Iron deficiency induces cluster (proteoid) root formation in Casuarina glauca

The effect of iron deficiency, phosphorus, NaHCO3, chelator supply and nitrogen source on the formation of cluster (proteoid) roots was investigated in Casuarina glauca growing in water culture. The addition of iron-binding chelators (e.g. EDDHA, DTPA, EDTA) or increase in nutrient solution pH with NaHCO3 resulted in the formation of cluster roots when plants were grown in solution lacking iron. Phosphorus supply even at a concentration of 500 mu M did not inhibit cluster root formation if EDDHA was added to the iron-deficient medium. Cluster root formation was influenced significantly by nitrogen source and occurred only in nitrate-fed plants. C. glauca seemed to be very sensitive to iron deficiency as shown by plant chlorosis when grown on alkaline soil. The symptoms of chlorosis decreased as the chlorophyll content in shoots and the number of cluster roots increased, suggesting that the alleviation of iron deficiency in plant tissues was correlated with cluster root formation. It appears that iron deficiency is more important than phosphorus deficiency in inducing the formation of cluster roots in C. glauca.