Open questions for future research
Taken as a whole, key questions that are still unanswered and deserve
more research can be summarized as follows:
- 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.
- 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?
- 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.
- 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.