ALUMINATE-INDUCED CHANGES IN MORPHOLOGY AND ULTRASTRUCTURE OF THINOPYRUM ROOTS

The effects of aluminate [Al(OH)4-] on the morphology and ultrastructure of root cells were studied in the salt-tolerant grasses Thinopyrum bessarabicum (Savul. and Rayss) A. Love (2 x ) and Thinopyrum. junceum (L.) A. Love (6 x ) by light and transmission electron microscopy. Seedlings were grown in nutrient solution containing 1 mol m-3 [Al] and 5 mol m-3 Na2CO3 at pH 10.0. Light microscopy revealed that root tips of [Al]-treated plants displayed bending. Many cells of the cortex in the elongating region contained a fibrillar/granular material which rendered them densely staining. Radial (anticlinal) walls of the epidermal cells were either cleft apart of unusually thickened. Amyloplasts of the central root cap cells contained fewer starch grains, while their distribution was disturbed. Electron microscopy showed that the most serious effects of [Al] toxicity occurred at the cell walls of the epidermal and root cap cells, as they lost their fibrillar fine structure and contained an amorphous electron-dense material distributed all over the wall section. Electron-opaque droplets were encountered at the plasma membrane region of epidermal cells, while the electron-dense material observed in the vacuoles of cortex cells could be aluminate which had accumulated there. Thus, despite the presence of a barrier to aluminate uptake, some [Al] does enter the symplast. However, the cytoplasm of many epidermal cells displayed a normal fine structure and contained the usual subcellular components. Dictyosomes, in particular, were abundant and surrounded by many vesicles denoting an active state. These observations stress the role of cell walls as the major [Al] pool and of the plasma membrane as the ultimate barrier that protects the cytoplasm. Results are further discussed in relation to the findings in other plant species and it is concluded that. although aluminate is less toxic than Al3+, it causes morphological, structural and. presumably. functional damage to the roots of the species investigated.