Phloem sap may contain nitrate
In fact, there are noticeable exceptions where nitrate is not absent
from phloem sap composition (Fig. 1). It is the case of cereals (wheat,
rice), with phloem sap nitrate of up to 8 mM (Hayashi & Chino, 1985;
Hayashi & Chino, 1986). Also, in palm trees, nitrate has been found in
phloem exudates for either trunk and inflorescence peduncles (van Die &
Tammes, 1975). Extensive analysis of castor bean phloem sap has shown
that average nitrate phloem concentration is 0.59 mM, ranging from no
detectability to 2.4 mM (Peuke, 2010). This is of course much less than
in xylem, which contains up to 25 mM nitrate. Interestingly, nitrate has
been found to be much more abundant in phloem (0.29 mM) than xylem sap
(3 µM) in Western candle tree (Banksia prionotes ) (Jeschke &
Pate, 1995). These findings cannot be explained by the technique of sap
collection (i.e. potential adulteration by other tissues upon sampling)
since the above-cited studies used very different techniques
(aphid-stylet and pure phloem sap bleeding) and therefore a systematic
contamination is not a plausible explanation.