants Rej; from the in vitro microcuttings IVM of the same clone and also from the in vitro juvenile seedlings IVJ, whereas outdoor seedlings Juv exhibited intermediate SAM dome height. SAM domes from the rejuvenated material Rej, from the in vitro mature IVM and juvenile IVJ origins were also narrower than those from the outdoor seedlings Juv and to lesser extent than those from the mature trees Mat. Overall, the mature source Mat displayed bigger and somehow sharper hemispherical domes than those from Rej and Juv, physiologically more juvenile, or those from the in vitro origins IVM and IVJ which looked flatter and smaller. SAM dome height, diameter D and H/D values varied also significantly according to the plastochron. More specifically, H, D, and H/D SAM differences between the five origins were not significant during the early plastochron phase corresponding to leaf initiation, to become more salient as leaf structures started to elongate and to differentiate. This was particularly obvious for mature tree Mat SAM dome shapes which showed at this stage much higher H/D values than the other SAM sources. A shape index S used for characterizing more accurately dome shape confirmed these trends. These observations provide additional arguments to the view that juvenility in trees becomes more and more time- and shoot-tip restricted as ageing increases in the course of time during the ontogenetical process and could be ultimately confined to the most organogenic phase of SAM, from which shoot characteristics derive.