A short story about a group of rural people who start to develop the ability to photosynthesize. They start to grow tendrils which reach into the dirt, and create energy from the sun.
Your story description has similarities to "Whatever Happened to the McGowans?", published in the May 1970 Galaxy. The protagonist is living on a rural planet with his wife and son, on a homestead left behind by the McGowans family of the title. The protagonist is dreading the impending visit of his in-laws (whom he finds annoying).
First the little boy, and then his parents, start developing an affliction: they experience extreme lassitude, and the only time they feel better is in the sun. In the end, it's only the arrival and intervention of the in-laws that prevents them from transforming fully and permanently into vegetation. The protagonist realizes that the McGowans never left their homestead; instead they're still on it, in the form of large plants.
There are some similarities to "Piper in the Woods" by Philip K. Dick, enough for it to possibly be the story you're thinking of. People in that story come to believe that they are plants. They bask in the sun, and at night they sleep on dirt. However it is unlikely that they actually photosynthesize, they only believe that they do. Also they are soldiers stationed on an asteroid, not rural people. http://www.gutenberg.org/ebooks/32832