Protective effects of melatonin on cadmium subcellular distribution, biochemical, and anatomical alterations in Malva parviflora roots.

Plants use various mechanisms to cope with cadmium (Cd) stress. Melatonin's impact on the root response to Cd stress, which is the first organ to detect its presence, has been less studied. This study aims to investigate the effects of melatonin pretreatment on the root strategies of Malva parviflora in response to Cd stress, focusing on the mechanisms of heavy metal tolerance. The plants, 42 days post-germination, were subjected to a 48-h pretreatment with 50 μM melatonin in a complete nutrient solution under controlled growth conditions. Following pretreatment, plants were exposed to a nutrient solution containing 50 μM Cd for 8 days. Comparative analyses were conducted on root length, weight, anatomical features, Cd content, Cd subcellular distribution, nutrient absorption, glutathione, and lignin. Melatonin pretreatment significantly enhanced root length and weight under Cd stress. It also increased the xylem and phloem area in the roots and promoted the absorption and translocation of essential nutrients such as Fe, Zn, Ca, and Mg to the shoots. Additionally, there was a marked increase in glutathione content and Cd proportion in the cell wall and organelle fractions in melatonin-pretreated roots. Notably, melatonin reduced overall plant Cd content and its translocation from roots to shoots, while decreasing root lignin content. This study demonstrates that melatonin plays an important role in managing Cd toxicity by improving morphological, anatomical, and biochemical characteristics of roots under Cd stress. The findings suggest that melatonin pretreatment can effectively alter Cd subcellular distribution, thereby mitigating its harmful effects in plants.