Introduction
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). Furthermore, experiments with15N-nitrate labelling in legumes have shown that nitrate could not be transported from xylem to phloem at detectable levels (Pate, Sharkey, & Lewis, 1975). There are more recent reports where nitrate was found to be undetectable, for example in maize (Lohaus et al., 2000). It is thus widely accepted that phloem sap nitrate is of negligible importance and in particular, that nitrate circulation from shoots to roots does not occur. For example, in a recent review, the backflow of nitrate via the phloem to regulate root development is not mentioned (Fig. 3 in (Tegeder & Masclaux-Daubresse, 2018)). Also, in textbooks and university lectures, it is often reported that nitrate is absent from phloem sap and thus does not flow back from shoots to roots, see for example (Taiz, Zeiger, Moller, & Murphy, 2015). However, many pieces of recent evidence suggest the contrary and provide possible reasons explaining why phloem nitrate concentration is usually low. They are presented in this Opinion.