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 et al. , 1975). There are more recent reports where
nitrate was found to be undetectable, for example in maize (Lohauset 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 et al. , 2015). However, many pieces
of recent evidence suggest the contrary and provide possible reasons
explaining why phloem nitrate concentration is usually low.