What was going on when I was born?

Some scientists now suspect that trees can in effect warn one another of danger by releasing chemicals into the air. Researchers are preparing new experiments that they hope will confirm the tentative findings of their initial tests, the National Science Foundation announced yesterday. If they are successful, the science foundation said, they will probably be the first to show that plants emit signals that are received and responded to by other plants.

International Leaders of the O.A.U. will try again, for the third and possibly last time, to end rifts that threaten the role the Organization of African Unity has sought to play for two decades in Africa. There was uncertainly about whether the attempt would succeed as African leaders began arriving in Addis Ababa, Ethiopia, for preliminary negotiations to determine whether the O.A.U. twice-postponed 19th summit meeting can take place. (Page A1, Column 3.) An attempt to stop further sabotage in South Africa by the underground African National Congress will be made by shutting the group's military wing out of neighboring black states, South Africa's highest-ranking military official said. Gen. Constand Viljoen, chief of the Defense Force, said this isolation could be accomplished through a combination of military and diplomatic pressure. (A1:3.)

An article in the Business People column Friday gave an incorrect street number for the New York branch of Creditanstalt-Bankverein. It is at 717 Fifth Avenue.

''There were always a lot of snide implications that we came to Princeton for social reasons.

Any Democratic attempt to modify the 10 percent income tax cut scheduled for July 1 will be defeated, Howard H. Baker Jr. of Tennessee, the leader of the Republican majority in the Senate, predicted. In a television interview, he also supported the reappointment of Paul A. Volcker as chairman of the Federal Reserve, a view that has been opposed privately by some Administration officials. (Page A1.)

Ana Brewster pinned a red carnation corsage to her graduation gown, and with the happiest of smiles said, ''I thought I was going to wake up this morning and somebody would say it was all a dream.'' But it was not a dream, and yesterday Mrs. Brewster, a New York City housing assistant and a member of Teamsters Local 237, was one of 17 students to be awarded bachelor's degrees from City College as part of an unusual program for working people without college degrees. Mrs. Brewster, who is 48 years old, believes that she may enroll in law school. Deborah Edwards, 27, a legal assistant, eventually wants to teach history. James Calvin, 46, a massage therapist, leaves soon for Upper Volta, where he will teach English with the Peace Corps.

African leaders began last-ditch negotiations today in an effort to save the Organization of African Unity's twice-postponed 19th summit meeting, which is scheduled to open here Monday. The outcome was still uncertain tonight. Some leaders fear the effort may be the last chance to end the rifts in the organization, which threaten the role it has sought to play for two decades as the voice of a continent. African diplomats said the formal start of the meeting may be postponed if the preliminary talks falter.

The much-maligned ''kids'' of the Mets did some fancy work today before 48,214 fans in Dodger Stadium as they defeated the Dodgers, 4-2, and earned a split of the four-game series against the team with the best record in baseball. Part of the work was done by Scott Holman, who outpitched Jerry Reuss for his first victory of the season. When he got into trouble in the ninth inning, Jesse Orosco went to the rescue and supplied two strikeouts that ended the threat. But the fanciest work of all was performed by Mookie Wilson. He opened the afternoon in a 4-for-40 slump and had lost 95 points in his batting average in a month. But he singled home the deciding run in the eighth inning, stole his 20th base and then scored an insurance run. And he made two dazzling catches in center field, robbing the Dodgers of a home run in the first inning and a leadoff double in the ninth.

Howard H. Baker Jr. of Tennessee, the leader of the Republican majority in the Senate, predicted today that Reagan Administration forces there would defeat any Democratic attempt to modify the 10 percent income tax cut scheduled for July 1. On Monday the Speaker of the House, Representative Thomas P.O'Neill Jr., Democrat of Massachusetts, is expected to propose limiting the cut to $700 for each individual taxpayer or couple. Mr. O'Neill has promised that the House would send such a bill to the Senate. Support For Paul Volcker On the CBS News program ''Face the Nation,'' and later in a telephone interview, Senator Baker made these statements: - He repeated and strengthened his earlier support for the reappointment of Paul A. Volcker as chairman of the Federal Reserve Board. Alternatively, Senator Baker said, he would like to see the job go to Alan Greenspan, the New York economic consultant, who was chief economic adviser to President Ford.

In victory, Yannick Noah was his bouyant, irrepressible self. First he fell to his knees with his head thrown back and his arms stretched upward. Then he sprang to his feet and jumped into his father's outstretched arms. Finally he ran through the line of ball boys and onto the award platform with none of the shy decorum that usually accompanies such moments. Noah had just beaten Mats Wilander, the defending champion, by the unexpected scores of 6-2, 7-5, 7-6; had become the first Frenchman to win the French Open tennis title since 1946, and had won the first Grand Slam tournament of his career. It was the biggest day of his life.

Landlords of rent-stabilized apartments in New York City yesterday proposed monthly rent increases of 10 percent for one-year leases beginning Oct. 1 on apartments renting for more than $400. The landlords, represented by the Rent Stabilization Association, are asking for additional 4.5 percent rises on those apartments during the second and third years of a lease. The landlords also said at a news conference at the New York Sheraton Hotel that raising all stabilized apartment rents by the same percentage was outdated. They called for specific increases on lower-priced apartments that would raise many of those rents even more sharply.

Controversy over whether the Reagan Administration made a formal commitment at the Willamsburg economic summit meeting to reduce interest rates in the United States threatens to undermine the cooperative spirit the conference was intended to foster. The dispute is marked by apparently contradictory statements about the Administration's interest rate policy. Moreover, the situation is complicated by the unexpected strength of the United States economic recovery, which is already leading to increased demand for credit from the private sector. Last Monday evening, in the waning hours of the summit meeting, a reporter asked a top Administration policy maker about the most important effect of the conference on American policy. The official, who was speaking at a background briefing on the condition that he not be identified by name, said, ''I think it will reinforce our determination to try to do something about interest rates and whatever it is that is causing interest rates to remain high.''