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

Summary

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  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.
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  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 by ¹³CO₂ labelling and gas exchange measurements.
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  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 (Δw_i). The growth coefficient g (R_new: A_n) was c. 0.32. In the most severely shaded subordinate plants, g was < 0.2, but low g stimulated r_old. The contribution of R_new to total respiraton (f_{R, new}) and the carbon use efficiency CUE (1 – R/(A_n +R_new)) were c. 0.68 and 0.62 for Δw_i > 0.1, respectively. For Δw_i < 0.1, f_{R, new} and CUE decreased with decreasing Δw_i in both dominant and subordinate plants.
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  The results suggest that R_old was closely related to maintenance, whereas R_new was primarily involved in growth.