INTERACTION OF THE YIELD RESPONSE SURFACE OF LETTUCE WITH HIGH AND TOXIC CONCENTRATIONS OF ZINC AND NICKEL

Abstract

We germinated lettuce seeds in nutrient solutions with different high concentrations of Zn and Ni singly and together and estimated growth by measurement of root length at 120 h. The high concentration zone of the dose-response curve of lettuce to Zn consisted of a tolerance zone and one toxic phase. The toxicity threshold for Zn was 63 μeq L−1 Zn. In contrast, the dose-response curve for Ni had a tolerance zone, but the toxic zone comprised two phases of Ni toxicity, 20 μeq L−1 Ni, with 45 μeq L−1 Ni for the second phase. The two toxic phases for Ni were expressed both as a yield function and as a metabolic function where abrupt changes in the tissue concentration of Ca, Cd, and Na corresponded to changes in Ni status, as indicated in the dose-response curve developed from root length measurements. The response of lettuce to a matrix of both Zn and Ni concentrations was portrayed as a response surface. The Zn-Ni dose-response surface indicated that an interaction of Ni and Zn in lettuce occurred only when both metals were present at concentrations greater than their toxicity thresholds. The interaction of Zn and Ni was multiplical in form.