THE RECOVERY OF THE ROOTS OF ZEA-MAYS L FROM VARIOUS ALUMINUM TREATMENTS - TOWARDS ELUCIDATING THE REGULATORY PROCESSES THAT UNDERLIE ROOT-GROWTH CONTROL

Controlled environment studies have demonstrated the potential of roots exposed to Al (5 mg/dm3) for various times (up to 48 hr) to recover and resume primary growth subsequent to their transfer to Al-free recovery solutions. Ultrastructural investigation of the stages involved in recovery showed that the resumption of root elongation rates during recovery coincided with the presence of a morphologically distinctive secretory activity in peripheral root cap cells. This suggested links for this activity in regulating root growth responses. Changes in the volume of the root cap, attributed to reactivation of the cap meristem initially made inactive by Al, and the recovery of root meristem cells involving nucleolar reaggregation and decreased vacuolation were characteristic of the early stages of root recovery, and preceded by several days a resumption of root elongation rates. Recovery was concomitant with root growth stimulation. Activation of the cells of the quiescent centre to form a new cap was not an early response of roots recovering from Al. In contrast, "continuous" root exposure to Al prevented recovery of the primary root meristem and resulted in severe reductions in root elongation rates and a loss of apical dominance. The stage involved in the recovery of roots from Al are used to discuss root growth regulation and the physiology of Al toxicity.