FORTY-EIGHTH SUPPLEMENTAL INDENTURE

Exhibit 4(x)

KANSAS GAS AND ELECTRIC COMPANY

TO

THE BANK OF NEW YORK TRUST COMPANY, N.A.

(successor to BNY Midwest Trust Company)

and

JUDITH L. BARTOLINI

(successor to W. A. Spooner, Henry A. Theis, Oliver R. Brooks,

Wesley L. Baker, Edwin F. McMichael and R. Amundsen)

as Trustees under Kansas Gas and Electric Company’s

Mortgage and Deed of Trust, Dated as of April 1, 1940

FORTY-EIGHTH SUPPLEMENTAL INDENTURE

Providing for the Amendment of the Indenture

Dated as of July 10, 2007

FORTY-EIGHTH SUPPLEMENTAL INDENTURE

INDENTURE, dated as of July 10, 2007, between Kansas Gas and Electric Company, a corporation of the State of Kansas (formerly named KCA Corporation and successor by merger to Kansas Gas and Electric Company, a corporation of the State of Kansas, hereinafter sometimes called the “Company-Kansas”), whose post office address is 120 East First Street, Wichita, Kansas 67202 (hereinafter sometimes called the “Company”), The Bank of New York Trust Company, N.A., a national banking association, whose post office address is 2 North LaSalle Street, Suite 1020, Chicago, Illinois 60602 (successor to BNY Midwest Trust Company (the “Corporate Trustee”)), and Judith L. Bartolini (successor to W.A. Spooner, Henry A. Theis, Oliver R. Brooks, Wesley L. Baker, Edwin F. McMichael and R. Amundsen, and being hereinafter sometimes called the “Individual Trustee”), whose post office address is 2 North LaSalle Street, Suite 1020, Chicago, Illinois 60602 (the Corporate Trustee and the Individual Trustee being hereinafter together sometimes called the “Trustees”), as Trustees under the Mortgage and Deed of Trust, dated as of April 1, 1940 (hereinafter called the “Mortgage”), which Mortgage was executed and delivered by Kansas Gas and Electric Company, a corporation of the State of West Virginia to which the Company-Kansas was successor by merger (hereinafter sometimes called the “Company-West Virginia”), to secure the payment of bonds issued or to be issued under and in accordance with the provisions of the Mortgage, reference to which Mortgage is hereby made, this Indenture (hereinafter sometimes called the “Forty-eighth Supplemental Indenture”) being supplemental thereto;

WHEREAS, the Company-West Virginia caused the Mortgage to be filed for record as a mortgage of real property and as a chattel mortgage in the offices of the Registers of Deeds in various counties in the State of Kansas, and on April 25, 1940 paid to the Register of Deeds of Sedgwick County, Kansas, that being the County in which the Mortgage was first filed for record, the sum of $40,000 in payment of the Kansas mortgage registration tax as provided by Section 79-3101 et seq. , General Statutes of Kansas 1935; and

WHEREAS, by the Mortgage, the Company-West Virginia covenanted that it would execute and deliver such supplemental indenture or indentures and such further instruments and do such further acts as might be necessary or proper to carry out more effectually the purposes of the Mortgage and to make subject to the lien of the Mortgage any property thereafter acquired, intended to be subject to the lien thereof; and

WHEREAS, an instrument, dated May 31, 1949, was executed by the Company-West Virginia appointing Oliver R. Brooks as Individual Trustee in succession to said Henry A. Theis, resigned, under the Mortgage, and by Oliver R. Brooks accepting the appointment as Individual Trustee under the Mortgage in succession to said Henry A. Theis, which instrument was filed for record in the offices of the Registers of Deeds in various counties in the State of Kansas; and

WHEREAS, an instrument, dated March 3, 1958, was executed by the Company-West Virginia appointing Wesley L. Baker as Individual Trustee in succession to said Oliver R. Brooks, resigned, under the Mortgage, and by Wesley L. Baker accepting the appointment as Individual Trustee under the Mortgage in succession to said Oliver R. Brooks, which instrument was filed for record in the offices of the Registers of Deeds in various counties in the State of Kansas; and

WHEREAS, an instrument, dated November 20, 1969, was executed by the Company-West Virginia appointing Edwin F. McMichael as Individual Trustee in succession to said Wesley L. Baker, resigned, under the Mortgage, and by Edwin F. McMichael accepting the appointment as Individual Trustee under the Mortgage in succession to said Wesley L. Baker, which instrument was filed for record in the offices of the Registers of Deeds in various counties in the State of Kansas; and

WHEREAS, by the Twenty-seventh Supplemental Indenture mentioned below, the Company-Kansas, among other things, appointed R. Amundsen as Individual Trustee in succession to said Edwin F. McMichael, resigned, under the Mortgage, and by R. Amundsen accepting the appointment as Individual Trustee under the Mortgage in succession to said Edwin F. McMichael; and

WHEREAS, by the Thirty-second Supplemental Indenture mentioned below, the Company-Kansas, among other things, appointed W. A. Spooner as Individual Trustee in succession to said R. Amundsen, resigned, under the Mortgage, and by W. A. Spooner accepting the appointment as Individual Trustee under the Mortgage in succession to said R. Amundsen; and

WHEREAS, by the Fortieth Supplemental Indenture mentioned below, the Company-Kansas, among other things, appointed Judith L. Bartolini as Individual Trustee in succession to said W.A. Spooner resigned, under the Mortgage, and by Judith L. Bartolini accepting the appointment as Individual Trustee under the Mortgage in succession to said W.A. Spooner; and

WHEREAS, the Company-West Virginia executed and delivered to the Trustees a First Supplemental Indenture, dated as of June 1, 1942 (which supplemental indenture is hereinafter sometimes called the “First Supplemental Indenture”); and

WHEREAS, the Company-West Virginia caused the First Supplemental Indenture to be filed for record as a mortgage of real property and as a chattel mortgage in the offices of the Registers of Deeds in various counties in the State of Kansas, but paid no mortgage registration tax in connection with the recordation of the First Supplemental Indenture, no such tax having been payable in connection with such recordation; and

- 2 -

WHEREAS, the Company-West Virginia executed and delivered to the Trustees the following supplemental indentures:

which supplemental indentures are hereinafter sometimes called the Second through Twelfth Supplemental Indentures, respectively; and

WHEREAS, the Company-West Virginia caused the Second through Eighth Supplemental Indentures to be filed for record as a mortgage of real property and as a chattel mortgage in the offices of the Registers of Deeds in various counties in the State of Kansas, and caused the Ninth through Twelfth Supplemental Indentures to be filed for record as a mortgage of real property in the offices of the Registers of Deeds in various counties in the State of Kansas and as a chattel mortgage in the Office of the Secretary of State of Kansas, and on the following dates paid to the Register of Deeds of Sedgwick County, Kansas, that being the County in which the Second through Twelfth Supplemental Indentures were first filed for record as a mortgage of real property, the following amounts:

- 3 -

such amounts being in payment of the Kansas mortgage registration tax as provided by the then currently applicable sections of the statutes of the State of Kansas in effect on those dates; and

WHEREAS, the Company-West Virginia was merged into the Company-Kansas on May 31, 1973; and

WHEREAS, in order to evidence the succession of the Company-Kansas to the Company-West Virginia and the assumption by the Company-Kansas of the covenants and conditions of the Company-West Virginia in the bonds and in the Mortgage contained, and to enable the Company-Kansas to have and exercise the powers and rights of the Company-West Virginia under the Mortgage in accordance with the terms thereof, the Company-Kansas executed and delivered to the Trustees a Thirteenth Supplemental Indenture, dated as of May 31, 1973 (which supplemental indenture is hereinafter sometimes called the “Thirteenth Supplemental Indenture”); and

WHEREAS, the Company-Kansas caused the Thirteenth Supplemental Indenture to be filed for record as a mortgage of real property in the offices of the Registers of Deeds in various counties in the State of Kansas and as a chattel mortgage in the Office of the Secretary of State of Kansas, but paid no mortgage registration tax in connection with the recordation of the Thirteenth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas executed and delivered to the Trustees the following supplemental indentures:

- 4 -

which supplemental indentures are hereinafter sometimes called the Fourteenth through Forty-seventh Supplemental Indentures, respectively; and

WHEREAS, the Company-Kansas caused the Fourteenth Supplemental Indenture to be filed for record as a mortgage of real property in the offices of the Registers of Deeds in various counties in the State of Kansas and as a chattel mortgage in the Office of the Secretary of State of Kansas; and

WHEREAS, the Company-Kansas caused the Fifteenth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on December 10, 1975, Film 169, page 363), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on December 10, 1975 and indexed as No. 325,911); and

WHEREAS, the Company-Kansas caused the Sixteenth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on September 29, 1976, Film 211, page 363), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on September 29, 1976 and indexed as No. 363,835); and

WHEREAS, the Company-Kansas caused the Seventeenth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of

- 5 -

Sedgwick County, Kansas (filed on March 16, 1977, Film 234, page 492), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on March 1, 1977 and indexed as No. 384,759); and

WHEREAS, the Company-Kansas caused the Eighteenth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on May 26, 1977, Film 246, page 655), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on May 26, 1977 and indexed as No. 394,573); and

WHEREAS, the Company-Kansas caused the Nineteenth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on August 31, 1977, Film 263, page 882), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on September 1, 1977 and indexed as No. 406,577); and

WHEREAS, the Company-Kansas caused the Twentieth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on March 29, 1978, Film 297, pages 635-656), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on March 30, 1978 and indexed as No. 434,072); and

WHEREAS, the Company-Kansas caused the Twenty-first Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on January 9, 1979, Film 345, page 648), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on January 10, 1979 and indexed as No. 470,851); and

WHEREAS, the Company-Kansas caused the Twenty-second Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on April 2, 1980, Film 413, page 1,468), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on April 3, 1980 and indexed as No. 533,415); and

WHEREAS, the Company-Kansas caused the Twenty-third Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on July 1, 1980, Film 425, page 1,003), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on July 2, 1980 and indexed as No. 546,185); and

WHEREAS, the Company-Kansas caused the Twenty-fourth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on August 28, 1980, Film 435, page 266), and as a chattel

- 6 -

mortgage in the Office of the Secretary of State of Kansas (filed on August 29, 1980 and indexed as No. 554,543); and

WHEREAS, the Company-Kansas caused the Twenty-fifth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on June 30, 1981, Film 483, page 1,512), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on June 30, 1981 and indexed as No. 601,270); and

WHEREAS, the Company-Kansas caused the Twenty-sixth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on December 30, 1981, Film 510, page 300), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on December 31, 1981 and indexed as No. 628,293); and

WHEREAS, the Company-Kansas caused the Twenty-seventh Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on May 6, 1982, Film 526, page 1,141), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on May 7, 1982 and indexed as No. 650,115); and

WHEREAS, the Company-Kansas caused the Twenty-eighth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on March 22, 1984, Film 645, page 1,524), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on March 23, 1984 and indexed as No. 796,449); and

WHEREAS, the Company-Kansas caused the Twenty-ninth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on September 5, 1984, Film 681, page 763), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on September 6, 1984 and indexed as No. 852,425); and

WHEREAS, the Company-Kansas caused the Thirtieth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on September 12, 1984, Film 682, page 1,087), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on September 13, 1984 and indexed as No. 854,284); and

WHEREAS, the Company-Kansas caused the Thirty-third Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on June 18, 1991, Film 1177, page 0876), and as a security

- 7 -

agreement in the Office of Secretary of State of Kansas (filed on June 18, 1991 and indexed as No. 1,693,446); and

WHEREAS, the Company-Kansas caused the Fortieth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on June 28, 2000, Film 2062, page 0053), and as a security agreement in the Office of Secretary of State of Kansas (filed on June 28, 2000, and indexed as No. 3756913); and

WHEREAS, the Company on the following dates paid to the Register of Deeds of Sedgwick County, Kansas, that being the County in which the Fourteenth through Thirtieth Supplemental Indentures, the Thirty-third Supplemental Indenture and the Fortieth Supplemental Indenture were first filed for record as a mortgage of real property, the following amounts:

such amounts being in payment of the Kansas mortgage registration tax as provided by the then currently applicable sections of the statutes of the State of Kansas in effect on those dates; and

WHEREAS, the Company-Kansas caused the Thirty-first Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of

- 8 -

Sedgwick County, Kansas (filed on February 1, 1985, Film 707, page 378), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on February 4, 1985 and indexed as No. 895,468), but paid no mortgage registration tax in connection with the recordation of the Thirty-first Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Thirty-second Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on April 16, 1986, Film 791, page 1,336), and as a chattel mortgage in the Office of the Secretary of State of Kansas (filed on April 17, 1986 and indexed as No. 1,048,212), but paid no mortgage registration tax in connection with the recordation of the Thirty-second Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, in order to evidence the succession of the Company to the Company-Kansas and the assumption by the Company of the covenants and conditions of the Company-Kansas in the bonds and in the Mortgage contained, and to enable the Company to have and exercise the powers and rights of the Company-Kansas under the Mortgage in accordance with the terms thereof, the Company executed and delivered to the Trustees a Thirty-fourth Supplemental Indenture, dated as of March 31, 1992 (which supplemental indenture is hereinafter sometimes called the “Thirty-fourth Supplemental Indenture”); and

WHEREAS, the Company-Kansas caused the Thirty-fourth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on March 31, 1992, Film 1236, page 987), and as a security agreement in the Office of Secretary of State of Kansas (filed on March 31, 1992 and indexed as No. 1,780,893), but paid no mortgage registration tax in connection with the recordation of the Thirty-fourth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company caused the Thirty-fifth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on December 16, 1992, Film 1301, page 0104), and as a security agreement in the Office of Secretary of State of Kansas (filed on December 16, 1992 and indexed as No. 1,861,886), but paid no mortgage registration tax in connection with the recordation of the Thirty-fifth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Thirty-sixth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on August 10, 1993, Film 1364, page 0515), and as a security agreement in the Office of Secretary of State of Kansas (filed on August 11, 1993 and indexed as No. 1,936,501), but paid no mortgage registration tax in connection with the

- 9 -

recordation of the Thirty-sixth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Thirty-seventh Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on January 18, 1994, Film 1411, page 0710), and as a security agreement in the Office of Secretary of State of Kansas (filed on January 18, 1994 and indexed as No. 1,985,104), but paid no mortgage registration tax in connection with the recordation of the Thirty-seventh Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Thirty-eighth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on February 28, 1994, Film 1422, page 1046), and as a security agreement in the Office of Secretary of State of Kansas (filed on February 28, 1994 and indexed as No. 1,997,743), but paid no mortgage registration tax in connection with the recordation of the Thirty-eighth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Thirty-ninth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on April 27, 1994, Film 1440, page 855), and as a security agreement in the Office of Secretary of State of Kansas (filed on April 27, 1994 and indexed as No. 1,377,915), but paid no mortgage registration tax in connection with the recordation of the Thirty-ninth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Forty-first Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on June 6, 2002, Film 2460, page 1), and as a security agreement in the office of Secretary of State of Kansas (filed on June 6, 2002, and indexed as No. 5264221), but paid no mortgage registration tax in connection with the recordation of the Forty-first Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Forty-second Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on March 12, 2004, Film 2854, page 8731), and as a security agreement in the office of Secretary of State of Kansas (filed on March 12, 2004, and indexed as No. 5760673), but paid no mortgage registration tax in connection with the recordation of the Forty-second Supplemental Indenture, no such tax having been payable in connection with such recordation; and

- 10 -

WHEREAS, the Company-Kansas caused the Forty-third Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on June 10, 2004, Film and Page 28578510), and as a security agreement in the office of Secretary of State of Kansas (filed on June 10, 2004, and indexed as No. 5820311), but paid no mortgage registration tax in connection with the recordation of the Forty-third Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Forty-fourth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on May 6, 2005, Film and Page 28671438), and as a security agreement in the office of Secretary of State of Kansas (filed on May 6, 2005, and indexed as No. 5981824), but paid no mortgage registration tax in connection with the recordation of the Forty-fourth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Forty-fifth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on March 17, 2006, Film and Page 28764552), and as a security agreement in the office of Secretary of State of Kansas (filed on March 17, 2006, and indexed as No. 6122576), but paid no mortgage registration tax in connection with the recordation of the Forty-fifth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Forty-sixth Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on June 1, 2006, Film and Page 28785638), and as a security agreement in the office of Secretary of State of Kansas (filed on June 1, 2006, and indexed as No. 6168504), but paid no mortgage registration tax in connection with the recordation of the Forty-sixth Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-Kansas caused the Forty-seventh Supplemental Indenture to be filed for record as a mortgage of real property in the office of the Register of Deeds of Sedgwick County, Kansas (filed on March 16, 2007, Film and Page 28865277), and as a security agreement in the office of Secretary of State of Kansas (filed on March 16, 2007, and indexed as No. 6326219), but paid no mortgage registration tax in connection with the recordation of the Forty-seventh Supplemental Indenture, no such tax having been payable in connection with such recordation; and

WHEREAS, the Company-West Virginia, the Company-Kansas or the Company has from time to time caused to be filed in the respective offices of the above-mentioned Registers of Deeds and Secretary of State affidavits executed by the Trustees under the Mortgage,

- 11 -

preserving and continuing the lien thereof either as a chattel mortgage in accordance with the provisions of K.S.A. 58-303 (Section 58-303 of the General Statutes of Kansas 1935) or as a security agreement under the provisions of K.S.A. 84-9-401 et seq.; and

WHEREAS, in addition to the aforesaid filings for record in the respective offices of the above-mentioned Registers of Deeds, the Company-West Virginia, the Company-Kansas or the Company has filed copies of the Mortgage and the First through Forty-seventh Supplemental Indentures, certified as true by it, with the Secretary of State of Kansas; and

WHEREAS, the Company-West Virginia, the Company-Kansas or the Company has heretofore issued, in accordance with the provisions of the Mortgage, as heretofore supplemented, the following series of First Mortgage Bonds:

- 12 -