Modeling C and N transport to developing soybean fruits.

Xylem sap and phloem exudates from detached leaves and fruit tips were collected and analyzed during early pod-fill in nodulated soybeans (Glycine max (L.) Merr. cv Wilkin) grown without (-N) and with (+N) NH(4)NO(3). Ureides were the predominant from (91%) of N transported in the xylem of -N plants, while amides (45%) and nitrate (23%) accounted for most of the N in the xylem of +N plants. Amino acids (44%) and ureides (36%) were the major N forms exported in phloem from leaves in -N plants, but amides (63%) were most important in +N plants. Based on the composition of fruit tip phloem, ureides (55% and 33%) and amides (26% and 47%) accounted for the majority of N imported by fruits of -N and +N plants, respectively.C:N weight ratios were lowest in xylem exudate (1.37 and 1.32), highest in petiole phloem (24.5 and 26.0), and intermediate in fruit tip exudate (12.6 and 12.1) for the -N and +N treatments, respectively. The ratios were combined with data on fruit growth and respiration to construct a model of C and N transport to developing fruits. The model indicates xylem to phloem transfer provides 35% to 52% of fruit N. Results suggest the phloem entering fruits oversupplies their N requirement so that 13% of the N imported is exported from fruit in the xylem.