BOCA RATON — It's the best thing I've heard the Boca Raton Symphonia play.

Max Reger's arrangement of the Bach chorale, O Mensch bewein dein Sünde gross (O man, bewail thy grievous sin), is a mere seven minutes long, written for strings alone. Yet for its season opener Sunday afternoon, the Symphonia lavished on it not only soulful grief but also quiet elegance and unusually warm sound.

Responding remarkably to new Principal Conductor Alexander Platt, players shaped the little-known piece into their most expressive and smoothly polished playing, the sum of many excellent points in a thoughtful, inventive program that included the national anthem and Bach, Schumann and Mendelssohn.

Platt conducted the Reger without a baton, shaping every measure with his hands. It came right after new board chairman Harry Shuford announced that Platt, who will lead only one other concert this season, would conduct all five concerts in 2008-09, far more than expected.

Guest pianist Roberta Rust gave another of the performances that stood out as remarkable. Making her Symphonia debut, the Lynn University Conservatory of Music professor lent a lyrical grace and velvety warmth to Schumann's beloved Piano Concerto in A Minor.

There were a few keyboard splotches and orchestra uncertainties, tempo shimmies and breath-holding spots. But Rust lived and breathed the Schumann. Her powerful connection to the work was clear in her fluent singing and beautifully weighted tone. She draws you into the work's strength of character as well as its distinctively solemn joy. Even after the music ends, she makes you want to hear more.

In a lovely, romanticized Bach Suite for Small Orchestra arranged by Mahler, and the triumphant concluding Mendelssohn Symphony No. 5 (Reformation), it was clear: The Symphonia has grown. A 3-year-old, but not a toddler, it is maturing at a surprising, but gratifying pace.