Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availab ility of these nutrients is also a major factor influencing longterm plant and ecosystem responses to rising atmospheric CO2 levels, i.e., the commonl y observed short-term increase in plant biomass may not be sustained over t he long-term. Therefore, it is critical to obtain a mechanistic understandi ng of whether elevated CO2 can elicit compensatory adjustments such that ac quisition capacity for minerals increases in concert with carbon (C) uptake . Compensatory adjustments such as increases in (a) root mycorrhizal infect ion, (b) root-to-shoot ratio and changes in root morphology and architectur e, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency c an enable plants to meet an increased nutrient demand under high CO2. Here we examine the literature to assess the extent to which these mechanisms ha ve been shown to respond to high CO2. The literature survey reveals no cons istent pattern either in direction or magnitude of responses of these mecha nisms to high CO2. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that i n addressing nutrient uptake responses to high CO2 most investigators have examined these mechanisms in isolation. Because such mechanisms can potenti ally counterbalance one another, a more reliable prediction of elevated CO2 responses requires experimental designs that integrate all mechanisms simu ltaneously. Finally, we present a functional balance (FB) model as an examp le of how root system adjustments and nitrogen-use efficiency can be integr ated to assess growth responses to high CO2. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights i n determining overall plant responses to high CO2. For example, while chang es in root-to-shoot biomass allocation, r, have a small effect on growth, a djustments in uptake rate per unit root mass, <(<nu>)over bar>, and photosy nthetic N use efficiency, p*, have a significantly greater leverage on grow th responses to elevated CO2 except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGR(max)).