Authors claim respiration-mass relationship differs between plants and animals, but others argue "universal"
By Melissa Lee Phillips | January 26, 2006
A paper in this week's Nature claims that plants do not show the same mass-to-respiration ratio widely documented in animals. According to Peter Reich of the University of Minnesota and his colleagues, plant mass scales in a one-to-one ratio with respiratory metabolism, while most animal respiration is proportional to an animal's mass raised to the 3/4 power. The authors claim that this discrepancy arises because plant respiration is constrained by nitrogen availability, rather than by delivery of nutrients through vascular networks.
Reich and his co-authors make a "strong argument" that respiration rate is not dependent on the delivery of metabolites through plant vascular systems, said John Damuth of the University of California, Santa Barbara, who was not involved in the study.
But not all scientists agree. Brian Enquist of the University of Arizona in Tucson and his colleagues have theorized that metabolism?as well as other physiological processes?scale in a consistent way with an organism's mass because of a similar anatomical feature in both plants and animals: hierarchical, branching vascular networks. According to the theory, these networks form fractal-like patterns, selected by evolution as the optimal geometry to deliver resources to a three-dimensional body. Enquist argued that this theory actually predicts deviations from 3/4 power scaling among the small plants that Reich and his co-authors sampled, suggesting that the findings actually support the theory, rather than contradicting it.
However, after collecting data on around 500 plant seedlings and saplings from 43 species, Reich and his colleagues found that respiratory metabolism did not scale with mass to the 3/4 power ? showing instead a one-to-one, or isometric, relationship.
The researchers found that the isometric relationship held regardless of the plant's environment, but total respiration rate depended on whether a plant lived indoors or outdoors. "A field-grown plant had a different respiration for the same size as a lab-grown plant, but the relationship within each group of respiration to size had the same proportionality," Reich said.
These parallel, but different, relationships likely arise because nitrogen content in plants appears to also scale isometrically with respiration ? and this value did not depend on environment. In plants, Reich said, "respiration scales with nitrogen, not with mass."
According to Reich, plant metabolism likely scales better with nitrogen content rather than mass because nitrogen content?not vascular architecture?is the limiting factor in plant respiratory metabolism. Because plant leaves can exchange gases by diffusion, plant metabolism is not as constrained by their vascular delivery systems as metabolism in animals, Reich said.
"It makes perfect sense that, if they're not constrained by the vascular system, then the rates at which plant tissues respire would just be proportional to the amount of respiring tissue," said Damuth.
However, Enquist listed a number of reasons why the data are, in fact, still consistent with a universal relationship between mass and respiration based on vascular constraint. For instance, gas diffusion through leaves is still related to the vascular network, because this network determines leaf branching patterns, Enquist said. Additionally, the theory laid out by Enquist and colleagues in previous work predicts that small plants like saplings and seedlings deviate from the 3/4 relationship between mass and metabolism, Enquist said.
The relationship between branch radius and length in larger plants?that radius must increase with increasing length so that the plant will not topple under its own weight?leads to a vascular network that confers the 3/4 rule on plant metabolism, Enquist said. These biomechanical constraints only apply to larger plants, however, he noted, not to the seedlings and saplings examined by Reich and his colleagues. Had they extended their studies to large trees, they would have found that those plants follow the 3/4 rule, Enquist said.
It has been known for a while that small plants' mass likely scales isometrically with metabolism, agreed Karl Niklas of Cornell University in Ithaca, N.Y. "The scaling exponent varies as a function of size and therefore there is no 'universal' scaling exponent," Niklas said, but "there is still a basic metabolic theory that explains these results."
According to Enquist, Reich's results are "consistent with what our original network-based model predicts: an isometric relationship between size and metabolism for very small saplings and seedlings, curving over and changing to a 3/4 exponent once you get up to small trees to very large trees," he said. "Their own data show this."
Links within this article
P.B. Reich et al., "Universal scaling of respiratory metabolism, size, and nitrogen in plants," Nature 439:457-461, 26 Jan 2006.
V.M. Savage et al., "The predominance of quarter-power scaling in biology," Funct Ecol. April, 2004.
P. Hunter, "The power of power laws," The Scientist, April 21, 2003.
S. Bunk, "Do energy transport systems shape organisms?" The Scientist, December 7, 1998.
G.B. West et al., "A general model for the origin of allometric scaling laws in biology," Science, April 4, 1997.
G.B. West et al., "A general model for the structure and allometry of plant vascular systems," Nature, August 12, 1999.
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