2.4. Determination of soluble sugar
Soluble sugar was extracted using the method of Dubois et al.
(1956). A total of 0.5 g lettuce tissue (shoot FW) were homogenized
with 2 mL of 80% ethanol solution, and the homogenate
were heated in a 75 ◦C water bath for 10 min. The precipitate were
removed by centrifuging at 5,000 × g for 10 min. The residues were
washed again with 2 mL of 80% ethanol at 75 ◦C and re-centrifuged.
The supernatants were mixed and dried under a stream of hot
air, and the residue was resuspended in 1 mL of distilled water
and desalted through a column of ion-exchange resin (Shanghai
Resin Factory Co., Ltd). The soluble sugar concentrations were
determined using the filtrate with a spectrophotometer (UV-1200,
MAPADA, Shanghai, China) at 485 nm according to the phenolsulphuric
acid method.