Hello Ahmed and Welcome on PPRuNe.
I had a quick look but can't find it in the latest FCOM/FCTM, But in the FCOM from November 2014, it's written:
When passenger numbers permit, LO ( ECON) modes saves up to 0.5% of fuel.
Applicable to A319/320/321.
FCOM: Procedures- Supplementary Procedures- Green operating Procedures- Flight phases- After Start.
The Green procedure has now been moved in the FCTM but I could not find that reference anymore.