Statistical analysis
All data analyses were performed in SAS 9.2 (SAS Institute Inc., NC, USA), and graphs were produced with Origin 8.0 (Origin Lab, MA, USA). Data were presented as mean ± SEM. Analysis of variance was performed using the ANOVA procedure in SAS 9.2. If the differences between treatments were significant, multiple comparisons were performed using the Tukey method. Statistical significance was considered P < 0.05.