Open questions for future research
Taken as a whole, key questions that are still unanswered and deserve more research can be summarized as follows:
  1. What is the rationale of phloem sap nitrate variation between species? As stated above, the range of concentration is very large (from non-detectability to nearly 10 mM). Uncertainty remains as to whether this is related to N allocation, efficiency of nitrate remobilization from leaves to developing organs, or different strategies in N status signaling.
  2. Does phloem nitrate contribute effectively to nitrate signaling and growth regulation? For example, is the response to nitrate modified when phloem nitrate transport is compromised? Is the xylem-to-phloem ratio of nitrate concentration or phloem nitrate-to-amino acid ratio indicators of N status?
  3. Do the relationships between nutrients, xylem nitrate and phloem nitrate still hold in species other that castor bean? Up to now, there is no such extensive dataset like that obtained in this species.
  4. Is low nitrate concentration an essential factor to avoid protein aggregation in phloem sap? Can experiments be designed where nitrate is trapped or added to manipulate protein aggregation propensity?
Future studies are warranted to provide more insight on these aspects and thus to clarify the roles of nitrate found in phloem sap and also demonstrate why it is kept at a very low concentration in many cases.