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

•  Respiratory costs of Medicago sativa and Helianthus annuus individuals 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 (An , g C d-1 ) and respiration rates of new (Rnew , g C d-1 ) and old C (Rold , g C d-1 ) were assessed by 13 CO2 labelling and gas exchange measurements. •  Specific respiration rate of old C (rold , g C g-1 C d-1 ) decreased exponentially with increasing shoot biomass, but was not affected by the instantaneous relative growth rate (Δwi ). The growth coefficient g (Rnew : An ) was c. 0.32. In the most severely shaded subordinate plants, g was < 0.2, but low g stimulated rold . The contribution of Rnew to total respiraton (fR, new ) and the carbon use efficiency CUE (1 - R/(An  +Rnew )) were c. 0.68 and 0.62 for Δwi  > 0.1, respectively. For Δwi  < 0.1, fR, new and CUE decreased with decreasing Δwi in both dominant and subordinate plants. •  The results suggest that Rold was closely related to maintenance, whereas Rnew was primarily involved in growth.