Destruction Process of Plant Root Cells by Aluminum

Abstract

Observations with the scanning electron microscope revealed that Al induced various morphological changes in the root surface, i.e. decrease in the turgescence of epidermal cells of the tip and elongating regions (barley), occurrence of a large number of small depressions mainly in the elongating region (oats and rice), destruction of epidermal and outer cortex cells in the tip and the elongating regions (maize), and cross-sectional deep cracks in the inner cortex cells mainly in the elongating region (pea); whereas few morphological changes occurred in the proximal portion of the root. Vital staining test for the elongating region after Al treatment showed that cell damage occurred only in the epidermis in the Al-tolerant oat plants, epidermis and outer cortex in the Al-sensitive maize plants, and epidermis and almost all of the cortex in barley plants highly sensitive to Al. Al content was higher in the root tip; K content was lower in the tip of hardly growing root, but was lowest in the middle portion, i.e., elongating region, of the growing root. By the Al treatment, the number of protoplasts of maize roots decreased and the plasmalemma of the protoplasts was abnormal, shrank or was thickened. These results indicate that: (1) in Al-sensitive plants the destruction of the root cells involves the more proximal and the more inner cells, and (2) that the destruction of cells associated with Al toxicity is not restricted to the meristematic region of the root. The following mechanisms are postulated: Al may bind to various kinds of binding sites in the plasmalemma of root cells, and consequently leakage of K out of the plasmalemma and passive Al permeation through the plasmalemma may be induced.