The Montgomery County Council, in an effort to expand representation on the powerful Planning Board to residents of the fast-growing upper county, is expected today to reopen the nomination process for the board's open seat, according to council members. And county officials are encouraging upcounty residents to apply.

The council plans to seek a new appointment rather than try to override County Executive Charles W. Gilchrist's veto yesterday of Rosalie Silverberg, a resident of the southern part of the county who was named to the Planning Board by the council two weeks ago.

"I just don't think the votes are there," said County Council President William E. Hanna Jr., referring to the five out of seven votes needed to override an executive veto.

Gilchrist vetoed Silverberg's appointment after an intense 10-day lobbying effort by community groups and political leaders, angered that a Bethesda resident was appointed to a five-member board that already includes four residents of the Bethesda-Chevy Chase area.

As an advisory body to the council, the Planning Board approves or rejects development and zoning plans. By not putting on the board someone who lives in the most rapidly growing area of the county, opponents said, the council showed it was insensitive to traffic and other problems related to that growth.

Gilchrist reiterated that view yesterday.

"The insensitivity of this appointment is disturbing evidence of myopia," he said. "If this appointment were allowed to take effect, our Planning Board would have its entire membership drawn from a small geographical part of this county."

Gilchrist's veto -- his fourth since his election in 1981 -- comes on the heels of several other political victories he has secured in his last few months in office. Gilchrist, first elected executive in 1978, is stepping down in January to study for the priesthood.

A top contender for the board's open seat was Ruby Rubens, 51, a black woman with a background in housing who lives near the congested Rte. 29 corridor in eastern Montgomery County. Residents of the eastern part of the county are experiencing many of the same growing pains felt in the upper county. The Planning Board is currently all white.

Norman L. Christeller, Planning Board chairman, said it is difficult to find qualified people in the growth areas of the county who can serve on the board. "A Planning Board member has to put in about 35 to 40 hours a week to do the job properly," he said, "and in the growth areas of the county both husband and wife are working."

Gilchrist's veto, his first of a Planning Board nominee, took some council members by surprise.

"I can't tell you how surprised I am," said council member Esther P. Gelman. " Silverberg's qualifications are really the best in the county." Gelman added that the council looked hard for upcounty candidates. Two more seats become vacant in a year, she said, and representatives from the upcounty area could get on the board then.

Neither Gilchrist nor upcounty leaders disputed Silverberg's qualifications for the board. Silverberg, a retired biochemist, has sat on several public commissions and boards.

Gilchrist said he based his decision on a state law passed in the 1986 session of the General Assembly, which states that membership on the Montgomery County Planning Board should be representative of the county.

In another planning-related action yesterday, Gilchrist threatened to sue the council if it votes to place two charter amendments on the ballot in the general election.

The Christeller-proposed amendments, which the council voted in a straw tally to put on the ballot, would repeal two state laws increasing the power that future county executives will have over planning matters.

The laws grant new authority to the county executive to veto master land use plans, the basic blueprints for developments in Montgomery, and to appoint two of the five Planning Board members.

Gilchrist also said he supported a Charter Review Commission recommendation, which the council backed in a straw vote, to add two members to the seven-member council and require five of them to be elected from districts rather than from the entire county as they are now.