Hello, For my thesis research I would like to stain young tomato...

Hi Vicky, Thanks for reaching out- I think it should be doable, I didn't try this method on tomato petioles, but I know colleagues over the years who have tried a modified version of this protocol on non leaf tissue. It is worth trying to see if at least you get a DAB stain. Try de-staining a little longer, and yes a dilution series with DAB could help too. I'd be interested to hear about how it turns out for you.