Perspectives for N cycling in plants
While there are 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. Pioneering N mass balance in castor bean proposed that nearly 50% of nitrate translocated to the shoot cycled back to roots (Marschner et al. , 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 et al. , 2020). Although quantitatively minor, this flux is important to explain the natural15N-enrichment in root nitrate. It is also possible that variations in nitrate concentration, in addition to organic N, contribute to the diel pattern of δ15N of phloem in castor bean (Peuke et al. , 2013). The δ15N value of aphids (feeding on phloem sap) has been shown to be lower (depleted) compared to host plants and related to nitrate reduction capacity, suggesting that the aphid-host isotopic difference is partly explained by the 15N-enrichment in phloem nitrate – as opposed to the 15N-depletion in phloem amino acids (Wilson et al. , 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). Also, phloem sap nitrate increases when nitrate availability increases and declines with salinity (Peuke et al. , 1996). Surprisingly, 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 (Tillardet al. , 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. Future studies are warranted to provide more data and therefore to appreciate the generality and significance of the transport of nitrate in the phloem.