A Prince George's County Circuit Court jury began deliberations yesterday in the emotional trial of Derrick O. Holmes, 29, a former contender for the world super bantamweight boxing title, who is charged with robbing and attempting to murder a Christmas tree salesman on a Clinton lot last December.

Holmes, who has denied that he shot Roswell C. Howard, 40, of Owensboro, Ky., testified yesterday that he was with his girlfriend at the time of the shooting. The former girlfriend backed up his testimony in court.

Howard, who came to Maryland to sell Christmas trees for a friend after being laid off a job in Kentucky, testified during the trial that Holmes had worked at the Christmas tree lot the weekend before the shooting and he and Holmes "became good friends." The jury was shown pictures of Howard and Holmes clowning around on the Christmas tree lot.

Howard said that on Dec. 20, at about 10:50 p.m., Holmes and another unidentified man came to the van where Howard slept on the Christmas tree lot, robbed and then shot him four times with a gun Howard kept in the van for security.

"I begged him not to shoot me. I was crying and saying 'Oh, Derrick, please don't shoot me,' and that's when he shot me in the side," Howard said, quietly crying as he spoke.

Holmes, of 7900 Kreeger Rd., Adelphi, denied the allegations, testifying that he spent the night with his then-girlfriend, Nancy Lee, 24.

Lee corroborated Holmes' account, saying that she met Holmes that night at a New Carrollton restaurant at 9:45 p.m., and was with him until 7:30 the next morning.

Mary Lee also testified that Nancy Lee was home about 10:45, and that she heard Holmes downstairs.

Holmes, also crying during his testimony, said, "I did not shoot Howard , I thought he was a nice guy until I heard this. I don't understand why he would involve me in this."

In August 1980, Holmes fought super bantamweight champion Wilfredo Gomez for the world title, but was knocked out in the fifth round.

The jury will reconvene today.