Chemical defences against free radicals include compounds that are strong reductants such as glu-
tathione, phenols, flavonoids, and polyamines [25]. Enzymatic defenses against free radicals include su-
peroxide dismutases, catalase, peroxidases, phenol oxidase, and ascorbic acid oxidase [26]. Excess light
energy trapped by chlorophyll in a high-oxygen environment can do significant damage. The violaxan-
thin-zeaxanthin cycle plays a major role in helping to dissipate that energy [28]. A diminished capacity
to defend against free radicals is thought to play a major role in tissue senescence [29]. Is it possible to
learn from respiratory metabolism the influence of environmental stress before the plant shows visible
symptoms? The answer is yes, as shown in the following sections.A. Temperature StressCheatgrass (Bromus tectorumL.) is a weedy annual first introduced into the Great Basin of the western
United States in the late 19th century. It germinates in the fall, overwinters as seedlings, grows very
rapidly in the early spring when moisture is abundant, flowers in May, sets seed, and drops them, com-
pleting the life cycle by early June. The dead grass then serves as fuel for wildfires. Cheatgrass seed sur-
vives fire well whereas competing native perennials do not, creating conditions for further spread of the
weed.
Cheatgrass is a highly autogamous species with minimal levels of genetic variation. Nonetheless, ge-
netic differentiation may arise in response to general and predictable differences among habitats that
make a population-level response appropriate [30].
Characteristics of respiratory metabolism were examined in 11 subpopulations from different habi-
tats [31]. Seeds from each subpopulation were germinated and metabolic heat rates and respiration rates
determined calorimetrically at 5°C intervals from 5 to 45°C. From the experimental data, growth rates and
efficiency of carbon conversion were calculated. Results are summarized in Table 1. One might suppose
that the temperature response would follow the large range of altitudes of the 11 populations studied. That
was not the case as the lowest elevation and warmest site, St. George at 850 m, had the lowest optimal
growth temperature (10°C) and the lowest upper limit for growth (16°C). On the other hand, higher ele-
vation sites had higher optimal growth temperatures and higher upper limit temperatures (Table 1). The
explanation is that plants must be adapted to the microclimate in which they must survive. In St. George
at 850 m, cheatgrass can grow only in the winter and very early spring, when temperatures are cool but
water is available. In the dry, hot summer, survival is impossible. By contrast, mountain sites (2000 to
3000 m) have a shorter frost-free period, but water is available in summer when temperatures are often
very warm. Cheatgrass has thus adapted to grow in warmer temperatures at high elevations.
Corn (Zea maysL.) varieties are grown worldwide. Growth rates of some of the cultivars are pre-
dicted to increase at low temperature, go through a maximum in the “normal” growth range, and then de-TIME, PLANT GROWTH, RESPIRATION, TEMPERATURE 5TABLE 1 Metabolic Heart Rate (q) and Respiration Rate (RCO2) Measured Every 5°C from 0
to 45°C for Germinated Cheatgrass (Bromus tectorumL.) Seed from 11 Populations at
Different Elevationsa
Temperature response (°C)
Population (altitude, m) Low stress Optimal High stress
St. George (850) 3–4 10 16
Green River (1280) 6 10 and 30 35
White Rocks (1450) 3–4 15 30
Ephraim (1740) 0–3 5 and 20 25
Hobble Creek (1800) 2–3 15 and 25 30
Potosi (1850) 01527
Castle Rock (1980) 6 15 44
Salina (2040) 7 25 32
Strawberry (2400) 5 10 40
Fairview (2770) 15 20 26
Nebo Summit (2850) 5 15 20
aPopulations are listed in order of altitude with the low-stress and high-stress temperatures indicated as well
as the temperature for optimal growth.