Root morphological responses of three hot pepper cultivars to Cd exposure and their correlations with Cd accumulation.

Cultivars of hot pepper (Capsicum annuum L.) differ widely in their fruit cadmium (Cd) concentrations. Previously, we suggested that low-Cd cultivars are better able to prevent the translocation of Cd from roots to aboveground parts, but the corresponding mechanisms are still unknown. In this study, we aimed to improve understanding of the root morphological characteristics of the mechanisms involved in two low-Cd and a high-Cd cultivar. Seedlings were grown in nutrient solutions containing 0 (control), 2, and 10 μM Cd for 20 days, and Cd contents for the three cultivars were compared with changes in root morphology. The total root length (RL), root surface area (SA), number of root tips (RT), and specific root length (SRL) of all cultivars were decreased significantly by the 10 μM Cd treatment with the exception of the SA in JFZ, which showed no obvious change. For each cultivar, the 10 μM Cd treatment decreased significantly RL and SA specifically in roots with diameters (RD) of RD ≤ 0.2 mm or 0.2 mm < RD ≤ 0.4 mm, and increased significantly RL and SA specifically in roots with diameters of 0.6 mm < RD ≤ 0.8 mm. Hot pepper cultivars differ greatly in Cd accumulation and root morphology. In the 10 μM Cd treatment, root volume (RV), SA, and RT of all cultivars were negatively correlated with Cd concentration and amount in roots. However, RL, SA, RV, and RT of all cultivars were positively correlated with Cd concentration and amount in shoots, and translocation rate of Cd. The two low-Cd cultivars of hot pepper had less root tips, shorter root length, and smaller root surface area than the high-Cd cultivar in 10 μM Cd treatment, which may play a vital role in reducing root-to-shoot Cd translocation.