Abstract:

The aim of this study was to evaluate the copper （Cu²⁺） tolerance of different red clover germplasm lines and select the tolerant lines. To this end， we evaluated the growth characteristics of 30 red clover germplasm lines under Cu²⁺ stress at the germination and seedling stages. A subordinate function method was used to evaluate copper tolerance. It was found that， as the Cu²⁺ concentration increased， the germination rate gradually decreased， and the radicle became shorter and thicker. Compared with the germination rate， radicle growth was more sensitive to Cu²⁺ stress. At a Cu²⁺ concentration of 0.5 mmol·L^-1， the radicle was 40%-69% of the length of that in the control. At a Cu²⁺ concentration of 8.0 mmol·L^-1， the radicle stopped growing while the germination rate was 61%-93% of that in the control. Red clover seedlings could tolerate Cu²⁺ concentrations up to 20 mmol·L^-1， however， the above-ground biomass， underground biomass， total root length， root tip number， and root volume were significantly reduced compared with their respective values in the control. There were also significant differences among different Cu²⁺ concentrations in terms of their effects on other major traits of red clover （P<0.05）. The values for each trait differed significantly among red clover germplasm materials at the germination stage （treated with Cu²⁺ at 0.5 mmol·L^-1） and seedling stage （treated with Cu²⁺ at 20 mmol·L^-1） （P<0.05）. The correlations between radicle length and radicle diameter， aboveground biomass and underground biomass， underground biomass and root：shoot ratio， and total root length and root tip number were extremely significant （P<0.01）. A least squares model to estimate the Cu²⁺ tolerance of each clover germplasm line was established using four factors： radicle length， seedling survival rate， underground biomass， and total root length. The value predicted using this model was significantly correlated with the multivariate evaluation value （P<0.01）. According to the results of the multivariate evaluation， CF022167， CF022178 and CF022232 were identified as the most Cu²⁺-tolerant lines. These lines are recommended for use as breeding materials to produce new Cu²⁺-tolerant varieties or directly utilized in production.

Key words: red clover, copper tolerance, subordinate function, multivariate evaluation