The control of carbon acquisition by roots

Abstract

We review four hypotheses for the control of carbon acquisition by roots, and conclude that the functional equilibrium hypothesis can offer a good description of C acquisition by roots relative to shoots, but is deficient mechanistically. The hypothesis that import into roots is solely dependent on export from the shoot, itself determined by features of the shoot alone (the ' push ' hypothesis), is supported by some but not all the evidence. Similarly, the idea that root demand, a function of the root alone, determines import into it (the ' pull ' hypothesis), is consonant with some of the evidence. The fourth, general, hypothesis (the ' shared control ' hypothesis) -that acquisition of C by roots is controlled by a range of variables distributed between root and shoot -accords with both experiment and theory. Top-down metabolic control analysis quantifies the control of C flux attributable to root relative to source leaf. We demonstrate that two levels of mechanistic control, short-term regulation of phloem transport and control of gene expression by compounds such as sugars, underlie distributed control. Implications for the impact of climate change variables are briefly discussed.

Monosaccharide Absorption Activity of Arabidopsis Roots Depends on Expression Profiles of Transporter Genes under High Salinity Conditions

“…However, roots might also attempt to reduce carbon loss through the reabsorption of these metabolites (32). Although the biological significance of the absorption of sugars is a matter of continuing debate, it is clear that roots are able to uptake sugar from the rhizosphere.…”

Response of root respiration to changes in temperature and its relevance to global warming

“…Reductions in root growth at low temperatures might also result in part from reduced allocation of photosynthetic C to the roots, especially if photosynthetic CO # assimilation is decreased after a decline in air temperature. The ' sink-strength ' of roots is often poor, with assimilates exported to roots representing those left over by other sinks (Brouwer, 1963 ;Lambers & Atkin, 1995 ;Farrar & Jones, 2000).…”

Responses of tree fine roots to temperature

“…It seems clear that in some herbaceous plants, photosynthesis and root respiration are directly coupled on time scales that range from minutes to hours (Hansen, 1977 ;Massimino et al, 1981 ;Ryle et al, 1985 ;Amthor, 1994). After reviewing work with annual plants and conducting experiments with single-rooted soybean leaves, Farrar & Jones (2000) argue that C allocation to roots is controlled jointly by roots and shoots. However, the relationships between photosynthesis, C allocation to roots, and rates of root respiration are not well understood in trees and shrubs, which are much larger and longer-lived than annual plants.…”