Metabolism of ¹⁴C-aspartate during shoot bud formation in cultured cotyledon explants of radiata pine

Aspartate metabolism was investigated in excised cotyledons of radiata pine (Pinus radiata D. Don). These cotyledons were cultured under shoot-forming (plus N⁶-benzyladenine, SF), non-shoot-forming (minus N⁶-benzyladenine, NSF) and unresponsive (plus N⁶-benzyladenine, OLD) conditions, then incubated with [¹⁴C]-aspartate for 3-h pulse treatments followed by 3-h chase treatments with cold aspartate. The majority of label was recovered in the CO₂, amino acid, organic acid and pellet fractions. Uptake was greatest in all tissue types early in culture. Most (over 80%) of the [¹⁴C]-aspartate taken up by the tissues was converted to CO₂ at day 0 in SF and NSF tissues, CO₂ accounted for less than 50% of the total radioactivity in other tissues. Greater incorporation into fractions was observed in SF tissues during promeristemoid formation, while in NSF tissues the greatest incorporation was observed during a period of rapid elongation. Generally, less incorporation was observed in OLD cotyledons than in SF and NSF cotyledons. Analysis of the amino acid fraction showed that labelled aspartate was converted to other amino acids, mainly glutamate, glutamine, asparagine and 4-aminobutyric acid.