NITROGEN ASSIMILATION AND TRANSPORT IN VASCULAR LAND PLANTS IN RELATION TO INTRACELLULAR pH REGULATION

The chemical composition of exudate obtained from incisions made in the bark of the stem of actively growing Ricinus plants has been determined. The exudate had a high dry matter content (100-125 mg/ml), a high sugar content (80-106 mg/ml) which was solely sucrose, reducing sugars being absent. The amino acid composition was mainly glutamic and aspartic acids and threonine with a total amino acid concentration of 35.2 mM. The exudate had a pH of 8.0-8.2. Potassium was the major cation (60-112 mM) with sodium present at a lower concentration (2-12 mM). Of the divalent cations, calcium was at a low concentration (0.5-2.3 mM) and magnesium relatively higher (4.5-5.4 mM). Chloride was the major inorganic anion (10-19 mM). Phosphate concentration was relatively high (3.7-5.7 mM) and low concentrations of sulphate (0.3-0.5 mM) and bicarbonate (1.7 mM) were also present. Nitrate was absent. The ionic balance was maintained by the presence of relatively large quantities (30-47 meq/l) of organic anions, mainly malate. Bioassays revealed auxin, gibberellin and cytokinin activities in chromatographed exudate. Adenosine triphosphate was found in the exudate (0.40-0.60 mM). The analysis is dicussed with respect to the composition of phloem sap reported for other plant species.