Phospholipase D and Lipoxygenase of Young Loquat Fruits in Response to Low Temperature Stress

Container grafted seedlings of three-year-old loquat (Eriobotrya japonica ‘Zaozhong No.6’) were treated at 0℃, -1℃ and -3℃ in phytotron to determine the mechanism of responses of membrane phospholipid and related enzymes in young loquat fruits under low temperature stress. Results showed that the activities of phospholipase D (PLD, EC 3.1.4.4) and lipoxygenase (LOX, EC 1.13.11.12) increased in young loquat fruits under different low temperature stress. The contents of phosphatidylcholine (PC) and phosphatidylinositol (PI) decreased due to gradual degradation. Phosphatidic acid (PA) accumulations increased, and the membrane-associated Ca²⁺ contents were reduced to different degrees. With increasing stress time and decreasing stress temperature, the activities of PLD and LOX increased, resulting in accelerated PC, PI degradations and PA accumulation. The decrease in PC content was greater than that of PI content in the membranes of young loquat fruits under low temperature stress. There was a negative correlation between PLD, LOX activities and membrane-associated Ca²⁺ contents. The increase in PLD and LOX activities was caused by the decrease in membrane-associated Ca²⁺ contents. Meanwhile, the decrease in membrane-associated Ca²⁺ contents resulted in stable decreases in membrane structure, which aggravated membrane damage due to lipid degradation and peroxidation. The damage to the cell membranes of young fruits was most serious under low temperature stress at -3℃ for 4-6 h. The signal system of Ca²⁺·CaM may be involved in regulating PLD and LOX activities of young loquat fruits under low temperature stress.