Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO.

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass, [Formula: see text], and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGR).