Allocation of reserve-derived and currently assimilated carbon and nitrogen in seedlings of Helianthus annuus under subambient and elevated CO2 growth conditions

Here, we analysed the transition from heterotrophic to autotrophic growth of the epigeal species sunflower (Helianthus annuus), and how transition is affected by CO2. Growth analysis and steady-state (CO2)-C-13/(CO2)-C-12 and (NO3-)-N-15/(NO3-)-N-14 labelling were used to quantify reserve- and current assimilation-derived carbon (C) and nitrogen (N) allocation to shoots and roots in the presence of 200 and 1000 mu mol CO2 mol(-1) air. Growth was not influenced by CO2 until cotyledons unfolded. Then, C accumulation at elevated CO2 increased to a rate 2-2.5 times higher than in subambient CO2 due to increased unit leaf rate (+ 120%) and leaf expansion (+ 60%). CO2 had no effect on mobilization and allocation of reserve-derived C and N, even during the transition period. Export of autotrophic C from cotyledons began immediately following the onset of photosynthetic activity, serving roots and shoots near-simultaneously. Allocation of autotrophic C to shoots was increased at subambient CO2. The synchrony in transition from heterotrophic to autotrophic supply for different sinks in sunflower contrasts with the sequential transition reported for species with hypogeal germination.