Gene silencing, plant breeding , Genetic Engineering to make trangenic plants. What is your aim? what you want to do depends on that.. which method suits you best.
you can create genetic variation in plants by introducing a strange DNA , so you can introduce DNA by plasmid or by biolistic method or microinjection of DNA, electroporation.
i dont know if we can create spontaneous mutation in plants by physical technique , to expose protoplaste to UV or gama rayonnement or using chemical technique : Ethyl Methane Sulfonate and N-methyl N-nitro N-nitrosoguanidine.