ACQUISITION AND WITHIN-PLANT ALLOCATION OF C-13 AND N-15 IN CO2-ENRICHED QUERCUS-ROBUR PLANTS

We assessed the effects of doubling atmospheric CO2 concentration, [CO 2], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual (CO2)-C-13 and (NO3-)-N-15 labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 mu l l(-1) [CO2] stimulated plant growth and biomass accumulati on (+53% as compared with the 350 mu l l(-1) [CO2] treatment) but decr eased the root/shoot biomass ratio (-23%) and specific leaf area (-18% ). Moreover, there was an increase in net CO2 assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO2 respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO2]. C-13 acquisit ion, expressed on a plant mass basis or on a plant leaf area basis, wa s also markedly stimulated under elevated [CO2] both after the 12-h (C O2)-C-13 pulse phase and after the 60-h chase phase. Plant N content w as increased under elevated CO2 (+36%), but not enough to compensate f or the increase in plant C content (+53%). Thus, the plant C/N ratio w as increased (+13%) and plant N concentration was decreased (-11%). Th ere was no effect of elevated [CO2] on fine root-specific N-15 uptake (amount of recently assimilated N-15 per unit fine root dry mass), sug gesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the l eaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO2] in the stem and in the actively gr owing organs (fine roots and leaves of the third growth flush). Furthe rmore, leaf N content per unit area was unaffected by [CO2]. These res ults are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storag e compartments (coarse roots, leaves of the first and second growth fl ushes). After the 60-h C-13 chase phase, leaves of the first and secon d growth flushes were almost completely depleted in recent C-13 under ambient [CO2], whereas these leaves retained important amounts of rece ntly assimilated C-13 (carbohydrate reserves?) under elevated [CO2].