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True lipases beside phospholipases contribute to walnut kernel viability loss during controlled deterioration and natural aging

Pournik, Shirin, Abbasi-Rostami, Mobina, Sadeghipour, Hamid Reza, Ghaderi-Far, Farshid
Environmental and experimental botany 2019 v.164 pp. 71-83
Juglans regia, biochemical pathways, butanol, cyanides, enzyme activity, free fatty acids, germination, lipid bodies, pH, phospholipase A2, phospholipase D, phospholipases, seeds, temperature, triacylglycerols, viability, walnuts, water content
Little is known on the role of true lipases in aging of orthodox seeds. Walnut (Juglans regia L.) kernels rich in triacylglycerol stores lose viability after storage however, data on lipolytic activities and their correlation to kernel viability loss are lacking. Extracts from mature kernels contained acid (pH 5.0) and alkaline (pH 9.0) lipases, phospholipase A2 and fatty acyl-esterase activities. Kernels with moisture contents of 6, 15 and 20% were subsequently incubated for 3 and 6 days at 45 °C for controlled deterioration. Viability loss was greatest in kernels with 20% moisture content after 6 days. Lipase and phospholipase A2 activities increased but total lipid declined as kernels lost viability. Pre-treatements with diphenylmethyl-phosphanate or 1-butanol which specifically inhibited oil body mobilization and phospholipase D, respectively, inhibited viability loss of kernels after controlled deterioration. Diphenylmethyl-phosphanate inhibited in vitro lipase activity and kernels treated with this chemical also had reduced lipase activity. Declined kernel viability after one year storage was also associated with the increased lipase activities and the accumulation of free fatty acids in the neutral lipid fraction. Cyanide application increased germination and fatty acyl-esterase activity of non-aged kernels but resulted in viability loss of one-year stored kernels. Thus, the increased activities of lipases beside phospholipase might result in walnut kernel viability loss at higher temperature and moisture content while fatty acyl-esterase activity may be a marker of kernel ongoing metabolic activity.