U.S. markets were closed on Monday in recognition of the Martin Luther King Jr. Day holiday, but the announcement that CEO Steve Jobs was taking another medical leave of absence (with Chief Operating Officer Tim Cook again overseeing day-to-day operations at Apple), the company’s stock was down nearly 10 % in Frankfurt, Germany, for European trading. It closed the day down 6.2 percent, to finish at €244.05.

U.S. markets, including the NASDAQ, where AAPL is traded, will open at their regular time of 9:30 a.m. eastern on Tuesday. In a note to clients — as reported by “AppleInsider” (http://www.appleinsider.com), analyst Mike Abramsky said he expects Apple stock to be volatile in the near future, but also buoyed by the strong expected quarter for the start of fiscal 2011.

He noted that Apple and Jobs’ decision not to reveal the CEO’s condition could “sustain uncertainty.” However, the analyst noted that investors may also focus on the fact that Jobs has twice already successfully returned from health absences. And Abramsky believes Apple has a strong management team with Cook overseeing things.

Analyst Gene Munster told clients (again, as reported by “AppleInsider”) that, unlike Jobs’ departure in 2009, he announced on Monday that he is not relinquishing his title as chief executive officer. That could be an indication, he said, that Jobs expects his leave of absence to be shorter and less serious than his previous departure.

“Recall that during Jobs’ last medical leave (January 2009 to June 2009) Cook became interim CEO of Apple and the business continued to perform well,” Munster wrote.

However, “Steve is more important to his company’s success than most other CEOs are to their company’s success,” analyst Roger Kay, president of Endpoint Technologies Associates, told “Infoworld” (http://macte.ch/8MvKi). And Jobs, who survived a cancerous pancreatic tumor in 2004 and had a successful liver transplant in 2009, is key to Apple, independent analyst Jeff Kagan told the publication.

“Without him at the helm, we just don’t know whether the company can continue on its successful course,” Kagan said. “At this point we have so many questions and no one has any real answers. It’s all uncharted territory for now.”

Jack Gold, president of J. Gold Associates, told “Infoworld” that Apple’s product pipeline is set for the foreseeable future, with refreshes of the iPhone and iPad lines coming, and that Cook did a fine job during Jobs’ previous medical absence. What’s more, Jobs has built up a solid foundation for the companyand “Apple is running pretty well,” he said.

However, the analyst says a long-term absence would make things more challenging, because Jobs brings a vision to product strategy and technology development that would be hard or impossible to replicate, Kay said. The effects of a prolonged absence would begin to be felt in about two years.

“Once they’re through product pipeline and facing competitive threats they haven’t seen yet, that creates a new environment,” Kay told “Infoworld.”

“Fortune” Editor-at-Large Adam Lashinsky says (http://macte.ch/terij) if Jobs returns to work this will be merely another blip for Apple. “Assuming the worst, however, the company will glide along for some period of time, and the world will learn just how talented Apple’s senior management team is,” he adds. “It’s a cohesive group that has been together for years and already responds well to Cook’s leadership.”

Gleacher & Co. analyst Brian Marshall told clients — as noted by “The Mac Observer” (http://www.macobserver.co) — that a US$300 support level exists for Apple in the wake of concern over Jobs’ leave of absence. With Friday’s closing price of $348.48, that would represent a 15% hit should the stock dip to those levels.

— Dennis Sellers