The scaling of respiratory metabolism to body size in animals is considered to be a fundamental law of nature1-11, and there is substantial evidence for an approximate 3/4-power relation. Studies suggest that plant respiratory metabolism also scales as the 3/4-power of mass12-14, and that higher plant and animal scaling follow similar rules owing to the predominance of fractal-like transport networks and associated allometric scaling8-14. Here, however, using data obtained from about 500 laboratory and field-grown plants from 43 species and four experiments, we show that whole-plant respiration rate scales approximately isometrically (scaling exponent ≈ 1) with total plant mass in individual experiments and has no common relation across all data. Moreover, consistent with theories about biochemically based physiological scaling15-18, isometric scaling of whole-plant respiration rate to total nitrogen content is observed within and across all data sets, with a single relation common to all data. This isometric scaling is unaffected by growth conditions including variation in light, nitrogen availability, temperature and atmospheric CO2 concentration, and is similar within or among species or functional groups. These findings suggest that plants and animals follow different metabolic scaling relations, driven by distinct mechanisms.
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Acknowledgements We thank the Wilderness Research Foundation and the National Science Foundation Long-Term Ecological Research Program for major support of the research reported herein and the Gordon Conference ‘Metabolic Basis of Ecology’ for inspiring us in this endeavour.