Light microscope and transmission electronmicroscopy (TEM)were
used to assess the effects of nZnO on symbiotic development. A longitudinal
section of typical mature pea nodule (indeterminate nodule) had
an elongated shape, which included an apical meristem, an invasion
zone, an interzone and a nitrogen fixation zone, and later in development,
a senescent zone (Vedamet al., 2004). In controlled cells, the nodules
in the fixation zone at 7 dpi were enlarged and contained densely
packed bacteroids and the infection threads were narrow and linear
throughout cells (Fig. 6a). The infection process was delayed in nZnOtreated
plants and the nitrogen fixation zone was not seen (Fig. 6b–c).
For nodules treated with 250 mg/L of nZnO, the infected cells in the
invasion zone had lowdensity of differentiating bacteroids. Nodules exposed
to 750 mg/L of nZnO contained mostly small non-infected cells,
with a great portion of the cellular content being vacuoles (Fig. 6c).
Many bacteroids in the nitrogen fixing zone of controlled nodules
were mature and differentiated into the typical characteristic Y-shape
of R. leguminosarum bv. viciae 3841 (Fig. 7a) and the infection threads
were of normal shape with thinner walls (Fig. 7b). In contrast, the