Why is phloem sap nitrate kept low?
Résumé
It has now been 61 years since phloem sap composition has first been described, using sap extracted from willow (Peel & Weatherley, 1959). In that study, using colorimetric methods, nitrate was found to be undetectable. Several subsequent studies also reported the absence of nitrate in other species such as castor bean (Ricinus communis) (Hall & Baker, 1972) and overviews of phloem sap composition
established the generally very low, or undetectable, concentration in phloem sap nitrate (Ziegler, 1975).