The Carter administration's effort to win Senate approval for the Pamama Canal treaties now depends on five key Democratic senators, all publicly "undecided," who met privately yesterday with Vice President Mondale in the Capitol.

The administration feel that all five senators are disposed to vote for the treaties, but only if certain of their concerns are satisfied through some alterations of the pacts. If all five vote yes, the best available head-counts indicate that the treaties will receive 68 to 70 votes. It wll take 67 to approve them if all 100 senators vote.

The five key Domocratic senators are Dennis DeConcin (Ariz.), Wendell H. Ford (Ky!), Russell B. Long (La.), Sam Nunn (Ga.) and Herman E. Talmadge (Ga.).

All five ate lunch yesterday in the office of Majority Leader Robert C. Byrd (D-W. Va.) with Mondale, Deputy Secretary of State Warren M. Christopher, Byrd, Senate Majority Whip Alan Cranston (d-calif.) and Sen. Frank Church (D-Idaho).

The purpose of the lunch was to look for ways to satisfy the five senators without altering the treaties so substantially that they might have to renegotiated with Paname or resubmitted to the Panamanian electorate for reapproval.

Mondale and Christopher took the position that some new understandings or reservations might be acceptable, but urged the five senators not to seek new amendments to the body of the treaties, according to sources present at the lunch.

No deals were struck, and the five senators are to meet again with the Senate leadership and perhaps adminstration officals in the days ahead. Late yesterday, Doconcini went to the White House to meet with President Carter.

Mondale and Christopher did suggest to the senators ways to draft additions to the treaties that would cause the least possible disruption.

The five senators' concerns apparently center on two areas: the provision in the Panama Canal treaty committing the Unites States to build any new Atlantic-Pacific canal in Panama, and the articles of the so-called neutrality treaty that severely limit U.S. freedom of action in Panama after the year 2000, when the canal would become Panamanian property.

Long said yesterday that he was particularly concerned that the treaty would prohibit the United States from negotiating with any other Central American country about building a new canal. Adminstration arguments on this point are unpersuasive, Long said.

Deconcini has introduced an amendment to the neutrality treaty addressing the second point. Article 5 of that treaty says that, after 2000, only Panama will have the right to station troops on Panamanian territory. Deconcini and others in the group of five say this might make it difficult for the United States to exercise its right to defend the canal's neutrality after 2000 (a right expected to be stated in the treaty language). What if the United States needed to put troops in Panama after 2000 to keep the canal neutral? these senators ask.

DeConcini's amendment would add wording to Article 5 permitting both parties to make joint agreement"in accordance with their respective constitutional process" that could, in effect, circumvent the prohibition on stationing any foreign troops in Panama after 2000.

In other words, if the United States and Panama both agreed that U.S. troops should based there, this would not be percluded by the treaties.

The administration told DeConcini it could accept this as a reservation or understanding to the treaty, but urged him hot to try to make it an amendment. The senator said he hadn't yet decided what to do .

The five senators' concerns about the sea-level canal provision could be eliminated by voting to delete it. Through this would be a substantive change to the treaties, the government of Panama never liked the sea-level provision anyhow, and strongman Omar Torrijos has said he would be happy to drop it.

A number of pro-treaty senators think this is a good provision, and the Senate leadership might have some difficulty mustering a majority to eliminate it. But this could be necessary to win the needed 67 votes for final approval.

Administration officals were reliably described as pleased by yesterday's luncheon meeting because they felt all five senators want to vote for the treaties if they can. "They want to be helpful," as one source put it.

Interviews with three of the five suggest sympathy for the administration argument that a presidential defeat on the treaties would gravely debilitate President Carter and the United States generally in dealing with foreign countries.

Another undecided senator, Paul Hatfield (D-Mont.)., recently appointed to serve out the term of the late Lee Metcalf, said yesterday he is "leaning against" final approval, though he too left open the possibility that his concern could still be still be satisfied. Hatfield said he was worried about the sea-level canal and post-2000 situations, and also about whether the treaties should be mad explicit in binding Panama to maintain the canal and keep it open.