In a program in which riddles were the common thread, it was only fitting that last night's Pittsburgh Symphony Orchestra would be a head-scratcher at times. Heinz Hall overflowed with some gorgeous sounds, but questions mounted.

The concert featured a work examining Nietzche's "world-riddle" of life -- Richard Strauss' "Also sprach Zarathustra" -- and also Richard Danielpour's Concerto for Orchestra, "Zoroastrian Riddles." Mozart's Violin Concerto No. 5, with its mysterious Turkish intrusion, landed in-between. But why did the PSO present only two of the four movements of Danielpour's work? It was, after all, commissioned by the orchestra (premiered in 1997), and he is its composer of the year. I suppose the answer is length, but it would only have added about 18 minutes, and the concert ended at around 10 p.m.

Even if logistics did forbid finishing the piece, it's too bad because it begins to take shape in the third movement, which is a tender response to the raucous first two and a fertile motif is transformed in a profound way.

Another question was why the PSO brought back a conductor who didn't command the orchestra well in his debut in 2008. And the theatrical Andris Nelsons has only increased his distracting antics. Whereas last season he appeared to mimic his mentor and fellow Latvian Mariss Jansons, the former PSO maestro, now Nelsons practically danced on the podium as he called for expression. This can be forgiven if the musicians are inspired. But most had their heads in their scores, and ensemble issues abounded. Surely Nelsons is a musical and creative fellow, but the unfamiliar nature of guest conducting demands that you walk before you run.

While I can't blame the obnoxious warbling of the digital organ at the beginning of the "Zarathustra" on Nelsons, but the famous opening (think "2001: A Space Odyssey") arrived shaky and anything but epic because he didn't offer an obvious beat. Luckily, the PSO picked up some momentum and offered luscious collective sound, both in the tuttis and in the chamber sections. Violinist Steven Copes of the St. Paul Chamber Orchestra sat in as concertmaster and played several confident and bright solos.

Soloing in Mozart's Violin Concerto No. 5 was young American violinist Stefan Jackiw. The only question with this artist is how high he might climb. He has a light but ravishing tone and a bow arm that made even the fastest runs flow like liquid. He started out a bit out of sync with the PSO and his tempo dragged, but his golden-hued timbre was mesmerizing and he got his legs under him in the second movement. I prefer Mozart with more punch and texture, but his smooth delivery was something to behold.

The program repeats at 2:30 p.m. tomorrow.

Critics Andrew Druckenbrod and Scott Mervis talk about music on "The Beat," available exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.