The dynamics of fat, protein and sugar metabolism during walnut (<i>Juglans regia</i> L.) fruit development

Abstract

Walnut (Juglans regia L.) is named the “super food” in the 21^st century. In this study, the 9-year-old precocious walnut cultivar ‘Xiangling’ were used to exam the developmental process, and the dynamics of fat, protein and sugar content in the fruit, and the activities of enzymes related with sugar metabolism were further determined and analyzed. The result shows that the developmental process of walnut fruit could be divided into four stages: Slow growth {within 30 days after florescence, (DAF)}, fast growth (30 to 60 DAF), fat accumulation (60 to 100 DAF) and fruit maturity (100 to 140 DAF). Fat content in walnut fruit increased continuously and the maximum increment occurred within 60 to 90 DAF. It amounted to 50.11% at 100 DAF and maximized with 63.13% at 140 DAF when the fruit ripens. Sucrose and glucose content showed similar trend. They were both low at earlier stage, then gradually increased and reached the peak at 90 DAF, followed by gradual descending. Fructose content topped at earlier stage and then gradually decreased. Acid invertase (AI, EC 3.2.1.26), neutral invertase (NI, EC 3.2.1.26), sucrose phosphate synthase (SPS, EC 2.4.1.14) and sucrose synthase (SS, EC 2.4.1.13) activity were higher during the earlier stage and then decreased until the minimum point at 80 DAF, thereafter these enzyme activities turned to ascend the crest value at 90 to 100 DAF, and then kept decreasing until the late maturity stage. Our further analysis revealed soluble sugar content had significant positive correlation with fructose and glucose content (P＜0.05). Sucrose content positively correlated with glucose content at P＜0.01, with SS and SPS activity atP＜0.05. Protein content negatively correlated with NI and AI activity at P＜0.01, with fructose content at P＜0.05. Our result suggests that sucrose played an important role during carbohydrate metabolism of walnut fruit.

Key words: Walnut (Juglans regia L.), fruit development, sugar, fat, protein, invertase.