Plants like wheat, tomato, and lucerne can use nitrate and ammonium for food, but they react differently to ammonium. Wheat is very sensitive, tomato is somewhat sensitive, and lucerne is tolerant to ammonium. The more ammonium in the leaves, the less the plants grow. For tomato and lucerne, roots are better at using ammonium than leaves. This shows that where plants process ammonium affects how well they handle it.