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