SHARES IN APPLE have taken a tumble in after-hours trading after the resignation of co-founder and CEO Steve Jobs was announced.

Jobs, who has been on a medical leave of absence since January, tendered his resignation to the board on Wednesday.

Shares in the tech giant have fallen sharply on the announcement. In after-hours trading, stock traded at US$357.10 – down more than 5 per cent on yesterday’s closing price. At one point during the evening, shares fell more than 6 per cent, highlighting how important the markets believe the 56-year-old is to the company.

Investors had been spooked by the apparent deterioration of Jobs’ health back in January when he announced his leave of absence. However, they had come back on board after seeing “phenomenal” financial results. Driven by the continued success of the iPad and iPhone, revenues climbed 88 per cent in the company’s third quarter.

Before Jobs’ resignation was revealed, Apple shares had closed the day up 0.7 per cent. According to MarketWatch, the company’s shares have risen about 57 per cent in the past year.

Last week, Apple briefly became the most valuable company in the world, surpassing petrol firm Exxon Mobil.

The board held off announcing Jobs’ resignation until after the markets had closed in New York.

This is not the first time investors have reacted to news about the Jobs’ health. He is seen as the kingpin of Apple after turning its fortunes around when he rejoined the firm in 1996 and many link its success with his presence.

In 2008, a false story circulated that Jobs had suffered a heart attack and Apple shares dropped to a 17-month low, according to The Register.

Questions about Jobs’ health have influenced share price at various times since 2004 when he was originally treated for pancreatic cancer.

In January 2009, he took another leave of absence because of what he called a “hormone imbalance”. However, he returned to work six months later.

Jobs will be succeeded by current chief operating officer of Apple, Tim Cook. The momentous change will have Wall Street abuzz today, with much speculation about what could happen when the bell rings this morning.

The 56-year-old’s letter to the board and ‘Apple community’ tried to make it clear that there has been a succession plan in place.

He wrote: “As far as my successor goes, I strongly recommend that we execute our succession plan and name Tim Cook as CEO of Apple.”

Read: Steve Jobs resigns as CEO of Apple with immediate effect >