ROOT APICAL MERISTEMS OF ARBUSCULAR MYCORRHIZAE OF ALLIUM-PORRUM L

The apices of adventitious roots of leek plants have an open meristem with a quiescent centre localized at the pole of the stele. Mycorrhiza l apices do not change in structure with respect to those of uninfecte d apices but increase drastically in size under the growth conditions in the present study. This increase is due partly to a higher number o f meristematic cells, and partly to an increase in cell sizes which, i n the cortex, is principally due to greater cell diameters. The size a nd the complexity of the different zones within the root apex are corr elated: the mean thickness of the cortex is related to the diameter of the stele and to the mean length of the cap; the wider stele of mycor rhizal roots possesses a higher number of xylem poles. Despite their g reater size, root apex meristems of arbuscular mycorrhizae show a lowe r metabolic activity than controls, as indicated by the lower H-3-thym idine labelling index in the cortical meristem and by the lower degree and different pattern of labelling in the quiescent centre zone after H-3-uridine incorporation. These differences between apices of uninfe cted and infected roots probably depend on variations of the hormonal balance and are responsible for a different morphology in mycorrhizal and control root systems, which are longer and less branched in contro ls than in mycorrhizae.