I suggest in the way of your possibilities to carry out a dose-answer curve to determine the response of your plant to your plant growth regulators. Since, every plant has a unique response to a specific concentration of plant growth regulator. Try to find the stressing concentration for each plant growth regulator to carry out your experiments properly.
I suggest that you go to some literature and find out their recommendations on those specific plants, choose your plant, the recommended dose from literarure, then decrease and increase that dose, then you will find out your real concentration of that chemical acting best in your environment, that is for thses things are due to G x E interations! Regards