Question

While experimentation, why do you think it is difficult to assign any effect seen to any single hormone?

Answer 1

Phytohormones are synthesised by plant cells, itself and individually. These are auxin, GA, ABA, ethylene and cytokinin. There is not a separate system of their translocation in plants. So, their effects on plants are also intermixed. e.g., many effects of auxins GA shows the same function. Similarly, ethylene and ABA support each other for many roles in plants.
Secondly, the effects of phytohormone in vitro and in vivo will also be different.