STIMULATION OF RHIZOSPHERE IRON REDUCTION AND UPTAKE IN RESPONSE TO IRON-DEFICIENCY IN CITRUS ROOTSTOCKS

The effects of Fe deficiency on the extracellular Fe3+ reduction by ci trus seedling roots were characterized relative to the ability of citr us to acquire Fe in nutrient solutions. The roots of Fe-deficiency-str ess resistant rootstocks, when grown in minus-Fe solutions, have sharp ly increased rates of Fe3+ reduction and uptake compared to plants gro wn in complete nutrient solutions. The levels of stimulation in the re sistant rootstocks are much higher than those that occur in susceptibl e rootstocks. There was a correspondingly greater increase in Fe uptak e by the Fe deficiency resistant rootstocks than in Fe deficient, susc eptible rootstocks. Strong inhibition of Fe uptake by bathophenanthrol inedisulfonic acid indicated that Fe3+ reduction is essential for the Fe uptake by citrus. These findings indicate that degrees of Fe-defici ency-stress resistance in citrus rootstocks are determined by the leve ls of stimulation of root-associated Fe3+ reduction.