A healthy orchestra evolves - perhaps even mutates - almost imperceptibly in front of its loyal local audience.

If that assertion is even half way true, then the Royal Scottish National Orchestra is brimming with vitality. It opens its new season in Scotland's main cities under its music director Peter Oundjian from tonight until Saturday, with a programme that includes the UK premiere of the flute concerto of American composer Christopher Rouse, showcasing the talent of Linn recording artiste and the orchestra's principle, Katherine Bryan. But it may come as a surprise to many concert-goers to learn that Bryan, as the first of a whole sequence of changes on the front desks across all sections of the orchestra when she was appointed aged just 21, is now one the longest serving RSNO principals.

As such, one of her current tasks is to find a replacement for the most recent retiral among the principal wind players, the clarinet player who had become the father figure of the wind section and had been a mentor to her when she arrived more or less straight out of Julliard. There have been 200 applications to succeed John Cushing and sifting them and arranging trials for the contenders in concerts where all the decision-makers are in place may be a longish process, but the RSNO has business-like form in dealing with such regular eventualities and it is already underway, although Cushing only gave his last concert at the Edinburgh International Festival.

His retiral was acknowledged then, but Cushing is not a man for a big fuss, so his agreement to talk about his time with the orchestra surprised some of his RSNO colleagues, but as we did, it became clear that he is happy to depart - after 36 years he feels he had completed a purposeful arc of work.

Nor is he lost to the world of clarinettists in Scotland. Almost as soon as he arrived to work with the (then) Scottish National Orchestra in April 1978, he was embraced by the Royal Scottish Academy of Music and Drama (now the Royal Conservatoire of Scotland) as a teacher of the instrument and that work continues, without, he immediately points out, the scheduling headaches that previously came with combining the two jobs.

"The teaching at the academy had a much narrower focus then. Students now have a lot more academic work - and they are rehearsing all the time. But they need to have time for individual practice," he says, repeating an argument you feel sure he had made in the staff room before

The past year has been a particularly rewarding one at the conservatoire for Cushing because he has had two very talented young players to coach.

"It is very unusual to have two exceptional students in one year, but these things go in waves. The standard in Glasgow has always been high, the Academy was traditionally very good at winds. The selection process is very rigorous."

Cushing does suggest, however, that today's young players are rather more supportive of one another than in his more competitive student days at Royal Manchester College of Music, whe re his teacher was the London Symphony Orchestra's Sydney Fell.

"I remember when he took me to meet the principal of the college, he called the principal 'sir'. Those were different days."

Cushing came to the RSNO after five years at Welsh National Opera, where the orchestra was conducted by Richard Armstrong, who would later come to Glasgow as music director of Scottish Opera, presiding over the company's most recent, acclaimed, tilt at Wagner's Ring cycle.

Students may not have had such a busy curriculum in the late 1970s, but the musicians of the RSNO had a packed agenda.

"The winter season was 20 weeks long," recalls Cushing, "then there were three weeks of Glasgow Proms and two week of Edinburgh Proms in the summer." Add to that the Musica Nova mini-festival of contemporary composition at Glasgow University, Edinburgh Festival concerts and perhaps an overseas tour and the orchestral year was quite a full one.

Just as the RSNO is featuring its own as concerto soloist in the opening concerts this year, the SNO was quick to capitalise on its talented young principal, with a performance of Mozart's much-loved Clarinet Concerto in Dunfermline only months after Cushing's arrival in Scotland. The conductor was another highly-rated young talent by the name of Simon Rattle, and Cushing was paid £70 over and above his salary for the engagement.

Cushing thinks for a moment before naming his own favourite conductor as Neemi Jarvi, the orchestra's former principal conductor.

"I learned so much from him, about musical freedom and the importance of exercising your ability to express yourself in the orchestra. He was always engaging and never criticised."

As for a favourite piece, the answer is immediate, entirely understandable and part of the reason why Cushing chose this year to make his exit from the orchestra.

James MacMillan composed his clarinet concerto Ninian specifically for Cushing and dedicated it to him.

Although he had already written the phenomenally successful percussion concerto Veni Veni Emmanuel, premiered by Evelyn Glennie, and a Cello Concerto for Rostopovich, the composer had worked closely with neither of them in the creation of those pieces, the clarinettist notes.

"As a clarinet player you are not often able to exploit all the instrument can do, but in that piece you do."

Ninian demands great technical fluency and goes to the extremes of the clarinet's range. It is hard to play, and, at 40 minutes, perhaps a little too long to be regularly programmed, Cushing suspects, although he says publishers Boosey & Hawkes think it is a "sleeper" whose time is yet to come."

The man who premiered the piece helped hurry that process along when the orchestra revisited it last season, an opportunity that Cushing says gave him the spur to call a halt to his orchestral career.

"If you'd told me in 1997 that I'd be playing it again when I was 64 I'd not have believed you. That was a box ticked. I am not just agnostic, I'm an atheist, but that piece has a spirituality you can't help but grasp. The humanity of it is engrossing."

Cushing is also the dedicatee of a solo clarinet piece by MacMillan, entitled From Galloway, and chamber music is still part of his musical life, including membership of the wind quintet Primi, and the Phoenix wind trio with Bryan and bassoonist David Hubbard.

Bryan and Cushing have clearly talked about the role of a section principal in the wind section of the RSNO a great deal, that combination of being a team player and an outstanding soloist, a strong personality and skilled interpreter of the whim of a conductor. So while he will not be involved in the appointment of his successor, the Cushing influence will not be that far away.

"But we have to forget that we are replacing John," says Bryan. "We are looking for something special, but not the same."

The RSNO season opens in Aberdeen tonight, Edinburgh tomorrow and Glasgow on Saturday. rsno.org.uk