INFLUENCE OF BICARBONATE ON IRON DEFICIENCY CHLOROSIS IN RHODODENDRON

In order to study the influence of HCO₃^- on iron deficiency chlorosis and root growth in Rhododendron, greenhouse and in vitro experiments were carried out using cv. 'Cunningham's White' and two clones selected for Ca-tolerance (Preil and Ebbinghaus, 1994). In one experiment ('A') the plants were grown in 11 cm plastic pots in peat and conifer-needle compost (1:1, v/v) supplemented with different amounts of CaCO₃ (1, 3.5 or 17 g/l CaCO₃), resulting in pH 4.2, 6.4 and 7.0, resp.. In another experiment ('B') the plants were cultivated in quartz sand watered with solutions of Ca(HCO₃)₂ or CaSO₄ (15 and 20 meq/l).

In both experiments increasing concentrations of HCO₃^- induced typical iron deficiency chlorosis symptoms in young leaves. In experiment 'A' root formation was completely inhibited by high levels of CaCO₃, and the plants died after 4 months. The total iron content in the young leaves was low (25 – 33 μg Fe g^-1 dry weight), as well as the HCI-soluble ('active') Fe content (13 – 18 μg Fe g^-1 dry weight). Uptake of Mn by young leaves was also reduced at high levels of CaCO₃ (6.3 – 6.9 μg Mn g^-1 dry weight). Ca⁺⁺ supplied as CaSO₄ (experiment 'B') did not cause any chlorotic symptoms.

Investigating the effect of bicarbonate on in vitro root growth 2 meq/l NaHCO₃ significantly reduced root length. 10 meq/l NaHCO₃ completely inhibited root formation.