Aluminium Toxicity in Roots: An Investigation of Spatial Sensitivity and the Role of the Root Cap

Abstract

The primary symptom of aluminium (Al) toxicity in higher plants is inhibition of root growth. In this study, we investigated the spatial sensitivity of maize (Zea mays L.) roots to Al. A divided-chamber technique indicated that only exposure of the terminal 10 to 15 mm of the root to Al resulted in inhibition of growth. Application of Al to all but this apical region of the root had little or no effect on growth for 24 h and caused minimal damage to the root tissue. Small agar blocks infused with Al were then applied to discrete areas of the apex of maize roots to determine which section (root cap, meristem or elongation zone) was more important to Al-induced inhibition of growth. The terminal 2·0 to 3·0 mm of root (root cap and meristem) must be exposed to Al for inhibition. Application of Al to the 3·0 mm of root proximal to this terminal zone (elongation zone) resulted in damage to the root tissue but no significant inhibition of growth. Therefore, the visible injuries incurred by roots during Al-stress are not associated directly with the inhibition of root growth. Furthermore, removal of the root cap had no effect on the Al-induced inhibition of root growth in solution experiments and argues against the root cap providing protection from Al stress or serving an essential role in the mechanism of toxicity. We suggest that the meristem is the primary site of Al-toxicity.