When there is no transpiration, at night or in a decapitated plant the xylem can be at lower water potential than the soil solution because the endodermis pumps ions into the xylem. This can cause osmotic water uptake and a positive pressure from the roots, leading to guttation, the exudation of droplets from the leaves. Inorganic ions do not simply flow with the water, passing into plant roots. They are actively accumulated by epidermal cells, passed through cortical cells and finally secreted by the endodermis cells into the xylem stream. This is the route for all inorganic ions.
Plant metabolism is adapted to use the minimum amount of nitrogen.
The bulk of the dry weight of the plant is carbohydrate rather than protein. However, nitrogen is the nutrient that is most often limiting for plant growth. Some plants have developed ways of overcoming this problem. As we have seen the Fabaceae in general and a few other plants have developed symbiotic relationships with nitrogen fixing bacteria . Others acquire nitrogen by capturing insects in specially modified leaves.