EFFECTS OF EXTERNAL COPPER CONCENTRATIONS ON UPTAKE OF TRACE ELEMENTS BY CITRUS SEEDLINGS

Abstract

Many soils in Central Florida's citrus belt contain excess levels of copper (Cu) as a result of historical heavy use of Cu in citrus production. Citrus rootstocks vary considerably in their response to excess external Cu. This study was conducted to examine the effects of 0.1, 5, 10, or 20 μmol/L Cu concentrations in nutrient solution at pH 5.5 on growth, uptake, and partitioning of Cu, Zn, Fe, and Mn by seedlings of Cleopatra mandarin (CM) and Swingle citrumelo (SC) rootstock. With an increase in external Cu concentration, the shoot and root dry weights decreased significantly, whereas the concentration of Cu in both the plant parts increased. The increase in tissue Cu concentration was more marked in the root than in the shoot. Despite lack of significant difference between the two rootstocks with respect to short-term growth response to external Cu concentrations, this study demonstrated significant differences between the two rootstocks with respect to root and shoot Cu contents. Partitioning of total Cu into the shoot was significantly lower in CM than in SC seedlings. Accordingly, preferential accumulation of Cu in the roots of CM seedlings may be responsible for its increased tolerance, compared with SC rootstock, to high external Cu concentrations. Although increase in concentrations of external Cu decreased the uptake of Zn, Fe, and Mn by both rootstock seedlings, the effect of Cu was more pronounced on the uptake of Fe compared with that of Zn or Mn. The pronounced effect of Cu on Fe uptake could, in part, explain the development of Fe chlorosis symptoms, partic ularly, in citrus trees on Cu-sensitive rootstocks grown in high Cu soils