Chemical constituents of green peel of Persian walnut (Juglans regia L.) fruit.

Green peel of walnut (J. regia L.) is considered as one of the agricultural wastes, while it contains significant amounts of bioactive compounds and minerals. The aim was to evaluate the cytochemistry and the amount of minerals in these peels in order to estimate their possible uses. Mature Persian walnut fruits were collected from a village in the Kerman province of Iran. The green peels were used for manual sectioning and staining. Large amounts of starch and calcium oxalate were seen in peel cells. Carbohydrates were abundant in epidermal cells and superficial areas. Cells in the middle and inner layers had high levels of phenolic compounds. The concentration of tannins in cells of the epidermis, parenchyma, and phloem tissues was notably high. Assessment of mineral elements in peels was performed by the inductively coupled plasma mass spectrometry (ICP-MS) technique. Potassium had the highest amount; calcium was the second most abundant element, followed by phosphorus, magnesium, sulfur, silicon, iron, sodium, aluminum, strontium, titanium, copper, magnesium, selenium, zinc, molybdenum, and nickel, respectively. The great point was that walnut peel can be a complete plant fertilizer, a subject that has not been reported previously. Because fertilizers in the market contain the same elements and almost the same amount of each element similar to walnut peels, as future perspectives, the extraction of minerals and bioactive substances of these peels and investigation of their effects on the growth of model plants are needed. If the results are satisfactory, chemical fertilizer preparation from walnut peels can be economical. [ABSTRACT FROM AUTHOR]