Hello, I read one article related to your query that is,

         "The high concentration of GA3 increased the size of the midrib and its vascular bundle numbers. Both low and high concentrations of 2,4-D inhibited the formation of the midrib. 2,4-D in both low and high concentrations decreased the number of vessels in both protoxylem and metaxylem and also decreased their diameters, where as GA3 in low and high concentrations have less effect on the number of vessels and its diameters."

Go through the attached articles that may very useful to you.

Thank you Sisubalan Natarajan.

Application of IAA would also be helpful