I bought a 3 yesrs old Serissa in NewYork (Chinatown) 15 days ago but during
the journey back to Italy It suffered a lot...It lost almost all of the leaves.


You mean to tell me they don't sell Serissa in Italy?
Serissa are very prone to drop their leaves when stressed. Don't cut anything
off for now. Put it outdoors in the shade. Be careful not to overwater. When it
starts to come back, gradually put it out in part sun. Don't feed it until it
starts growing. Cu off dead branches when it is growing well & you know what is
dead.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)