No export duties shall be levied or collected on exports from Puerto Rico, but taxes and assessments on property, income taxes, internal revenue, and license fees, and royalties for franchises, privileges, and concessions may be imposed for the purposes of the insular and municipal governments, respectively, as may be provided and defined by the Legislature of Puerto Rico; and when necessary to anticipate taxes and revenues, bonds and other obligations may be issued by Puerto Rico or any municipal government therein as may be provided by law, and to protect the public credit.