Significance of the Apoplast for Al3+ Resistance
Cell walls, particularly pectin-rich primary cell walls, carry many negative charges which are usually compensated by bivalent ions (Mg2+, Ca2+). It is assumed that Al3+ replaces such bivalent ions from their binding sites, and that the cell wall thus acts as an aluminium trap. Such binding sites would quickly saturate with Al3+ ions, but upon continuous root growth new ion-binding sites originate so that protection of the protoplast by the cell wall does not seem unrealistic. However, so far differences in cell walls of sensitive and resistant cultivars have not been shown.
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