There’s an opening at Quarterback U.

Pete Carroll preaches that USC football is all about competition.

But when it comes to quarterbacks, the reality has mostly been succession.

Matt Leinart passed the torch, as expected, to John David Booty. Mark Sanchez took over, to the surprise of few, from Booty.

However, Sanchez’s early departure for the NFL broke the line.

For the first time in six years, there is no obvious heir apparent.

That’s why four players vying to become the new Trojans starter seem energized about the start of spring practice Saturday.

“It’s a more level playing field,” junior Mitch Mustain said.

Mustain, Aaron Corp, Matt Barkley and Garrett Green recognized a tectonic shift in January, the moment Sanchez announced during a campus news conference that he was turning pro.

They barely noticed Carroll’s nationally televised conniption. And with good reason: They were too busy celebrating their sudden good fortune.

“If ever there was a time where there’s a true competition, this would be it,” Green said later.

Not since 2003, when Leinart edged Matt Cassel in a spring battle to succeed Carson Palmer, has there been a real duel for what has become one of the most glamorous positions in college football.

Booty waited three years before taking the reins from Leinart. Sanchez bided his time until Booty exhausted his eligibility.

But Sanchez’s exit after only one season as a starter, plus the recent arrival of new quarterbacks coach and play-caller Jeremy Bates, has left this year’s competition wide open. “It doesn’t get dull,” Carroll said, chuckling. “It will be interesting and fun to watch Jeremy and the quarterbacks work their way through it.”

Carroll and Bates maintain that each quarterback will receive equal opportunity.

“All four of them have a chance . . . so it’s just a matter of who takes it over,” said Bates, who helped groom Jay Cutler into a Pro Bowl player for the Denver Broncos.

Bates, 32, is looking for leadership, command of the offense and huddle, and an ability to take care of the ball and avoid turnovers. Each quarterback, he said, would get chances to work with the first unit.

USC will practice 15 times over the next month, concluding with an April 25 scrimmage at the Coliseum.

A starter could be determined at any time.

“I don’t like going into the summer not knowing,” Carroll said.

Knowledge of the offense and exceptional mobility might give the fleet-footed Corp a slight edge.

The former Orange Lutheran High star began last season No. 2 on the depth chart but tumbled after a poor practice performance during an early-season off week and did not regain the backup position until the Trojans were preparing for the Rose Bowl.

A third-year sophomore, Corp has added 10 pounds of muscle to his 6-foot-4 frame, increasing his weight to 205 pounds.

He also stepped up as a leader, organizing off-season throwing sessions.

“He’s got the advantage as probably the most fluent guy with everything,” Carroll said.

That said, the 6-3, 210-pound Mustain has the most impressive college resume, having gone 8-0 as a freshman starter at Arkansas in 2006.

“Mitch has an advantage in that he’s played -- he’s not uncomfortable being out there,” Carroll said.

Barkley is the wild card.

The 6-foot-3, 220-pound freshman started for four seasons at powerful Santa Ana Mater Dei High. Had Sanchez returned to USC for a final season of eligibility, Barkley almost certainly would have redshirted.

With his strong arm and uncommon maturity, Barkley is expected to shine early in spring workouts. His challenge will come in the latter weeks when more of the offense is installed and information overload can derail a young quarterback.

“Matt is going to get a great look,” Carroll said. “He can just go for it like a sixth man coming off the bench.”

Green, a 6-2, 210-pound senior, will get a long-awaited opportunity. But after switching positions several times, he could be victimized by his versatility.

“He’s earned a chance,” Carroll said. “But he’s still a valuable utility guy.”

Carroll declined to predict who would be the starter when the Trojans play San Jose State in the Sept. 5 opener at the Coliseum.

“We’ll just see who functions the cleanest, who looks the most comfortable in the offense,” Carroll said. “And if there is someone who emerges, we’ll see how far we can go.”

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gary.klein@latimes.com

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Four for the show

There are four main candidates to become USC’s starting quarterback. Here’s what each must do to win the job:

Matt Barkley, freshman, 6-3, 220 -- Ready physically, he must show he can handle the mental demands of a complex, pro-style offense. And do it in 15 workouts.

Aaron Corp, sophomore, 6-4, 205 -- Knows the system, but needs to demonstrate he can run it making good decisions. Showcasing his ability to make plays with his feet could help.

Garrett Green, senior, 6-2, 210 -- Needs a strong start to offset coaches’ desire to use him at other spots. Maturity and three years of practice experience is in his favor.

Mitch Mustain, junior, 6-3, 210 -- Has the most game experience -- a season starting for Arkansas. Must play without the hesitation that hindered his performance last season.

-- Gary Klein