TOKYO - JAPANESE Emperor Akihito commemorated the 20th anniversary of his reign on Wednesday, but with no notable plans other than a trip to his father's grave.

The anniversary comes as Emperor Akihito, 75, has cut back his official duties due to an irregular pulse and bleeding from his stomach - symptoms imperial doctors have attributed to 'mental stress.' He has also undergone surgery and treatment for cancer.

The imperial couple had no special plans to mark Wednesday's anniversary other than their annual visit to his father's grave, palace officials said. Emperor Akihito ascended the Chrysanthemum Throne after his father, Hirohito, died on Jan 7, 1989.

On his birthday last month, Emperor Akihito assured the nation his health was improving.

Though no longer revered as living gods, members of the imperial family live tightly regimented lives and have been known to suffer from stress-related illnesses.

Emperor Akihito's 45-year-old daughter-in-law, Crown Princess Masako, withdrew from most of her official duties and appearances several years ago because of a nervous disorder attributed to the difficulties she felt adjusting to palace life and the pressure to bear a son to continue the imperial line.

Emperor Akihito's wife, Empress Michiko, 74, has also often complained of stress and suffered a breakdown in the early 1990s that rendered her unable to speak for months.

Succession concerns are also believed to be a major source of worry for the royal family.

Under a post-World War II law, only males are allowed to assume the throne. Crown Prince Naruhito, Princess Masako's husband and the eldest son of Akihito, is next in line, but they have no sons of their own.

When Princess Masako and Prince Naruhito had a daughter, a movement heated up to change the law to allow the girl to succeed her father.

That died down when Prince Naruhito's younger brother, Prince Akishino, had a baby boy two years ago. -- AP