Foliar application of nitrates limits lead uptake by Cucumis sativus L. plants

Lead is a toxic heavy metal, which accumulates in the soil and is readily absorbed by plant roots. The uptake of toxic elements by crops is a serious threat to human health. For this reason, it is important to prevent the incorporation of heavy metals into the food chain. Our previous study showed that foliar application of calcium nitrate reduces the intensity of lead uptake by different plant species. A significant decrease in metal concentration was observed both in the roots and in the shoots of three crops: tomato, cucumber, and flax. The present research investigated the mechanism for limiting lead accumulation in plant tissues. The experiments were conducted on Cucumis sativus L. seedlings, grown in hydroponic conditions. To compare the role of Ca^2 + and NO₃^- ions in the restriction of lead uptake three different calcium salts (nitrate, chloride, and formate), and two nitrates (calcium and potassium) were applied foliarly to plants. The results show that Ca(NO₃)₂ is more efficient in decreasing lead accumulation in tissues than other calcium salts which suggests an important role of NO₃^- ions in the process. In addition, the study demonstrated that the exogenous supply of nitrates helps compensate for nitrogen deficiency caused by lead action and supports the mineral balance. The reduction in lead toxicity to plants after foliar application of nitrates may be due to the stimulation of the biosynthesis of nitric oxide - a key molecule responsible for stress response.