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. Mycorrhizal apices do not change in structure with respect to those of uninfected apices but increase drastically in size under the growth conditions in the present study. This increase is due partly to a higher number of meristematic cells, and partly to an increase in cell sizes which, in the cortex, is principally due to greater cell diameters. The size and the complexity of the different zones within the root apex are correlated: 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 mycorrhizal roots possesses a higher number of xylem poles. Despite their greater size, root apex meristems of arbuscular mycorrhizae show a lower metabolic activity than controls, as indicated by the lower ³H-thymidine labelling index in the cortical meristem and by the lower degree and different pattern of labelling in the quiescent centre zone after ³H-uridine incorporation. These differences between apices of uninfected 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 controls than in mycorrhizae.