Phloem sap may contain nitrate
In fact, there are noticeable exceptions where nitrate is not absent from phloem sap composition (Fig. 1a-f). 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). The above-cited studies used very different techniques (aphid-stylet and pure phloem sap exudation) and therefore a systematic contamination is unlikely. In effect, nitrate can be found in phloem sap using the aphid stylet method (i.e. involving stylectomy), which gives access to very high purity phloem sap (van Helden, Tjallingh, & van Beek, 1994).