Insights into Cadmium-Induced Morphophysiological Disorders in Althea rosea Cavan and Its Phytoremediation through the Exogeneous Citric Acid

Cadmium (Cd) is taken in plants from soil and then travels through the food cycle, posing a major threat to all the units of the ecosystem. A pot experiment was conducted to understand the influence of citric acid (CA) on Cadmium (Cd) phytoextraction ability of hollyhock (Althea rosea Cavan.). A. rosea plants were exposed to Cd concentrations (100 and 200 mg·kg−1), either in simultaneous administration or without adding CA (5 mM·kg−1 dry weight). The results revealed that exposing A. rosea to different levels of Cd stress, i.e., 100 and 200 mg·kg−1, significantly decreased (p < 0.05) plant growth and biochemical attributes, such as root length (RL), shoot length (SL), fresh biomass (FW), dry biomass (DW), relative water content (RWC), and chlorophyll and carotenoid contents. Meanwhile, a net increase in MDA and REL indicated Cd-induced oxidative stress in plants. However, the application of citric acid (CA) as an organic chelator helped the plants to alleviate the phytotoxic effects of Cd stress on A. rosea, which is shown in terms of enhancing plant growth and biomass; that is, the root length (27.3% and 21.12%), shoot length (32.11% and 23.02%), fresh weight (39.66% and 29.8%), and dry weight (29.8% and 57.33%) under 100 and 200 mg·kg−1 of Cd stress, respectively, were observed. CA application also helped to alleviate the level of chlorophyll and carotenoid contents; foster high level of antioxidants, such as SOD, POD, CAT, and APX; and lower concentration of MDA and EL. In addition to enhancing plant-growth attributes, the application of CA also managed to increase the phytoextraction potential of the plants by enhancing the concentration of Cd in roots and shoots tissues. This is also demonstrated by rising levels of bioaccumulation (BAC) and translocation factors (TFs). These findings showed that CA application could be a practical strategy to apply to ornamental plants, such as A. rosea seedlings, cultivated in Cd-contaminated locations, opening ways to cope with Cd stress and enhanced phytoextraction.