Keeping in consideration most Indian weddings in both Northern and Southern India, one can notice that most mantras which are recited in weddings are actually same but the practice of showering rice is prevalent in South India.
If we look back at the era before Vedas were compiled. The only things South Indians were known for were Heat and Rice.
It is said that the rice cultivation was widespread all over the Central-south India. It is also said that at the time when agricultural produce was lagging behind its market demand, rice was considered worth equal to the copper coins for both farmers and Zamindars( landlords).
Old Legend: Many centuries ago, there was a local Zamindar who started throwing rice during his daughter’s wedding to display his love and pride for his only daughter which meant that his daughter is way precious than wealth(rice).