Growth and maintenance respiration for individual plants in hierarchically structured canopies ofMedicago sativaandHelianthus annuus: the contribution of current and old assimilates.

• Respiratory costs ofMedicago sativaandHelianthus annuusindividuals growing in hierarchically structured stands in a controlled environment were analysed with regard to the daily rate of carbon (C) assimilation.• Net assimilation of new C (A_n, g C d⁻¹) and respiration rates of new (R_new, g C d⁻¹) and old C (R_old, g C d⁻¹) were assessed by¹³CO₂ labelling and gas exchange measurements.• Specific respiration rate of old C (r_old, g C g⁻¹ C d⁻¹) decreased exponentially with increasing shoot biomass, but was not affected by the instantaneous relative growth rate (Δ w_i). The growth coefficientg(R_new:A_n) wasc. 0.32. In the most severely shaded subordinate plants,gwas< 0.2, but lowgstimulatedr_old. The contribution ofR_new to total respiraton (f_{R, new}) and the carbon use efficiency CUE (1 –  R/(A_n + R_new)) werec. 0.68 and 0.62 forΔ w_i > 0.1, respectively. ForΔ w_i < 0.1,f_{R, new} and CUE decreased with decreasingΔ w_i in both dominant and subordinate plants.• The results suggest thatR_old was closely related to maintenance, whereasR_new was primarily involved in growth.New Phytologist(2004) doi: 10.1111/j.1469-8137.2004.01170.x© New Phytologist(2004) [ABSTRACT FROM AUTHOR]