The Senate yesterday defied a veto threat by adding a controversial plant-closing provision to the trade bill, but reversed its earlier move to limit the president's discretion to deny import restraints sought by domestic industries.

On Tuesday the Senate placed limits on presidential authority to refuse to impose restraints to help industries hurt by fairly traded foreign goods. But with little debate and no roll call votes, it adopted two amendments last night that effectively reversed Tuesday's action.

The change came on amendments to the trade bill offered by Sens. Bill Bradley (D-N.J.) and Phil Gramm (R-Tex.) that gave the president added grounds for refusing to impose tariffs or quotas in such cases.

In its action Tuesday, the Senate allowed only two exceptions for the president: cases involving national security or instances in which industries that use the protected products in manufacturing would suffer from their increased cost. It defeated an amendment pushed by Sen. Bob Packwood (R-Ore.) that would have added the national economic interest as a reason for denying trade relief.

But Packwood said the Bradley and Gramm amendments adopted last night would accomplish that aim. The Bradley amendment said the president should consider the impact of trade relief on the poor, and Gramm's added the impact of possible retaliation on American farmers as a reason for denial.

Senators who wanted curbs on presidential power to deny trade relief said they decided against making an issue of the Bradley amendment because "no one wanted to fight" helping the poor.

"We decided to let it ride. It only has a small impact, not a substantial impact," said a Senate trade specialist.

The final approval of the plant closing measure came on a 60 to 40 vote yesterday morning that defeated a Republican effort to kill the measure. The provision requires companies employing more than 100 people to notify their workers and the community if they plan to close a factory.

The Senate had approved the plant closing provision on a voice vote Wednesday night as a parliamentary maneuver to give the Republicans a clear shot at trying to kill it with a roll call vote yesterday.

Major business organizations opposed the provision even after it was watered down by its proponents, led by Senate Labor Committee Chairman Edward M. Kennedy (D-Mass.) and Sen. Howard Metzenbaum (D-Ohio), to gain more Senate support.

The National Association of Manufacturers called the action on the provision "a hollow victory for labor's anticompetitive agenda." NAM President Alexander B. Trowbridge credited "well-focused efforts" by business for weakening the measure.

Metzenbaum said yesterday's vote doesn't mean "the labor agenda is off and running." But he acknowledged that the margin of victory was larger than expected and called the vote "a great victory" for labor