PATTERNS OF ROOT RESPIRATION ASSOCIATED WITH THE INDUCTION OF ALUMINUM TOLERANCE IN PHASEOLUS-VULGARIS L

Aluminium (Al) tolerance in an Al-tolerant cultivar of Phaseolus vulgaris L. ('Dade') was found to be an inducible trait. Upon exposure to 10-mu-M Al, the rate of root elongation was inhibited in comparison to controls. During the following 72 h, the rate of elongation returned to levels comparable to controls. In contrast, root elongation of an Al-sensitive cultivar ('Romano') did not recover after exposure to Al. In Dade, the resumption of root elongation following exposure to Al was accompanied by increased rates of root respiration, whereas respiration rates slowly declined over the 72 h treatment period in Romano. When partitioned into growth and maintenance expenditures, a larger proportion of root respiration of Dade exposed to Al was allocated to maintenance processes, potentially reflecting diversion of energy to metabolic pathways that offset the adverse effects of Al toxicity. Romano did not show such a pattern and respiration associated with both growth and maintenance was reduced after exposure to Al. Root and shoot growth of both cultivars were also measured to determine the effects of long-term (21 d) exposure to 10-mu-M Al. Dade plants exposed to Al exhibited enhanced growth in comparison to controls, whereas Romano plants were characterized by reduced shoot and root growth. Modelling the time-course of root respiration and measuring the long-term growth responses to Al is a valuable method of elucidating respiratory costs of stress tolerance.