CNB: Requiescat in pace?

Despite its failure to predict variation in secondary chemistry for many metabolites and taxa, the Carbon/Nutrient Balance (CNB) hypothesis continues to be invoked with regularity. According to Lerdau and Coley (2002), the value of the hypothesis is that it explains phytochemical phenotypic plasticity outside of an evolutionary context. We contend that the CNB hypothesis impedes a more comprehensive understanding of plant biology and that an approach divorcing plant chemistry from function is inherently limited in its ability to predict responses of plants to the environmental conditions in which they have evolved. Although it is possible for a mechanistically flawed hypothesis to predict phenomena, such hypotheses almost always suffer a lack of generality. Lerdau and Coley argue that the limitations of the CNB model are its strengths, not its weaknesses, and set forth refinements that, in essence, further limit its applicability. We address these refinements and explore alternative adaptive explanations for adjustments of secondary metabolism in relation to resource availability and environmental variation. In view of its flawed nature and the existence of alternative approaches, CNB no longer warrants consideration as a viable hypothesis.