EFFECT OF ELEVATED CO2 ON CARBON PARTITIONING AND EXUDATE RELEASE FROM PLANTAGO-LANCEOLATA SEEDLINGS

Plantago lanceolata L. seedlings were grown in sand microcosm units ov er a 43-day experimental period under two CO2 regimes (800 or 400 mu m ol mol(-1)) to investigate the effect of elevated atmospheric CO2 conc entration on carbon partitioning and exudate release. Total organic ca rbon (TOC) content of the collected exudate material was measured thro ughout the experimental period. After 42 days growth the seedlings wer e labelled with [C-14]-CO2 and the fate of the label within the plant and its release by the roots monitored. Elevated CO2 significantly (P less than or equal to 0.001) enhanced shoot, root and total dry matter production although the R:S ratio was unaltered, suggesting no altera tion in press carbon partitioning. The cumulative release of TOC (in m g C) over 0-42 days was unaltered by CO2 treatment however, when expre ssed as a percentage of net assimilated C, ambient-grown plants releas ed a significantly (P less than or equal to 0.001) higher percentage f rom their roots compared to elevated CO2-grown plants (i.e. 8 vs 3%). The distribution of C-14-label was markedly altered by CO2 treatment w ith significantly (P less than or equal to 0.001) greater per cent lab el partitioned to the roots under elevated CO2. This indicates increas ed partitioning of recent assimilate belowground under elevated CO2 tr eatment although there was no significant difference in the percentage of C-14-label released by the roots. Comparison of plant C budgets ba sed on C-14-pulse-chase methodology and TOC measurements is discussed.