The lemon is a species of small evergreen tree native to South Asia, primarily Northeast India. Citric acid may amount to 5 percent or more by weight of the lemon’s juice, which is also rich in vitamin C and contains smaller amounts of the B vitamins, particularly riboflavin, and niacin.The distinctive sour taste of lemon juice makes it a key ingredient in drinks and foods such as lemonade and lemon meringue pie.