Study on the physiological mechanism of postharvest seed coat browning of pecan

Abstracts:【Objective】Pecan [Carya illinoinensis (Wangenh.) K. Koch] is popular among consumers due to its high nutritional value and delicious flavor. However, the pecan seed coat would brown after harvest, resulting in the reduction of the nutritional and commercial value. The pecan cultivar“Pawnee”was used as the research material to understand the mechanism of seed coat browning during storage. Correlations between the color and physiological and biochemical indicators were also analyzed. 【Methods】Pecan nuts were harvested from the orchard of Institute of Botany, Jiangsu Province and Chinese Academy of Sciences, and were transported to the Key Laboratory of Plant resources and utilization in Jiangsu Province. Each biological replication was separated into 7 bags after peeling, and then was stored in a climatic cabinate set at 25 ℃, a relative humidity (RH) of 80%-90% and a light intensity of 100% with the cycle of 16 hours in the day and 8 hours at night. One bag of each replication was taken from the climatic cabinate at 0, 15, 30, 45, 60, 75 and 90 days of storage, scratched the seed skin and stored in the -80 ℃ refrigerator for the following research. The photos were taken and the differences of color were measured at the same time. The physiological and biochemistry indicators including browning degree, total phenolic content, poliphenol oxidade (PPO), superoxidase dismutase (SOD), catalase (CAT), hydrogen peroxide (H2O2) and malonaldeyde (MDA) were measured.【Results】The color of the skin became more and more brown with the prolonging of the storage time according to the photos and the △E values. The browning rate of the nuts was faster during the first 45 days, and then, it became slowly during the next 45 days. The MDA content indicating the degree of membrane lipid peroxidation increased gradually during the storage. The content of MDA on the 90th day was 1.861 fold higher than that in the beginning of the storage. The SOD activity had a decrease trend, the activity in the beginning of the storage was 1.449 times higher than that in the end of the storage, but it remained at a high level during the entire storage period. The activity of CAT increased from the first day to the 15th day and slightly decreased in the next 15 days, then increased up to the end of storage. The activity of CAT in the end was 1.886 times higher than that in the beginning. The content of H2O2 increased in the first 45 days and then decreased in the next 45 days, and it remained at a high level in the total storage, and the highest level of the content was on the 45th day with 29.707 μmol·g^-1 . The PPO activity had a increased trend in the total storage period with a short period of decrease on the 30th to the 60th day, and the maximum PPO activity was recorded on the last day (118.48 U· g^-1 ·min^-1 ), which was 2.195 times than that during the early storage period (53.98 U· g^-1 ·min^-1 ). The content of total phenols increased in the entire storage period. The content of 90th day was 1.523 times than that on the first day. The trends of changes of the total phenols and △E value were the same. The correction analysis showed that there was a significantly positive relationship between the browning degree and the MDA content (r = 0.829^**), the total phenols (r = 0.799^**), the PPO activity (r = 0.500^* ); and a significantly positive relationship between the total phenols and the PPO activity (r = 0.496^* ), the CAT activity (r = 0.584^**), the MDA conent (r = 0.788^**).【Conclusion】The dynamic change of pecan seed coat color during the accelerated storage was clarified. The browning rate of the pecan nuts during early days than that during later days. Postharvest browning of the pecan nut skin might be due to the enzymatic browning catalyzed by PPO, membrane lipid peroxidation and the disruption of the antioxidant system according to the analysis of physiological and biochemical indicators. The pecan nut skin still maintained the antioxidant capacity in the end of the storage. The early stage of storage would be the important period for prevention and control of the browning of the nuts.