Organ-specific rates of cellular respiration in developing sunflower seedlings and their bearing on metabolic scaling theory.

Fifty years ago Max Kleiber described what has become known as the "mouse-to-elephant" curve, i.e., a log-log plot of basal metabolic rate versus body mass. From these data, "Kleiber's 3/4 law" was deduced, which states that metabolic activity scales as the three fourths-power of body mass. However, for reasons unknown so far, no such "universal scaling law" has been discovered for land plants (embryophytes). Here, we report that the metabolic rates of four different organs (cotyledons, cotyledonary hook, hypocotyl, and roots) of developing sunflower (Helianthus annuus L.) seedlings grown in darkness (skotomorphogenesis) and in white light (photomorphogenesis) differ by a factor of 2 to 5 and are largely independent of light treatment. The organ-specific respiration rate (oxygen uptake per minute per gram of fresh mass) of the apical hook, which is composed of cells with densely packaged cytoplasm, is much higher than that of the hypocotyl, an organ that contains vacuolated cells. Data for cell length, cell density, and DNA content reveal that (1) hook opening in white light is caused by a stimulation of cell elongation on the inside of the curved organ, (2) respiration, cell density and DNA content are much higher in the hook than in the stem, and (3) organ-specific respiration rates and the DNA contents of tissues are statistically correlated. We conclude that, due to the heterogeneity of the plant body caused by the vacuolization of the cells, Kleiber's law, which was deduced using mammals as a model system, cannot be applied to embryophytes. In plants, this rule may reflect scaling phenomena at the level of the metabolically active protoplasmic contents of the cells.