Low night-temperatures affect the metabolism of raffinose-family oligosaccharides in melon (Cucumis melo L.) leaves during fruit expansion

This study served to reveal the effects and mechanism of low night-temperatures on the metabolism of raffinose-family oligosaccharides in melon (Cucumis melo L.) leaves. We investigated the carbohydrate contents and sugar-metabolising enzyme activities in leaves under normal (15 degrees C; control) and two low night-temperature (12 degrees C or 9 degrees C) conditions during fruit expansion. The results showed that the greatest difference in carbohydrate metabolism occurred between 9 degrees C at night and the 15 degrees C control. Compared to the normal night temperature, total carbohydrate accumulation, in particular raffinose, increased and the ratio of raffinose to total carbohydrates exhibited the greatest increase in melon leaves treated at 9 degrees C at night, while the ratio of sucrose to total carbohydrates declined at the end of the same treatment. These data suggested that 9 degrees C at night played an important role in partitioning carbon from sucrose to raffinose-family oligosaccharides (RFO), and that raffinose may serve dual purposes in stress protection and carbon storage. Meanwhile, the activities of sugar-metabolising enzymes were notably affected by 9 degrees C at night. Sucrose synthase (SS), sucrose phosphate synthase (SPS) and, in particular, galactinol and stachyose synthase activities decreased markedly, while acid invertase (AI) and neutral invertase (NI) activities increased sharply, which restricted sucrose and RFO synthesis. Overall, these data suggest that the decline in stachyose and galactinol synthase activities were the main limiting factor for RFO synthesis in melon leaves at 9 degrees C at night. They also indicated that the reduction in stachyose metabolism may be an important factor during the development of unmarketable fruit.