Physiological and biochemical responses of Ulva prolifera and Ulva linza to cadmium stress

Responses of Ulva prolifera and Ulva linza to Cd²⁺ stress were studied. We found that the relative growth rate (RGR), Fv/Fm, and actual photochemical efficiency of PSII (Yield) of two Ulvaspecies were decreased under Cd²⁺ treatments, and these reductions were greater in U. prolifera than in U. linza. U. prolifera accumulated more cadmium than U. linza under Cd²⁺ stress. While U. linza showed positive osmotic adjustment ability (OAA) at a wider Cd²⁺ range than U. prolifera. U. linza had greater contents of N, P, Na⁺, K⁺, and amino acids than U. prolifera. A range of parameters (concentrations of cadmium, Ca²⁺, N, P, K⁺, Cl^-, free amino acids (FAAs), proline, organic acids and soluble protein, Fv/Fm, Yield, OAA, and K⁺/Na⁺) could be used to evaluate cadmium resistance in Ulva by correlation analysis. In accordance with the order of the absolute values of correlation coefficient, contents of Cd²⁺ and K⁺, Yield, proline content, Fv/Fm, FAA content, and OAA value of Ulva were more highly related to their adaptation to Cd²⁺ than the other eight indices. Thus, U. linza has a better adaptation to Cd²⁺ than U. prolifera, which was due mainly to higher nutrient content and stronger OAA and photosynthesis in U. linza.