September 22, 1985

FOR almost a century, Columbus Circle has played the tortoise to the hare of Manhattan real-estate development, moving in brief bursts of energy only when expansion of other sections of the borough faltered. Now it is undergoing a period of rapid growth that may carry it nearer the long-ago expectations of some city planners who predicted that it would be the eventual hub of the city. The centerpiece is the multimillion-dollar twin-towered complex of offices, hotel, condominiums and stores that Boston Properties is planning on the site of the New York Coliseum. But dotted around it in the area bounded by Broadway, Lincoln Center, 55th Street and the Hudson River are at least 15 other projects in various stages of development. Indeed, in the now updated view of some real-estate developers, the Columbus Circle area - which the Dutch first named the ''Valley of the Flowers'' - is blossoming into a future that could make it one of Manhattan's prime residential and commercial neighborhoods.

ATTRACTED by Federal and state tax credits and the possibility of saving on energy costs, thousands of Long Islanders have built or are thinking of building ''solar'' rooms. But in many cases, consumer affairs officials warn, the buyer may be getting a lot less than he bargained for. Dealers say they expect a great deal of business this fall as people seek to build before the Dec. 31 deadline for Federal and state tax credits for solar energy systems. The combined 55 percent credit (40 percent Federal, 15 percent state) can bring the cost of a $6,000 solar room down to $2,700.

FOR the last decade, things were good for the Marriott Corporation. Few sizable companies in any business can grow at 20 percent year after year, but the aggressive company from the suburbs of the nation's capital made fast growth its credo. Marriott's business hotels sprouted in choice suburban, airport and downtown locations and the company - and its industry - seemed to be on a never-ending growth spiral. But times have changed. The hotel market has become overbuilt and business has slackened. And feisty Marriott, which anticipated many of these changes, is now switching gears within the industry and taking its name where it has never been before. Next month the doors officially open on the New York Marriott Marquis, a glittering $400 million atrium hotel plunked down in tawdry Times Square. Marriott is going into the convention business in a big way and will spend over $1 billion in a coast-to-coast effort to take on the heavyweight in conventions, the Hilton Hotels Corporation. The first try has not been easy: The Times Square location is not prime territory, and a new convention center nearby, which Marriott was betting on to feed guests into its 1,877-room hotel, is still in construction and far behind schedule. Marriott executives insist that the goliath New York hotel will eventually succeed, but they concede that profits are several years away.

THOUGH the state's apple growers faced some difficulties this year, including a late frost last spring and high heat and humidity in recent weeks, most expect to harvest a hearty crop of big and juicy apples. While the crop is expected to be slightly smaller than last year's, growers say prices will be about the same and the quality better. ''The quality of our apples is fantastic, the best in several years,'' said Dr. Alan J. Grout, who runs Aspetuck Valley Orchards in Easton. He was not as enthusiastic May 8, though, when a late frost threatened to destroy most of his crop. His apple trees were in full bloom, with blossoms at their most delicate change, when the temperature reached a critical 28 degrees.

A gigantic shift on the Pacific Ocean floor jarred Mexico's west coast last week, causing devastation in Mexico City, 230 miles away. Preliminary reports said the earthquake had killed several thousand people, leaving thousands more injured and homeless. Yesterday, a White House official predicted the final total of dead and injured would be in the tens of thousands. A powerful second earthquake compounded the destruction, much of it in the capital's historic center, which rises above an ancient lakebed, an insecure footing that made the destruction worse.