Since the first description of phloem sap composition nearly 60 years
ago, it is generally assumed that phloem sap does not contain nitrate
and that there is little or no backflow of nitrate from shoots to roots.
While it is true that nitrate can occasionally be absent from phloem
sap, there is now substantial evidence that phloem can carry nitrate and
furthermore, transporters involved in nitrate redistribution to shoot
sink organs and roots have been found. This raises the question of why
nitrate may or may not be present in phloem sap, why its concentration
is generally kept low, and whether plant shoot-root nutrient cycling
also involves nitrate. We propose here that phloem sap nitrate is not
only an essential component of plant nutritional signaling but also
contributes to physical properties of phloem sap and as such, its
concentration is controlled to ensure proper coordination of plant
development and nutrient transport.