Controlled environment studies involving the treatment of intact roots with Al (5 mg/dm3) and La3+ (25 mg/dm3 ) have identified some notable similarities in the early reactions by primary roots to these ions. These included root growth inhibition, changes in cap size and construction as well as a marked decline in cap secretory activity. The effects of La3+ nevertheless appeared to be less damaging to the root than comparable Al treatments (shorter lag periods and faster growth rates). Moreover, roots subjected to ''continuous'' La3+ treatments resumed growth with times greater-than-or-equal 6 days whereas roots subjected to ''continuous'' Al treatment did not. These similarities and differences in the response of root cells to La3+ and Al are discussed with a view to the possible development of La3+ as an ultrastructural tracer for Al. Roots which had previously been treated with La3+ or Al but which received Ca2+ during the recovery phase resumed growth during the course of the experiment (8 days). In contrast Ca2+ deprivation of roots initially subjected to Al or La3+ treatments delayed and in some cases prevented recovery. The effects of Ca2+ deprivation on the stages involved in recovery are discussed in relation to identifying an external (apoplasmic) Ca2+ requirement which influences the activity of the root cap and mediates the reversal of growth inhibition attributed to the effects of La3+ and Al. The requirement for an exogenous Ca2+ supply is combined with data pertaining to Al- and La3+-induced ultrastructural changes in the apoplasm of peripheral root cap cells to identify components of a stimulus-response system functioning in the root which is responsive to La3+ and Al. Changes in La3+- and Al-treated roots deprived of Ca2+ during recovery also coincided with a re-orientation of shoot growth. These effects are considered in relation to a possible external requirement, by the root, for Ca2+ which interacts with an endogenous programme involved in modulating shoot expansion and development.
