Nitrate cycling in plants
While there might be good reasons to explain why nitrate concentration must remain low in phloem sap (signaling and chaotropism), the occasional presence of nitrate raises the question of its potential role in nutrient cycling. Incubation with a nitrate-containing buffer (10 mM nitrate) of castor bean cotyledons (the endosperm being removed) have shown that nitrate can be found (at about 1 mM) in phloem sap exudated from cut hypocotyls, suggesting that nitrate cycling via the phloem is effectively possible (Schobert & Komor, 1992). Furthermore, pioneering N mass balance in castor bean proposed that nearly 50% of nitrate translocated to the shoot cycled back to roots (Marschner, Kirkby, & Engels, 1997). While this number is certainly overestimated, recent isotopic data (15N natural abundance, δ15N) also suggest that a small flux of a few percent of xylem translocation to shoots can cycle back to roots, in both sunflower and oil palm (Cui, Lamade, Fourel, & Tcherkez, 2020). Although quantitatively minor, this flux is important to explain the natural, considerable 15N-enrichment in root nitrate: in fact, nitrate reduction in shoots discriminates between N isotopes in favor of 14N and thus 15N-nitrate left behind is redistributed to roots via the phloem. Also, since nitrate redistributed to the phloem is naturally15N-enriched, it is possible that variations in phloem sap nitrate concentration along the day contribute to explaining the diel pattern of δ15N of phloem in castor bean (Peuke, Gessler, & Tcherkez, 2013). Interestingly, the δ15N value of aphids (feeding on phloem sap) has been shown to be lower (15N-depleted) compared to host plants and related to nitrate reduction capacity, suggesting that the aphid-host isotopic difference is partly explained by the natural15N-enrichment in phloem nitrate, as opposed to the15N-depletion in phloem amino acids (Wilson, Sternberg, & Hurley, 2011).
The backflow of nitrate from shoots to roots depends on growth conditions impacting on overall nutrition, since (Cui et al., 2020) showed it depends on K nutrition and root hypoxia (waterlogging). The interaction between K nutrition and nitrate absorption has been known for decades and has been mostly explained by the key role of K+ as a counteraction for nitrate (Casadesús, Tapia, & Lambers, 1995; Coskun, Britto, & Kronzucker, 2017; Wang & Wu, 2013). Accordingly, the nitrate transporter NPF7.3/NRT1.5 has been shown to act as an antiporter with K+/H+(Li et al., 2017). Nevertheless, there is no significant relationship between nitrate and K+ concentrations in phloem (Peuke, 2010) and this might not be surprising because K+ is present at very high concentration in phloem. Interestingly, phloem sap nitrate increases when nitrate availability increases and declines with salinity (Peuke, Glaab, Kaiser, & Jeschke, 1996). Meta-analyses have shown that phloem nitrate concentration does not correlate significantly to other cations and only correlates with xylem, leaf and root nitrate content (Peuke, 2010). However, the nitrate flow in the phloem (expressed in µmol nitrate g-1 FW d-1) correlates reasonably well with phloem carbon and Ca2+ flows (Peuke, 2010). The nitrate backflow thus depends on other nutrients and salinity and is maybe linked to metabolites (organic acids and amino acids) present in phloem sap. The supply of amino acids to roots via the phloem participates in the control of root N acquisition (for a specific discussion, see (Tillard, Passama, & Gojon, 1998)) and as discussed above, nitrate also plays a regulatory role. Thus, more than individual concentrations, the nitrate-to-amino acid ratio of phloem sap might be a crucial component of plant development, root growth and nitrogen assimilation. In the past years, there have been an increasing number of publications on phloem (including proteomics data) but due to the difficulty of phloem sap collection, there is limited information on phloem composition under varying conditions, including metabolite profiling (metabolomics), nitrate content and δ15N value.