4. Materials and Methods 4.1. Plant Materials The ‘Duli’ (Pyrus betulifolia Bunge) seedlings were collected from an orchard in Hebei province, China, and were grown in a growth chamber at Nanjing Agricultural University, under a 16 h/8 h light/dark photoperiod with 75% relative humidity at 25 °C for 30 days, and used for the sugar treatments. After thirty days of treatment, the leaves of the ‘Duli’ seedlings were flash-frozen and stored at −80 °C prior to analysis.