PHLOEM TRANSPORT OF SOLUTES IN CROP PLANTS 455
Figure 5 Chemical structures of (A) phloem-mobile sugars, (B) sugar alcohols, and (C) nitrogenous com-
pounds found in phloem saps.
glutamine [2]. The amino acid composition of the phloem sap can vary greatly depending on the species
(Table 2) or environmental conditions [34]. In addition to amides, some species, particularly the nitrogen-
fixing legumes, transport small quantities of the ureides allantoin and allatonic acid in the phloem [30].
Nonprotein amino acids, such as canavanine and ornithine, other ureides such as citrulline, and
polyamines such as putrescine may also be found in limited quantities in phloem saps of many plant