The Control of Carbon Acquisition by and Growth of Roots

Abstract

What controls the rate of growth of roots? Behind this deceptively simple question lie a very complex set of processes within the plant and a wide range of environmental variables that affect root growth. To begin to answer it, we will simplify by making the assumption that the question is nearly the same as this: what controls the rate of net acquisition of carbon by roots? A consideration of the gross fluxes of carbon (C) that together constitute the net flux into a root (Table 4.1) is thus central to our argument.We have recently provided a brief review of the main hypotheses for the control of C acquisition by roots (Farrar and Jones 2000). We concluded that each of the fluxes that contributes to the net acquisition of C exerts some degree of control over the process; we called this the 'shared control hypothesis'. Whilst many of these fluxes occur in the root, others are in the leaf or stern. We rejected two hypotheses wh ich are much simpler. These were the 'push hypothesis' , which suggests that net C acquisition is controlled by the supply of C from the shoot; and the 'pull hypothesis', where C acquisition is controlled by demand from within the root itself. Here we will concentrate on processes within roots, but these alone cannot give a complete understanding of control of fluxes within roots.Roots grow very quickly; young fibrous root systems can increase their dry weight by 25 % per day; root apices commonly elongate at 2 cm/day. To grow this quickly, the fluxes of assimilates and other compounds into, and metabolie rates in, the root must be correspondingly high. A root can gain C by import of carbohydrate and other compounds from the shoot, and by uptake of a variety of organic moleeules from the soil (Table 4.1). It can lose carbon by export to the shoot, by respiration, and by rhizodeposition involving loss of dead cells or exudation. The challenge is to determine just what controls each of the individual fluxes, and to evaluate just how important each is in the control of net C flux to the root.

Does plant biomass partitioning reflect energetic investments in carbon and nutrient foraging?

“…The supply of C to the soil microbial community is another factor that weakens the link between below‐ground biomass and energy partitioning in plants, and our results broadly reflect what is currently understood about the energy requirements of plant associations with mycorrhizal fungi and nitrogen‐fixing bacteria (Farrar & Jones, ; Minchin & Witty, ; Peng et al, ). Because the identity of microbial associates as well as nutritional and other rhizosphere properties varied significantly across studies, we did not attempt to analyse microbial effects in more detail (e.g., energy demands of arbuscular vs. ectomycorrhizal fungi, the latter represented by only three studies).…”

Potential effects of climate change on DOC release from three different soil types on the Northern Pennines UK: examination using field manipulation experiments

“…Root growth is known to be a key process governing inputs of carbon to soil (Fitter et al , 1998; see Fig. 4.3 in Farrar & Jones, 2003) and may be a main source of the DOC released to leaching waters.…”

Fine root production and litter input: Its effects on soil carbon

“…Based mainly on crop studies, Farrar and Jones (2003) generalized that 7.5% of net C fixation was released to soil via root exudates. Since the highest plant biomass (root and shoot) accumulation was found in pine seedlings (Figure 1), under which soil carbon declined, the root exudation rate per unit mass may be much lower from pine fine root.…”