Geoscience Reference
In-Depth Information
of the temperature range over which it is measured. However,
this is not true for plant respiration. In fact, the Q
10
increases
linearly upon short-term increases in temperatures from 40°C
to 0°C. This dynamic temperature response of Q
10
measures
the temperature-dependent change in the respiration rate; an
increase in ambient temperature will cause a greater change in
rates of respiration in plants native to cold, Arctic climates than
in plants native to hotter climates. Furthermore, other abiotic
factors such as irradiance and water deficit can also influence
the Q
10
for plant respiration. Short temperature exposures to low
temperatures reduces the flux of carbon through glycolysis and
the TCA cycle because low temperatures will reduce the activity
of the various enzymes involved in these pathways. In addition,
low temperatures will decrease the fluidity of the inner mem-
brane, which decreases the rate of respiratory electron transport.
As a consequence, short-term exposure to low temperatures will
reduce the rates of CO
2
evolution and O
2
consumption. However,
at moderate to high temperatures, it is not enzyme activity that
limits the rate of reaction but rather the availability of substrates
such as ADP and ATP, at high temperature, mitochondrial
membranes may become leaky to protons and, therefore, reduce
the capacity to synthesise ATP by chemiosmosis.
14.4
plants adjust osmotically in water stress
condition
Osmotic adjustment is the capacity of plant cells to accumulate
solutes and use them to lower Ψw (these sign means water poten-
tial) during periods of osmotic stress. The adjustment involves
a net increase in solute content per cell that is independent of
the volume changes that result from loss of water. The decrease
in ΨS (=osmotic potential) is typically limited to about 0.2-
0.8 MPa, except in plants adapted to extremely dry conditions.
Water stress in many plants is a decrease in osmotic poten-
tial resulting from an accumulation of solute. This process is
known as osmotic adjustment. While some increase in sol-
ute concentration is expected as a result of dehydration and
decreasing cell volume, osmotic adjustment refers specifically
to a net increase in solute concentration due to metabolic pro-
cesses triggered by stress. An osmotic adjustment generates a
more negative leaf water potential, thereby helping to maintain
water movement into the leaf and, consequently, leaf turgor.
Osmotic adjustment may also play an important role in par-
tially recoveries by helping to maintain leaf turgor. Osmotic
