Measurements of amino acids in the guttation fluid and in the xylem exudates of cut leaves from intact plants provide
evidence of the remarkable efficiency with which these nitrogenous compounds are reabsorbed from the xylem sap.
This could be achieved by mechanisms involving intercellular transport and/or metabolism. Developmental changes
in transcripts and protein showed that transcripts for phosphoenolpyruvate carboxykinase (PEPCK) increased from
the base to the leaf tip, and were markedly increased by supplying asparagine. Supplying amino acids also increased
the amounts of protein of PEPCK and, to a lesser extent, of pyruvate, Pi dikinase. PEPCK is present in the hydathodes,
stomata and vascular parenchyma of rice leaves. Evidence for the role of PEPCK was obtained by using 3-mercaptopicolinic
acid (MPA), a specific inhibitor of PEPCK, and by using an activation-tagged rice line that had an increase
in PEPCK activity, to show that activation of PEPCK resulted in a decrease in N in the guttation fluid and that treatment
by MPA resulted in an increase in amino acids in the guttation fluid and xylem sap towards the leaf tip. Furthermore,
increasing PEPCK activity decreased the amount of guttation fluid, whereas decreasing PEPCK activity increased the
amount of xylem sap or guttation fluid towards the leaf tip. The findings suggest the following hypotheses: (i) both
metabolism and transport are involved in xylem recycling and (ii) excess N is the signal involved in modulating xylem
hydraulics, perhaps via nutrient regulation of water-transporting aquaporins. Water relations and vascular metabolism
and transport are thus intimately linked
