Briefly, 0.25 g leaf samples were grinded to power and then extracted in 3% (w/v) sulfosalicylic acid for 10 min at 100◦C. After that, 2 ml ninhydrin reagent and 2 ml glacial acetic acid were added to the 2 ml extraction solution. The mixed solution was boiled at 100◦C for 40 min. After cooling to room temperature, the proline level of sample was determined by measurement of absorbance at 520 nm as described previously (Shi et al., 2012).