Source-sink relationships for carbon and nitrogen during early growth of Juglans regia L. seedlings:: analysis at two elevated CO2 concentrations

Assimilation and allocation of carbon (C) and nitrogen (N) were studied in seedlings (Jugalan regia L.) grown for 55 days under controlled conditions (22 degrees C, 12 h, 90 % relative humidity [RH]) using two CO, concentrations (550 and 800 mu L L-1 CO,). C and N decrease in seeds was unaltered by CO2. At the end of seed contribution (day 35), C and N accumulation in seedlings was favoured under 800 mu L L-1 [CO2], resulting in an increase of about +50 % for C and +35 % for N. Growth enhancement was larger in roots than in shoot, resulting in a higher root:shoot ratio (R:S = 0.62) with respect to 550 mu L L-1 CO, (R:S = 0.40) at day 55. These results were due, in order, to: 1) a shoot respiration temporarily depressed by [CO2], 2) a reduction by 46 % of the root + soil respiration, 3) a stimulation by 14 % of the C assimilation and 1) an increased uptake and assimilation of N coming from the rooting medium. An increased use of N originated from the seed was observed in leaves and lateral roots, suggesting optimisation of distribution of stored N pools by seedlings. These changes finally gave rise to an increased C:N ratio for taproot (+27 %), roots (+20 %), stem (+28 %), and leaves (+12 %), suggesting a N dilution in the tissues. ((C) Inra/Elsevier, Paris.).