Since I am a very recent Maple user, I do not know why I obtained a wrong answer with the following two comands:

Exp:= Psi(rho(x,t),theta(x,t));
                  Psi(rho(x, t), theta(x, t))
Exp1:= diff(Exp,t);
     / d           \                                  
     |--- rho(x, t)| D[1](Psi)(rho(x, t), theta(x, t))
     \ dt          /                                  

          / d             \                                
    + |--- theta(x, t)| Psi(rho(x, t) + 1, theta(x, t))
          \ dt            /                                
 

Can someone, please, help me?

Thank you.