Plants are observed to regulate their uptake of nutrients in elaborate patternsaccording to their environment of growth and their stage of development.Among the common patterns is that plants reduce their nutrient uptakecapacities (per mass of root,n, or of the whole plant,nplant) as nutrient concen-trations increase (Clements et al. 1979; Godwin and Blair 1991; Youssefi et al.1999; see Chap. 6, this Vol.). Similarly, changes in the shoot’s environment,such as in CO2partial pressure, also induce changes in nutrient uptake rates(n). A number of questions arise – for one, why should a ‘good’ such as nutri-ent acquisition ever be curtailed, or not expressed at a maximal rate? Onemust infer that downregulating the acquisition of a beneficial resource con-fers a net benefit in Darwinian fitness, for which most plants or their immedi-ate ancestors have been heavily selected. Admittedly, long-domesticatedplants may diverge from the fitness functions of wild plants (Gutschick 1987,1997, 1999; Jackson and Koch 1997). In some cases, the explanation lies at theimmediate physiological level, in that some nutrients in excess are toxic, suchas boron (Nable et al. 1990) and even phosphate (e.g., Romera et al. 1992).Nonetheless, such downregulation occurs even for nutrients that show noapparent toxicity in luxury consumption, such as nitrogen. Plant performancedoes not follow the guidelines espoused by the actress Mae West, “Too muchof a good thing is wonderful”. Some experimental evidence shows that over-expression of nontoxic nutrient acquisition is deleterious to plant growth andfitness – witness the stunting of supernodulating legumes (Carroll et al.1985), which can perhaps be attributed to excess diversion of photosynthateto N2fixation. Nonetheless, downregulation occurs even at modest, physiolog-ical nutrient content, most markedly in woody plants (Gessler et al. 1998).