The National Symphony Orchestra opens its 48th season on Tuesday night in a program of Schubert and Tchaikovsky and then leaves next Sunday for a week's visit to Mexico, where it is to play six concerts in six days. The brief tour will be the orchestra's first out-of-the-country trip under conductor Mstislav Rostropovich.

At the beginning of every new orchestral season, there are four basic questions: 1) What will the orchestraplay? 2) Who will be its conductors? 3) Who will be its soloists? 4) How will be play?

The first question is the most important, for the obvious reason that an orchestra those musical experiences that can come only from hearing familiar and unfamiliar music, new and old, in first-rate performances. There is never any agreement about what music a conductor should choose for a season. Only a few weeks ago a letter appeared in a Letters column in which the writer objected because the National Symphony "required," he said, that if he wanted to hear Isaac Stern, he "had to listen to Schubert and Messiaen."

Now there's an interesting pairing of names. As a rule symphony subscribers are pleased when offered Schubert. But it is wise to remember that there is no composer whose music is at all well known who is not loathed by some music lovers, just as there are always some who rate that same man as their absolute favorite.

But it is unusual to find someone who dislikes both Schubert, that first of the wholly romantic composers, and Olivier Messiaen, the great French composer-teacher-organist whose 70th birthday the world of music is celebrating this year.

While we're on this particular subject, let's discuss that letter writer's objections a little more. He is griping about the subscription system. Apparently he wants to be sure of a seat for one of the concerts at which Isaac Stern is going to play the Second Violin Concerto by Prokofiev. It will be played on Oct. 24, 25, 26. With good reason, the writer is afraid that those concerts may be entirely sold out before he can snag a single ticket.

The system to which he objects, however, is used by every symphony, opera and ballet organization in the country, and by most of its established theater groups. It is the means by which those artistic outfits can be assured of the largest number of listeners at most of their events. If the writer had a subscription to the Metropolitan opera, he might very well not be especially interested in hearing last Tuesday's "Billy Budd" by Benjamin Britten, but he knows that if he wants to be sure of hearing the following Tuesday nights' "Tannhauser," "Otello," "Werther," "Fidelio," "Carmen," "Aida" and "The Bartered Bride," he will have to buy "Billy Budd" as well. To accuse a symphony or opera management, as people sometimes do, of being high-handed, or even blackmailers, is to oppose a vital and valuable method of supporting the arts.

What is more important from the artistic point of view is that it is mere whim on the part of this country's music directors to play more Schubert this season thatn usual, and to plan special programs in honor to Olivier Messiaen. It is one of the basic reasons for their existence in precisely this way to single out those, dead and living, who have given music some of its greatest monuments. Some conductors are doing far more than Rostropovich in this year, the 150th anniversary of the death of Schubert, and are programming all nine of his symphonies. In Detroil Antal Dorati will present to Detroil Symphony subscribers the first U.S. performance of one of Schubert's finest works, his opera, "Alfonso und Estrella," and I want to make the trip to that city to hear it.

As for Messiaen, no man living today has had a wider or more enriching influence upon younger musicians, composers and performers, than this remarkable genius. The French government has declared an extraordinary period of official salutes to Messiaen, with performances of every single one of his compositions to be given in Paris within a few weeks' span. For those performances, many of the world's leading musicians will gather.

Within the next few months, nearly every major American orchestra will offer its subscribers programs made up either wholly or in part of the French composer's works. In Washington Rostropovich will present the early-and highly romantic suite. "The Ascension," in company with the later "Exotic Birds." These will share an evening with the Fourth Symphony of Brahms.

It is his balancing of works that have often been played by the National Symphony with those that are new to this orchestra that Rostropovich has made the coming season interesting. For those who tend to run in the opposite direction the instant "unfamiliar works" are mentioned, it should be pointed out that these have been selected from music by Purcell, Britten - hardly names to frighten the timorous - along with newer music by Makris, Milhaud, Dutilleux, Hovhaness, Schuller, Montsalvatge and Dorati.

Rostropovich will be on hand to conduct 10 of the orchestra's fall-winter-spring season's concerts. His guests will include Aaron Copland, Lorin Maazel, Leonard Slatkin, Erich Leinsdorf, Claudio Abbado, Antal Dorati, Bernstein, Christian Badea and James Conlon.

Among those names you will see some of the most famous in the worlds of orchestral and operatic conducting, together with those of several outstanding younger men. The season's soloists include pianists Murray Perahia, Yvonne Loriod - the distinguished wife of Messiaen - Rudolf and Peter Serkin and Steven de Groote; violinists Isaac Stern, Yehudi Menuhin. Ruggiero Ricci, Miran Kojian, and Henryk Szeryng; cellists Rostropovich, John Martin and Lluis Claret; Jean-Pierre Rampal, flute; Lewis Lipnick, contrabassoon; Nicanor Zabaleta, harp; and singers Maureen Forrester, Galina Vishnevskaya, Peter Pears, John Shirkley-Quirk, Kiri Te Kanawa, Hearther Harper, Clarice Carlson, Rolfe Johnson and Peter Lagger.

There will be a special Thursday night program in which Rostropovich's desire to further the career of young soloists will be advanced. With James Colon as guest conductor, soprano Mary Burgess will sing Britten's "Les Illuminations," tenor Philip Creech will sing the Britten Serenade for tenor, horn and strings; and Mezzo Fredda Bakusin will sing the Chausson "Proeme de I'amour et de le mer." All three works, gorgeously written for voice and orchestra, are rarely heard on symphony programs.

Other special Rostropovich enthusiasms in the season will include the Britten "War Requiem" whose soloists will include two - Vishnevskaya and Pears - for whom it was written; the Lutoslawski Cello Concerto for which the composer will conduct with Rostropovich as soloist; and the new concertos for flute, by Hovhaness, and for contrabassoon, by Schuller, commissioned by the orchestra.

No - there is no avant-garde music on any concerts. There are symphoniese never before played by the National Symphony from such names as Haydn, Dvorak and Mahler. There will be a brand new work by the orchestra's composer-vilinist Andreas Makris, music called "Chromatokinesis." There will be the "Four Last Songs" by Richard Strauss, written in 1948; and Purcell fantasies written 250 years earlier. That's not a bad range.

How will the orchestra play? No one knows. But this season there will be a new first flutist named Toshiko Kohno and 14 other new players (including eight additional positions): five violins, two violas, two cellos, ttwo double basses, two horns and one trombone. For the first time in it history, the National Symphony's permanent membership will be the same as that of the orchestras of New York, Chicago, Boston and Philadelphia. There will be a difference.