mic reaction and releases respiratory energy that
activity, and stellar differentiation in roots (Ren-
is utilized for many plant processes. The disap-
dig and Taylor 1989). An ethylene concentration
of ≤ 0.4 ppm can inhibit nodule formation on le-
pearance of molecular oxygen triggers a se-
quence of changes in the physico-chemical prop-
gume roots. Furthermore, ethylene has some ad-
erties of the soil (Gambrell and Patrick 1978,
vantageous impact on roots and stimulates later-
Ponnamperuma 1984). Such changes include ac-
al branching as well as root hair initiation
cumulations of reduced metal ions, organic acids,
(Rendig and Taylor 1989).
and volatiles that are potentially harmful to plant
roots (Drew and Lynch 1980). Such accumulation
Soil temperature
to phytotoxic levels requires time. The absence of
Temperature influences plant processes at the
oxygen alone is sufficient to profoundly alter the
cellular level, such as osmotic potential, hydra-
plant metabolism (Drew 1983).
tion of ions, stomatal activity and transpiration,
In cold climates, cool autumn or early spring
Gibbs free energy available for work, membrane
temperatures result in a slow rate of depletion of
permeability, solute solubilities, diffusion, and
dissolved soil oxygen by respiration of roots and
enzymatic activities (Voorhees et al. 1981). The fi-
soil organisms (Drew 1992). The root apical mer-
nal shape of the root system is determined from
istem, the zone of fastest oxygen consumption, is
root branching and elongation of individual root
particularly sensitive to oxygen deficiency and
axes. Several reviews indicate that most plant
species exhibit an optimum temperature for max-
the sequence of metabolic events leading to cell
imum root elongation rates (Nielson and
death (Roberts et al. 1984a,b). In summer, crops of
Humphries 1966, Cooper 1973). Furthermore, the
cold climates consumed oxygen at the rate of 1.7
L m3 day1; furthermore, oxygen consumption
species within a genus (Heinrichs and Nielsen
rates reduce to 1.5 L m3 day1 in winter, and to
1966) and cultivars within a species (Johnson and
Hartman 1919) varied in root elongation and
about half in the absence of plants (Currie 1970).
branching rates in response to temperature. For
Suboptimal oxygen concentrations in the soil
instance, Brar et al. (1990a) reported high vari-
air occur because of interactions among soil prop-
ability among legume cultivars in main axis root
erties such as porosity, water content, tempera-
length at different specific temperatures (Fig. 8).
ture, surface water movement, and continuity of
Low temperature reduces water absorption by
air-filled pores with biotic activity (Drew 1983,
increasing the water viscosity and decreasing cell
Grable 1966). Jones et al. (1991) developed a root
membrane permeability. Furthermore, low tem-
growth simulation model and considered the ef-
peratures also decrease metabolic activity and
fect of soil water content, bulk density, texture,
decrease the root growth (dry matter produc-
and plant genotype on soil air. Increased soil wa-
tion). The addition of fertilizer, particularly phos-
ter content or bulk density reduce the oxygen dif-
fusion rate and affect air-filled porosity. The
phorus, may compensate to some extent for the
diffusion coefficient of oxygen in air is approx-
reduced growth in cold soils (Nielsen and
imately 10,000 times greater (0.23 cm2 s1) than
Humphries 1966).
that in water (0.26 104 cm2 s1) at 26C (Rendig
Snow cover has a large influence on soil tem-
and Taylor 1989).
perature. Snow cover decreases heat conduction
A mathematical model for the relationship of
to the soil surface (Legget and Crawford 1952)
root respiration, soil oxygen supply, and internal
and serves as a sink for heat fluxes at both upper
oxygen supply was first developed by Luxmoore
and lower surfaces of the cover (Granger et al.
et al. (1970). Measurements used to predict the ef-
1977). Relationships between air temperature and
fect of a reduced soil oxygen supply on root
snow depth can be used to predict the minimum
growth and function are
soil temperatures (Fig. 9). The pattern of seasonal
1. Oxygen diffusion rate;
warming of air temperature is predominantly
2. Oxygen concentration in the soil;
temporal at the ground surface; however, its ef-
3. Air-filled porosity percentage;
fect is temporal as well as spatial below ground
4. Redox potential;
and causes temperature limitations on root
5. Air permeability.
growth (Fig. 10). During the growing season, as
Soil air that is low in oxygen can contain high
the warming front moves downward, tempera-
concentrations of ethylene, which is a growth reg-
tures of the deeper soil layers become suitable for
ulator rather than a toxin. Elevated concentration
root growth. The depth of soil thermally suitable
of ethylene inhibits root elongation, cambial
for appreciable root growth is a function of both
12
Previous Page
