is placed in the calorimeter ampule with the tissue. Again, a 15–20 min thermal equibration is necessary,
followed by measurement of the respiration rate (RCO2) for 15–20 min. As the CO 2 and NaOH react in so-
lution, additional heat is produced (108.5 kJ mol^1 is the heat of reaction for carbonate formation), giv-
ing the rate of CO 2 evolution (RCO2) by the plant tissue. Next, the NaOH is removed and the heat rate (q)
is measured as before. The tissue may then be run at another temperature. The difference in qand 455RCO2
[21] can then be used to predict growth rate changes with temperature [see Eqs. (2) and (4)] under the as-
sumption that carbohydrate is the substrate for reaction (2).
Substrate carbon conversion efficiency , described in Eq. (2), is related to the ratio q/RCO2as in Eq.
(12) [21].
(/1)HBq/RCO2(1 p/4)HO2 (12)whereHBis the enthalpy change for the formation of biomass from photosynthate [Eq. (4)], (^) pis the
mean chemical oxidation state of the substrate carbon oxidized to CO 2 , and HO2is Thornton’s constant,
with a value of 455
15 kJ mol^1 of O 2.
Incorporating Thornton’s constant and assuming carbohydrate substrate with (^) p0, the specific
growth rate of structural biomass (RSG) is related to the two measured variables as in Eq. (13).
RSGHB455RCO2q (13)
VI. CATABOLISM AND ANABOLISM
Photosynthesis transforms energy from sunlight into energy-rich organic matter, i.e., carbohydrates.
This organic matter then serves as the energy source for all life on earth. The energy is partially liber-
ated in glycolysis (fermentation) or in the oxidative pentose phosphate cycle, both in the cytoplasm.
Substrate-level ATP and reduced pyridine nucleotides are produced. This may have been the extent of
energy conservation in anoxic early earth [3]. Once oxygen began to increase, mitochondrial activity
provided a much higher rate of energy turnover, resulting in explosive adaptive radiation [5]. The key
to rapid expansion of life on earth as well as growth of a single plant is rapid turnover of ATP/ADP—
perhaps as much as 50% of the dry biomass of active tissues every 24 hr [22]. If an inhibitor blocks
the cytochrome oxidase pathway or an uncoupler destroys the proton gradient across the inner mito-
chondrial membrane, there is a rapid increase in oxygen uptake and CO 2 production in response to the
drop in ATP production.
Louis Pasteur showed that yeast cells would produce more CO 2 in nitrogen than in air. Plant bio-
chemists showed that tissues committed to rapid growth (e.g., germinating seeds, meristematic tissue)
would show the Pasteur effect whereas mature or senescing tissue would not. The control mechanism for
respiration proposed was the ATP/ADP ratio [23]. In growing tissues, oxidative phosphorylation rapidly
produces ATP, which is utilized just as rapidly in anabolic activities. Plants store energy not as ATP but
rather as sucrose, starch, protein, or lipid. Of interest is that chloroplasts do not export ATP but mito-
chondria do. For growth, both ADP and ATP must be present.
VII. STRESS
Plants are subject to many forms of environmental stress. Some are abiotic, physicochemical, or density
independent, such as temperature, drought, fire, and air pollution. Other sources of stress are biotic or den-
sity dependent, such as competition, herbivory, disease, and parasitism [24]. For each of these environ-
mental factors there is a range or life zone that the plant can tolerate. If the tolerance range for a given
stress factor is exceeded, the plant will suffer stress, and if the stress is severe enough, the plant may die.
Short-term acclimation may be possible, and given enough time, natural selection may result in adapta-
tion to the stress.
At the cellular and molecular level, the common theme of stress is the formation of free radicals—
strong oxidants that can do significant damage to membranes and DNA. Free radicals include superox-
ide, hydrogen peroxide, and superhydroxide [25,26]. Air pollutants may themselves be strong oxidants,
such as ozone, peroxyacetyl nitrate (PAN), and oxides of sulfur and nitrogen [27]. Heavy metals with
more than one possible valence state can also serve as strong electron donors.
4 SMITH ET AL.