Metabolic heat rate

Calorimetry shows that the rates of metabolism of plant tissues vary widely with species, with cell types, and with environmental conditions. This provides a means of exploring the mechanisms by which various agents influence the health of a plant community. Studies are being done on beneficial agents such as growth promoters and detrimental ones such as atmospheric pollutants. For example, a correlation has been found between the metabolic heat rates and the extent of damage to pine needles by ozone. 

Figure 26. Microbial growth (a) and metabolic heat rate (b) fix>m milk kept at 30 as affected by pasteurization (P), lactoperoxydase system (LPS) activation and untreated raw milk (C). A highly significant interaction (P < 0.001) was found between milk treatment and incubation period [40]. (With permission from Elsevier.)...

The garment containing PCM was tested in a climatic chamber at various temperatures to determine the thermo-regulating effect from heat absorption and heat release of the PCM. In this test, the garment samples were attached to a simulated skin apparatus, the temperature of which was measured over time at various ambient temperatures and metabolic heat rates. Based on these tests, time intervals were estimated within which the skin temperature could be stabilized within a desired temperature range. The test results for the two garments under ambient temperature exposures of — 20 °C and -I- 20 °C are summarized in Fig. 3.5. 

Changes in metabolic heat rates over the range from 0° to 30° were completely reversible. Experimental points collected for Fig. 2 were obtained in random order. Repeated points at selected temperatures were reproducible within 5% whether approached from higher or lower temperatures. Measured effects of temperature on cell metabolism correlate well with generally observed long-term temperature responses of the Intact plants. 

As one method of obtaining Rcoi data, measurement of heat rates from trapping CO2 in NaOH solutions is employed. Sorption heat rates measured for CO2 are typically about 20% of the metabolic heat rate. Thus, tissue samples producing metabolic heat rates of 100 pW will produce only 20 pW from reaction of CO2 to form carbonate. Thus the 2 pW baseline error becomes a 10% error in CO2 rate measurements. To maintain a 2% maximum error in / co2, sufficient plant tissue must be used to produce < > values above 500 pW. 

Rapid adjustment of temperature for isothermal studies. Stepwise adjustment of temperature during isothermal detennination of plant metabolic heat rates on the same tissue sample is essential for interpretation of metabolic properties. This generally requires a calorimeter with both rapid scanning and isothermal capabilities. 

Temperature scanning capabilities. While many of the measurements required for plant calorimetry studies are best done in an isothermal mode, measurements of metabolic heat rates while. scanning over the viable range of plant growth adds an important dimension for interpretation of plant properties. Studies are facilitated by the ability to scan temperature both up and down. 

The studies of Criddle et al. [17] on carrot and tomato cell cultures outlined basic procedures for isothermal heat rate measurements of plant tissues. Samples are placed in an ampule, sealed to prevent any water vapor loss, placed in the calorimeter at the desired temperature and the heat rates recorded directly. Figure la shows the type of thermogram obtained. There is an initial rapid change in recorded heat rate while sample and ampules are thermally equilibrated. Following equilibration, (about 45 min in this example) the amplitude of the thermal signal is corrected for baseline values obtained with empty ampules to yield the sample metabolic heat rate. Temperature may then be adjusted to new values to establish temperature coefficients of heat rate or the ampules may be opened and the sample environment modified before the ampule is resealed and re-equilibrated for evaluation of effects of the modification on plant activities. Because plants are ectotherms that live in a variable temperature environment, temperature dependence studies using sequential i.sothermal mea.surements are essential for characterization of plant physiological properties. 

Figure 1. Isothermal measurement of plant metabolic heat rates. Plant tissue samples are placed in the calorimeter ampules, sealed, and heat rates are followed over time. The initial rapid changes in heat rates observed in this figure illustrate thermal equilibration of ampules and samples. This is followed by heat rate values that, when adjusted for any baseline corrections, represent the metabolic heat rate of the tissue preparation. When tissue metabolic rates are stable over the time course of the experiment, constant heat rates are obtained (a). When tissues are unstable or are stressed so that metabolic activity decreases during the calorimetric measurement, a decreasing heat rate is ob.served (b).

Batch studies. Methods and equipment have been described for isothermal measurement of metabolic heat rates and determination of the flux rates of both O2 and CO2 [21, 22, 39]. Isothermal heat rates are determined as in section 1.8.1. O2 rates are determined by pressure change. CO2 rates are determined by two methods, one measuring heat rate increases in the presence of a CO2 trap and the other by measuring pressure change. 

A scanning calorimetry study of tomato cells at elevated pressures by Criddle et al. [39 was able to identify important elements related to high temperature inactivation. Metabolic heat rate measurements were conducted at temperatures from 25 to 60 "C and at pressures from ambient to 12 MPa. Elevated pressure increased the inactivation temperature for the tomato cells. The combined calorimetry and pressure results thus show that a reaction with a positive volume change is associated with high-temperature inactivation of tomato cells. 

Heat conduction differential scanning calorimetry methods for the measurement of metabolic heat rates of plant tissues as a continuous function of temperature were developed by Hansen and Criddle [19]. Thermally induced transitions and heat rates can be determined simultaneously. 

A determination of sample heat rate requires a minimum of two sets of temperature scans a ba.seline scan with both ampules empty and a scan with. sample present in one ampule. Because reference ampules and contents cannot be exactly matched to the mass and heat capacity of the sample, baseline corrections for precise determination of sample metabolic heat rates are complex. The ab.solute determination by DSC of both the metabolic heat rate... 

Scanning calorimetry allows rapid assessment of metabolic heat rates as a continuous function of temperature. This is a significant improvement over traditional methods of determining metabolic and growth rate responses to temperature that are often slow, allow measurements at only a few temperatures, and require interpolation to predict rates at other temperatures. The thermograms often show fine structure that would not be identified by the traditional methods. 

Figure 3 illustrates the major features of typical metabolic heat rate vs. temperature curves from a scanning analysis [50]. The shapes of the... 

Figure 3. Metabolic heat rate vs. temperature curves and features that may be used for species comparisons. The dashed curve in Figure 1 was obtained from data collected on Callistemon during mid May. The segment of the curve from 15 to 30 "C (A-B) shows an approximately exponential increase in metabolic rate as temperature is increa.sed. B is an inflection point termed the low shoulder temperature (Ti ) above which metabolic rates no longer increase exponentially and the slope decreases with temperature increase. C indicates T m the temperature at which maximum rate is achieved. The exothermic peak at D depends on the amount of O2 remaining in the calorimeter ampule when this temperature is reached. A second scan of Callistemon tissue examining a tissue sample collected from the same plant two months later in the season is also shown (solid line).

Plant respiratory properties are stable genetic traits that differ among species and to a lesser extent within species. Measurements of properties such as metabolic heat rates, COi rates, and energy efficiencies produce particular combinations of respiratory properties sufficiently unique to separate Eucalyptus species by canonical analysis of the respiratory variables [68-70]. Canonical discriminant analysis using the measured respiratory properties 0, Rqoi. 0IRcoi. and and... 

Suwanagul [91] developed microcalorimetric methods for early detection of weed resistance to herbicides by measurement of differences in metabolic responses between herbicide resistant and susceptible biotopes. Herbicides were applied to young weeds and, after appropriate times, metabolic heat rates of meristematic tissue was measured. Three weed species and three herbicides with different modes of action were examined atrazine, metsulfuron-methyl and diclofop-methyl. Susceptible weed metabolic rates were inhibited at lower concentration of herbicides than the resistant biotopes. Calorimetry offered a rapid way to screen for resistant weed populations. Suwanagul concluded that early detection of weed resistance via calorimetry is an important tool for assisting farmers in dynamic managing of weed resistance. 

Respiratory parameters in species of eucalypts change with maturation. state of the trees. Metabolic heat rates, C02 production rates, and temperature coefficient of heat rate all showed systematic changes with tree age. Therefore, maximizing economic returns also depends on understanding and quantifying the growth rate as a function of tree age. 

Pines. Calorimetric studies of growth rates and temperature responses have not been employed to examine pine trees. Two studies were conducted to analyze effects of air pollutants on the respiration properties of Ponderosa and Jeffrey pine needles. Bower [104] used one-cm needle segments and demonstrated a correlation between the extent of ozone damage, measured as the number of lesions on the needles, and isothermal metabolic heat rates. He also measured increases in metabolic heat rates resulting from acid and nitrate deposition on the needles. Momen et al. [105] conducted a more controlled study of acid rain and ozone effects on Ponderosa pine with defined applications to plantation grown plants. In seedlings, metabolic heat rates increased in response to ozone and combinations of ozone and acid rain. Mature tree metabolic activities showed no response to ozone, acid, or combinations of the two. No studies were made to determine whether metabolic efficiencies were altered by these treatments. Thus the results show that calorimetry can be used to monitor pollutant effects on trees, but more definitive experiments must be done to identify how the ob.served responses relate to growth and survival of the trees. 

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