Histocytological characteristics of Eucalyptus urophylla × Eucalyptus grandis shoot apical meristems of different physiological ages

Histocytological characteristics of Eucalyptus urophylla × Eucalyptus grandis shoot apical meristems (SAMs) were described, comparing five outdoor and in vitro sources of akin genotypes differing in their physiological age. The size and the number of cells of the five zones identified within each SAM, i.e. the two tunica layers (L1 and L2), the central mother cells (CMC), the peripheral zone (PZ) and the combination of these four zones (4CZ) varied according to physiological age and plastochron phase. These five zones were significantly larger with higher numbers of cells for SAMs from mature and juvenile trees than for those from physiologically rejuvenated, in vitro mature and in vitro juvenile plants. However, these origin-related differences were not significant for SAMs in their early plastochron phase, to become obvious in a more advanced plastochron stage. Individual cell and nuclear measurements confirmed the rationale of distinguishing within SAM zones, characterized by specific cell and nuclear sizes liable to vary according to physiological age. The various histocytological investigations carried out established that SAM cell characteristics appeared to be the more reliable indicators of phase change. This was particularly true for the nucleoplasmic ratio and for more qualitative differences observed also at the nuclear level. SAM nuclei of the two in vitro origins were more evenly stained by naphtol blue-black, uniformly light for the juvenile source, whereas the mature source showed also darker nuclei. In contrast, SAM nuclei from outdoor origins had more chromocenters, darker and diffusely spread for the mature source than for the rejuvenated and the juvenile origins, where they were more peripherally distributed and where the nucleoli appeared more clearly. These results were discussed with respect to physiological ageing and in vitro culture influence, and suggest a determining influence of SAM cell nuclei on phase change phenomenon of arborescent species.