*4.5. Determination of GABA Content*

The GABA content in the leaf tissues was measured using GABAase commercial enzyme preparation (Sigma chemical Co., St. Louis, MI, USA) as previously described by Ma et al. [21].

#### *4.6. Assay of Phenolic Metabolism-related Enzymes*

The activity of PAL was determined according to the method of Zheng et al. [95] with slight modification. Fresh leaves (2g) were homogenized with 2.5 mL of solution containing 100 mM K3PO4 buffer, 2 mM EDTA, 1% (*m*/*v*) PVP, and 1 mM phenyl-methylsulfonyl fluoride (PMSF). Then, the homogenates were centrifuged at 12,000× *g* for 15 min at 4 ◦C, and the supernatant fractions were used for enzyme analysis. The absorbance was spectrophotometrically measured at 290 nm. The activities of PPO and SKDH were measured according to the method of Sheteiwy et al. [82]. The activity of CAD was measured according to the method of Wyrambik and Grisebach [96].
