If you shoot a tall stalk of beans with an arrow, the bean plant will go from tall to short. When the beans are hit with that arrow, it severs the plant in two, and you are left with a stalk of beans that is much shorter than before.
All this shooting of arrows to make the bean plants more short is taking a long time. Since you’re out in the sun, you’re starting to get a tan (たん). After the day is finally over, your tan is really, really dark.