Physiological Responses of Boerhavia diffusa  L. to the Heavy Metal Stress: Reflections on Chlorophyll, Proline and Nitrate Reductase Activity

The present study investigates the physiological responses of Boerhavia diffusa L., a traditionally valued medicinal plant, to heavy metal stress induced by Cadmium (Cd), Chromium (Cr), Mercury (Hg) and Lead (Pb). The study focuses on the accumulation of proline, alterations in chlorophyll content and the activity of nitrate reductase across different plant tissues (root, stem, and leaf) during developmental stages. Results revealed that proline content significantly increased in all tissues of B. diffusa exposed to heavy metal treatments compared to their respective controls. Notably, on the 20th day, root tissues under cadmium treatment exhibited more than a twofold increase in proline content. Similar trends were observed in mercury and chromium treatments, whereas lead induced a comparatively lower accumulation. Stem tissues showed maximal proline levels in chromium- and lead-treated plants. Leaf tissues displayed the highest proline accumulation under lead exposure over eight times that of the control followed by mercury and cadmium. Interestingly, chromium treated leaves recorded relatively lower proline levels than other metal treatments. Chlorophyll analysis indicated a marked decline in chlorophyll a and b content in cadmium, chromium and mercury treated plants, with the most significant reduction observed under chromium stress. The chlorophyll a/b ratio remained largely unaffected across treatments. Lead exposure showed only a marginal effect on chlorophyll content. Nitrate reductase activity, essential for nitrogen assimilation, was significantly reduced in all heavy metal treatments, particularly in leaf tissues. Cadmium had the most pronounced inhibitory effect, followed by mercury, chromium and lead. The activity was consistently higher in control leaves, with root and stem tissues showing notably lower levels. The findings underscore the differential tolerance and physiological adaptability of B. diffusa under heavy metal stress and highlight its potential as a bioindicator or candidate for phytoremediation in contaminated environments.