Seeds from the aluminum tolerant inbred line Cat-100-6 and from the aluminum sensitive somaclonal variant S1587-17 were germinated at 30?C and after 3 days were transferred to an aerated nutrient solution containing aluminum in the form of AlK3(SO4)3 at a concentration of 6 mg/l. Seedlings were removed at different exposure times (0, 1, 2, 4 and 6 days) and the primary roots were fixed and embedded in paraffin. For comparison, 10 mm root sections were taken from the sensitive and tolerant plants and stained with toluidine blue and hematoxylin.

Histological sections (Fig. 1) show the effect of aluminum at the tissue level. At 0 days, both tolerant (Fig. 1A) and sensitive (Fig. 1F) plants presented normal morphology, with the tolerant plants continuing to present normal root apex morphology after 6 days exposure to aluminum (Fig. 1B-E). Figs. 1D and 1E show abscission of cap cells from the tolerant root apex, which is indicative of actively growing roots. After 1 day of exposure to aluminum the sensitive plant root (Fig. 1G) showed an increase in cap volume; after 2 days of exposure the epidermis and the cortex were severely damaged, and the root cap cells exhibited grossly swollen cells (Fig. 1H); after 4 days the peripheral cap and central cap were completely destroyed and only the cap meristem, although severely damaged, could be observed (Fig. 1I). After 6 days of aluminum exposure (Fig. 1J) all components of the meristematic zone were completely destroyed, and secondary root meristems appeared in the elongation region.

Aluminum infiltration in the roots was detected by hematoxylin staining of the primary root sections. The sensitive plant showed a gradual accumulation of aluminum within the root tissue with time. After 1 day of exposure the staining was confined to the root exterior; after 2 days the nuclei of the root cells were stained, indicating the presence of aluminum even within the nucleus of the affected cells; after 4 and 6 days of exposure there was a more intense staining within the root indicating massive infiltration of aluminum into the root tissue. The tolerant root sections showed very little staining after 1 day and minimal staining over the rest of the time period.

Figure 1. Toluidine blue stained maize primary roots sectioned through the elongation zone from tolerant Cat-100-6 (A to E) and sensitive S1587-17 (F to J) plants. Controls without Al (A and F); 6 mg/l Al for 1 day (B and G); 6 mg/l Al for 2 days (C and H); 6 mg/l Al for 4 days (D and I); 6 mg/l Al for 6 days (E and J). The arrow in (E) indicates cap cells abscission, a signal of actively growing root; in (I) indicates complete destruction of root cap and in (J), the appearance of lateral root meristems after disruption of root cap. Sections were photographed with bright field illumination with a magnification factor of x80.

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