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.