Jose-Luis Machado

Universal scaling of respiratory metabolism, size and nitrogen in plants

The scaling of respiratory metabolism to body size in animals is considered to be a fundamental law of nature, and there is substantial evidence for an approximate ¾-power relation. Studies suggest that plant respiratory metabolism also scales as the ¾-power of mass, and that higher plant and animal scaling follow similar rules owing to the predominance of fractal-like transport networks and associated allometric scaling. 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 scaling, 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.

Scaling of respiration to nitrogen in leaves, stems and roots of higher land plants

Using a database of 2510 measurements from 287 species, we assessed whether general relationships exist between mass-based dark respiration rate and nitrogen concentration for stems and roots, and if they do, whether they are similar to those for leaves. The results demonstrate strong respiration-nitrogen scaling relationships for all observations and for data averaged by species; for roots, stems and leaves examined separately; and for life-forms (woody, herbaceous plants) and phylogenetic groups (angiosperms, gymnosperms) considered separately. No consistent differences in the slopes of these log-log scaling relations were observed among organs or among plant groups, but respiration rates at any common nitrogen concentration were consistently lower on average in leaves than in stems or roots, indicating that organ-specific relationships should be used in models that simulate respiration based on tissue nitrogen concentrations. The results demonstrate both common and divergent aspects of tissue-level respiration-nitrogen scaling for leaves, stems and roots across higher land plants, which are important in their own right and for their utility in modelling carbon fluxes at local to global scales.

Drought acclimation among tropical forest shrubs (Psychotria, Rubiaceae)

SummaryMechanisms of dry-season drought resistance were evaluated for five evergreen shrubs (Psychotria, Rubiaceae) which occur syntopically in tropical moist forest in central Panama. Rooting depths, leaf conductance, tissue osmotic potentials and elasticity, and the timing of leaf production were evaluated. From wet to dry season, tissue osmotic potentials declined and moduli of elasticity increased in four and five species, respectively. Irrigation only affected osmotic adjustment by P. furcata. The other seasonal changes in leaf tissue properties represented ontogenetic change. Nevertheless, they made an important contribution to dry-season turgor maintenance. Small between-year differences in dry season rainfall had large effects on plant water status. In 1986, 51 mm of rain fell between 1 January and 31 March, and pre-dawn turgor potentials averaged <0.1 MPa for all five Psychotria species in March (Wright 1991). In 1989, 111 mm of rain fell in the same period, pre-dawn turgor potentials averaged from 0.75 to 1.0 MPa for three of the species in April, and only P. chagrensis lost turgor. The relation between leaf production and drought differed among species. P. limonensis was buffered against drought by the lowest dry-season conductances and the deepest roots (averaging 244% deeper than its congeners) and was the only species to produce large numbers of leaves in the dry season. P. chagrensis was most susceptible to drought, and leaf production ceased as turgor loss developed. For the other species, water stress during severe dry seasons may select against dry-season leaf production.

Leaf demography and growth rates of Espeletia barclayana Cuatrec. (Compositae), a caulescent rosette in a Colombian paramo

The production of leaves, inflorescences, and growth rates were measured during 1985 for 547 plants of Espeletia barclayana, a giant rosette species in the high Andes of Colombia. There were differences in leaf production between individuals of different sizes, with a maximum for plants 61-90 cm in height (27.9 leaves yr −1 ) and a minimum for plants less than 30 cm (5.1 leaves yr −1 ). For plants more than 61 cm in height there was a minimum in leaf production during February, coinciding with the minimum in rainfall, and also during June coinciding with a decrease in rainfall, low temperature, and a peak in fog and condensation. There was a great variation in the number of reproductive plants from one year to the next (...)

Biological scaling: Does the exception prove the rule? (Reply)

Enquist et al. raise several points that they claim cast doubt on our findings and interpretation regarding whole-plant relations of respiration, R, with plant mass, M, and total plant nitrogen content, N. We agree with Enquist et al. that R does not scale isometrically with M across all plants. However, their assertion that we claim that isometric scaling (R ∝ Mθ, with θ = 1) is universal in plants of all sizes is incorrect — in fact, we conclude the opposite, noting that there is isometric scaling within individual experiments, non-isometric scaling of respiration versus mass across all data pooled, and no common relation across all data.

Biological Scaling: Does The Exception Prove The Rule?

Enquist et al. raise several points that they claim cast doubt on our findings and interpretation regarding whole-plant relations of respiration, R, with plant mass, M, and total plant nitrogen content, N. We agree with Enquist et al. that R does not scale isometrically with M across all plants. However, their assertion that we claim that isometric scaling (R ∝ Mθ, with θ = 1) is universal in plants of all sizes is incorrect — in fact, we conclude the opposite, noting that there is isometric scaling within individual experiments, non-isometric scaling of respiration versus mass across all data pooled, and no common relation across all data.

Corrigendum: Universal scaling of respiratory metabolism, size and nitrogen in plants

This corrects the article DOI: 10.1038/nature04282

Does the exception prove the rule? (Reply): Biological scaling

Enquist et al. raise several points that they claim cast doubt on our findings and interpretation regarding whole-plant relations of respiration, R, with plant mass, M, and total plant nitrogen content, N. We agree with Enquist et al. that R does not scale isometrically with M across all plants. However, their assertion that we claim that isometric scaling (R ∝ Mθ, with θ = 1) is universal in plants of all sizes is incorrect — in fact, we conclude the opposite, noting that there is isometric scaling within individual experiments, non-isometric scaling of respiration versus mass across all data pooled, and no common relation across all data.

Erratum: Corrigendum: Universal scaling of respiratory metabolism, size and nitrogen in plants

This corrects the article DOI: 10.1038/nature04282

Erratum: Universal scaling of respiratory metabolism, size and nitrogen in plants (Nature (2006) 439, (457-461))

This corrects the article DOI: 10.1038/nature04282