Heavy metal stress reduces the deposition of calcium oxalate crystals in leaves of Phaseolus vulgaris

Calcium oxalate (CaOx) crystals in plants may serve as a sink for the absorption of excess calcium, and they could play an important role in heavy metal detoxification. In this study, the effect of heavy metals and different calcium concentrations on the growth of calcium oxalate crystals in leaves of Phaseolus vulgaris was investigated. Different analytical techniques were used to determine the influence of exogenous lead and zinc on CaOx deposition and to detect a presence of these metals in CaOx crystals. We found a positive correlation between the calcium concentration in the nutrient medium and the production of calcium oxalate crystals in leaves of hydroponically grown plants. On the other hand, addition of the heavy metals to the nutrient medium decreased the number of crystals. Energy dispersive X-ray spectrometry did not detect the inclusion of heavy metals inside the CaOx crystals. Our investigation suggests that CaOx crystals do not play a major role in heavy metal detoxification in P. vulgaris but do play an important role in bulk calcium regulation.