ROLE OF CALCIUM IN AMELIORATION OF COPPER PHYTOTOXICITY FOR CITRUS

Abstract

Excessive application of Cu to Florida citrus has resulted in accumulation of Cu, causing Cu toxicity. Maintaining soil pH levels at 6.5 to 7.0 has been recommended to minimize the Cu toxicity problem. In this study, the role of Ca, independent of pH, on Cu toxicity was investigated. Cleopatra mandarin and rough lemon rootstock seedlings were grown in Myakka sand (sandy, siliceous, hyperthermic, Aeric Haploquods) treated with a factorial combination of four Cu rates (0, 50, 100, and 200 mg Cu/kg), two Ca sources (CaCO3 and CaSO4), and four Ca rates (0, 200, 400, and 800 mg Ca/kg). Increasing the rate of Ca from 0 to 800 mg/kg increased the soil pH (1:1, soil:water) by 1.5 unit and decreased the pH by 0.3 unit when using CaCO3 and CaSO4, respectively. Top and root growth were greater for seedlings grown in CaCO3- compared with CaSO4- amended soil. Increasing rates of Ca application, as either CaCO3 or CaSO4, increased top and root growth of both rootstocks at some Cu rates. The effects of increasing rates of Cu on top and root weights in the absence of Ca amendment (i.e., “pure-effect” of Cu) were much greater on Cleopatra mandarin than on rough lemon rootstock. Fibrous root Cu content for both rootstocks was highly correlated with soil Cu extractable by Mehlich 3 (r2 = 0.883 to 0.891) and with root weight (r2 = 0.256 to 0.521). Increasing rates of Ca, applied either as CaCO3 or CaSO4, significantly decreased concentrations of Cu in the fibrous roots. The results demonstrate that increased Ca availability in the rooting environment ameliorates the effects of Cu phytotoxicity. The concentration of Cu in fibrous roots is a better index of Cu phytotoxicity than is that in the leaves.