A cooperative may not sell, mortgage, lease or otherwise dispose of or encumber all or any substantial portion of its property unless such sale, mortgage, lease, or other disposition or encumbrance is authorized at a duly held meeting of the members thereof by the affirmative vote of not less than two-thirds of all of the members of the cooperative, and unless the notice of such proposed sale, mortgage, lease, or other disposition or encumbrance shall have been contained in the notice of the meeting; provided, however, that notwithstanding anything herein contained, or any other provisions of law, the board of trustees of a cooperative, without authorization by the members thereof, shall have full power and authority upon an affirmative vote of not less than two-thirds ( ²/₃ ) of the board of trustees to authorize the execution and delivery of a lease and leaseback transaction only where the board of trustees determines that such transaction will not impair the ability of the cooperative to use the assets as needed to serve the members; provided, however, that such transactions shall apply only to the physical assets of a cooperative and shall not be used to effect a sale or other disposition of the cooperative business entity itself; and further, shall have full power and authority to authorize the execution and delivery of a mortgage or mortgages or a deed or deeds of trust upon, or the pledging or encumbering of, any or all of the property, assets, rights, privileges, licenses, franchises and permits of the cooperative, whether acquired or to be acquired, and wherever situated, as well as the revenues and income therefrom, all upon such terms and conditions as the board of trustees shall determine, to secure any indebtedness of the cooperative to the United States of America or any instrumentality or agency thereof or to any corporation or financial institution authorized to assist in the credit and financial needs of rural electric cooperatives.