Notwithstanding any provision of law to the contrary, any municipal corporation of this state having a population of more than 300,000 according to the United States decennial census of 1970 or any future such census is authorized, in the discretion of its governing authority, to enter into valid and binding leases and contracts with private persons, firms, associations, or corporations for any period of time not to exceed 50 years to provide for the operation and maintenance of botanical gardens on municipal property.