PORTLAND — Maine Medical Center is leaning on the power of plants to grow a crop the state could always use more of — doctors. A new preventive medicine residency program at the hospital hopes to build up the number of doctors in the state who are dedicated to helping Mainers avoid disease through practices like healthy eating, lifestyle modification and early detection of health problems. It’s the only one of about 70 preventive medicine residencies in the country to put a special focus on “plant-based medicine,” said Paul Bonta, associate director of the American College of Preventive Medicine. The plant-based portion of the program is focused on health and nutrition.