The use of fossil fuel is predicted to cause an increase of the atmosp
heric CO2 concentration, which will affect the global pattern of tempe
rature and precipitation. It is therefore essential to incorporate eff
ects of temperature and water supply on the carbon requirement for roo
t respiration of plants to predict effects of elevated [CO2] on the ca
rbon budget of natural and managed systems. There is insufficient info
rmation to support the contentention that an increase in the concentra
tion of CO2 in the atmosphere will enhance the CO2 concentration in th
e soil to an extent that is likely to affect root respiration. Moreove
r, there is no convincing evidence for a direct effect of elevated atm
ospheric [CO2] on the rate of root respiration per unit root mass or t
he fraction of carbon required for root respiration. However, there ar
e likely to be indirect effects of elevated [CO2] on the carbon requir
ement of plants in natural systems. Firstly, it is very likely that th
e carbon requirement of root respiration relative to that fixed in pho
tosynthesis will increase when elevated [CO2] induces a decrease in nu
trient status of the plants. Although earlier papers have emphasized t
hat elevated [CO2] favours investment of biomass in roots relative to
that in leaves, these are in fact indirect effects. The increase in ro
ot weight ratio is due to the more rapid depletion of nutrients in the
root environment as a consequence of enhanced growth. This will decre
ase the specific rate of root respiration, but increase the carbon req
uirement as a fraction of the carbon fixed in photosynthesis. It is li
kely that these effects will be minor in systems where the nutrient su
pply is very high, e.g, in many managed arable systems, and increase w
ith decreasing soil fertility, i.e. in many natural systems. Secondly,
a decrease in rainfall in some parts of the world may cause a shortag
e in water supply which favours the carbon partitioning to roots. Wate
r stress is likely to reduce rates of root respiration per unit root m
ass, but enhance the fraction of total assimilates required for root r
espiration, due to greater allocation of biomass to roots. Increased t
emperatures are unlikely to affect the specific rate of root respirati
on in all species. Broadly generalized, the effect of temperature on b
iomass allocation is that the relative investment of biomass in roots
is lowest at a certain optimum temperature and increases at both highe
r and lower temperatures. The root respiration of some species acclima
tes to growth temperature, so that the effect of global temperature ri
se is entirely accounted for by the effect of temperature on biomass a
llocation. The specific rate of root respiration of other species will
increase with global warming. In response to global warming the carbo
n requirement of roots is likely to decrease in temperate regions, whe
n temperatures are suboptimal for the roots' capacity to acquire water
. Here global warming will induce a smaller biomass allocation to the
roots. Conversely, the carbon requirements are more likely to increase
in mediterranean environments, where temperatures are often supraopti
mal and a rise in temperature will induce greater allocation of biomas
s to the roots.