A horticulturalist is a person whose work is horticulture: growing or studying flowers, fruit, and vegetables. You might meet one at a garden shop, a park, a plant lab, or a university. The word names the person, not the science; the science is horticulture. Though actually you will see two spellings. Horticulturist is the more common one; horticulturalist means the same job. Both are rather formal. In casual talk people often just say gardener, though a horticulturalist may have more training than that everyday word suggests.