Change in citrus leaf lipids during freeze-thaw stress

Fatty acids in the leaves of the citrus hybrid B5-9-68, a BC₁ progeny [(Citrus paradisi x Poncirus trifoliata) x C. sinensis], were compared with those in P. trifoliata and C. sinensis. Total lipid and triacylglycerol fatty acid profiles of the cold hardened hybrid were similar to the profiles of the hardy P. trifoliata and different from the profiles of the less hardy C. sinensis. When subjected to a freeze regime, the unhardened hybrid lost 22% leaf fatty acids during the freeze stage and 13% during the thaw stage. Linolenic acid accounted for 98% of the fatty acid decrease. Degradation was greatest in phosphatidylcholine (89%), phosphatidylglycerol (83%), monogalactosyldiglyceride (79%) and digalactosyldiglyceride (50%). Phosphatidic acid increased 4-fold over the two stress stages. Total leaf fatty acids during the freeze-thaw regime increased 12% in the cold hardened hybrid. Three molecular species of triacylglycerol which were rich in linolenic acid increased in the hardened hybrid during the freeze-thaw regime. The increase in highly unsaturated triacylglycerol species under freeze-thaw stress suggests that triacylglycerol has a role in maintaining the fluidity of biomembranes during freezing conditions.