Effect of Cadmium on Vegetative Traits, Physiological and Biochemical Indexes of Radish (Raphanus sativus L.)

Among wide variety of soil pollutants including heavy metals, acidic precipitation and other toxicants, the importance of heavy metals due to their pollution capacity has received growing attention in recent years. Heavy metals are important environmental pollutants and their toxicity is a problem of increasing significance for ecological, evolutionary, nutritional, and environmental reasons. Of all non-essential heavy metals, cadmium (Cd) is perhaps the metal that has attracted the most attention in soil science and plant nutrition due to its potential toxicity to humans, and also its relative mobility in the soil–plant system. The uptake of ions takes place in competition with that of elements such as Zn, P, Cl–, Ca, and Cu. Soil, environmental, and management factors impact the amount of Cd accumulated in plants (Hart et al., 1998). Much of the Cd taken up by plants is retained in the roots, but a portion is translocated to the aerial portions of the plant and into the seed. The amount of Cd accumulated and translocated in plants varies with species and with cultivars within species. Cd toxicity causes inhibition and abnormalities of general growth in many plant species. After long-term exposure to Cd, roots are mucilaginous, browning, and decomposing; reduction of shoots and root elongation, rolling of leaves, and chlorosis can occur. Cd was found to inhibit lateral root formation while the main root became brown, rigid, and twisted. The changes in the leaf included alterations in chloroplast ultrastructure, low contents of chlorophylls, which caused chlorosis, and restricted activity of photosynthesis. Radish (Raphanus sativus) is a root vegetable grown and consumed all over the world and is considered as a part of the human diet, even though it is not common among some populations. Usually, people eat radishes raw as a crunchy vegetable, mainly in salad, while it also appears in many European dishes. Some people, at least in the Middle East, prefer to drink its juice in pursuit of certain health benefits. Radishes have different skin colors (red, purple, black, yellow, and white through pink), while its flesh is typically white. In addition, the edible root of radish varies in its flavor, size, and length throughout the world.

In this study, we investigated the influence of Cd application rates on vegetative parameters, and physiological and biological indexes of radish. The experimental design was a factorial with randomized block with two treatments and three replications carried out at the Research Farm of College of Agriculture, Shahid Chamran University. Treatments included three rates of Cd application of 0 (control), 30 and 60 mg kg-1, and two harvesting dates of commercial maturity (CM) and a week after CM, hereafter referred to as 1st and 2nd harvesting dates. Measurements included vegetative parameters such as wet and dry weights, leaf area, length and width of leaves, leaf numbers and root length. Physiological indexes of electrolyte leakage and relative humidity, and biochemical indexes of chlorophyll a, b and total, Cartonoeid, Proline and vitamin C were also determined.

The results indicated that the Cd application reduced all of the vegetative parameters. Application of 60 mg kg-1 of Cd increased the electrolyte leakage by 28.2% and Proline concentration by 48.8 mg g-1. Cd application increased the relative humidity. All biochemical indexes decreased as the Cd application rates increased. The maximum concentration of Cd in plant was observed at 60 mg kg-1 Cd contamination. It seems that decrease of physiological indices due to increased Cd concentration reduced the growth properties.

Application of different Cd concentrations affected the vegetative, physiological and biochemical properties. By increasing Cd concentration of soil, the Cd accumulation in the plant increased. Increasing the Cd concentration increased the electrolyte leakage and proline concentration and reduced the content of relative humidity, chlorophyll, vitamin C in radish. In addition, it decreased yield including fresh and dry weights, root length, leaf area, leaf length and width, and number of radish leaves. Further, the effects of degradation on vegetative, physiological and biochemical characteristics of radish were one week after commercial maturity more than the first time (commercial maturity). Therefore, the phosphorus-containing Cd for the cultivation of vegetables, especially tubers, such as radishes, as well as harvest management, should be carefully applied.