The effects of aluminium on root cap function and root development in Zea mays L.

Abstract

Aluminium concentrations ⩽1.25 mg dm −3 promoted a positive growth response in the primary root of Zea mays and this coincided with an increase in mean cap volume. Cap volume correlated with cap cell numbers. Significant alterations in mean cell volume were limited to the cap periphery where Al caused a decrease in cell volume. This implied that the action of Al was directed at differentiation of peripheral cap cells and that the observed changes in cap volume arose through interference by Al in mechanisms which control cell loss from the cap. This observation was supported by a reduction in the number of detached, mature cap cells surrounding Al-treated roots. Ultrastructural investigation revealed that disruption of Golgi apparatus activity in the peripheral cap cells occurred at the lowest Al concentration (0.5 mg dm −3). A decrease in amyloplast numbers coincided with diminished Golgi apparatus activity in the peripheral cap and these changes preceded reductions in mitotic activity, indicated by decreases in cap volume and root length which were only evident at Al concentrations in excess of 1.25 mg dm −3. Al treatment also resulted in a redistribution of amyloplasts to the proximal halves of central cap cells as well as alterations to the linear arrangement of these cells. The presence of Al was associated with an efflux of H +. Some of the more adverse responses to Al, including inhibition of cell division, were observed to be initiated at Al concentrations where the molar ratio of charge between Ca 2+ and Al 3+ favoured Al. This may attach relevance to the electrical properties of the Al ion in the initiation of an electro-physiological plant response to Al.