The new U.S.-Japanese trade agreement signed in Tokyo yesterday constitutes an important turning point in trade relations between the two nations.

Although the accord isn't expected to result in instant redress for import-plagued U.S. industries, it does signal a major change in direction on Japan's part.

It means, in effect, that after months of negotiations, Japan finally will begin moving from the export-dependent economy it has been since World War II to a full international trading partner willing to buy other nations' goods as well as its own products overseas.

Officials here concede that could tak years - and further rounds of negotiations - to achieve. But at least the two sides have made a start.

By far the most important agreement for the short run is the pledge by the Japanese to trim their massive trade-and-investment surplus according to a preset timetable - and ultimately accept a deficit if it comes.

The pledge carries with it a promise to take a series of significant steps to increase imports from other countries - specifically citrus fruits and hotel-grade beef important to the United States.

Most important, however, Tokyo is to launch a massive new economic stimulus program designed to spur the Japanese economy to a 7 per cent growth rate - enough to heighten demand there for U.S. exports.

Moreover, in a significant development, the Japanese also have promised to work in the multilateral trade talks in Genevat his year toward making their markets as open as those of the United States.

That means a significant reduction in tariffs and a major easing in import quotas on a widevariety of items. Japan already has pledged some tariff cuts, and is promising to proposed others.

Finally, the accord carries a pledge to unsnart Japan's complicated - and often frustrating - way of handling foreign products once they enter Japan and to eliminate bureaucratic red tape and seemingly endless delays.

That move alone should help assuage American exporters U.S. businessmen have complained for years that even if they get their products past Japanese customs officials they're almost impossible to sell.

And, significantly, Japan is to take steps to seek out U.S. exports in a number of key products - among them forest products, electric power plant machinery and nuclear plant components and equipment.

The forest products category refers to finished lumber, such as plywood - not logs, which the japanese have been buying for years. The nuclear components category was approved at the request of Japan, which is searching for new energy sources.

Tokyo already is talking about sending delegations of would-be Japanese buyers to discuss possible deals with American exporters. And the two nations are stepping up their trade-promotion efforts.

Two industries that should benefit immediately are the citrus and hotel-grade beef exports Quotas on both have been raised substantially. The new level for beef should guarantee American firms a wide - and lucrative - market.

To be sure, U.S. officials are not sanguine about the agreement. Alan W. Wolff, deputy to special trade representative Robert S. Strauss, conceded in an interview yesterday that persistent "follow-ups" will be needed.

But compated wth previous U.S. efforts to change the Japanese outlook, this one appears to have opened a new door.