Transpiration ratio

A related quantity is the transpiration ratio, which is the reciprocal of the water-use efficiency and hence represents the water lost per CO2 fixed. 

On a mass basis, this corresponds to a WUE of 6.1 g CO2 (kg H20)-1 (the molar masses of CO2 and H20 are 44.0 and 18.0 g mol-1, respectively). We also note that the transpiration ratio in this case is 1/(0.0025) or 400 H2O/CO2. This substantial water loss per CO2 fixed is generally not a problem when plenty of water is available for transpiration. Plants in such environments often have a high g Jal, which leads to a somewhat higher and somewhat higher rates of photosynthesis than is the case for plants with a moderate g al. 

Table 4.8. CO2 exchange and transpiration in obligate CAM plants in response to drought and its relation to dark acidification, transpiration ratio, and growth rate ...

The effectivity of water use by plants is conveniently expressed in terms of transpiration ratio (TR), the ratio of grams water loss into grams CO2 assimilated. Using the gas-transfer equation 5.1, transpiration can be expressed as ... 

The transpiration ratio is, as defined here, the ratio of T to P. A main problem in interpretation, however, is that T is tightly coupled to the environment through A. Hence, a graph of T versus A would be linear if the diffusion resistances are constant. Similarly, a graph of T P versus A would be linear and hence, the transpiration ratio (T P) will vary with the evaporative demand of the environment. 

In comparing transpiration ratio, it is assumed that A is constant among the test species. Actually, the slope of the graph T P versus A gives a better indication of water use efficiency in terms of carbon assimilated. 

Transpiration ratios have been estimated for a variety of CAM plants. Joshi et al. (1965) studied pineapple growing in a glasshouse in North Carolina during the winter. They estimated a CO2 uptake rate of 0.5-0.7 mg dm" h and transpiration of 20-50 mg dm h This gives a TR range of 40-72. For an 80-day period, pineapple growing at a rate of 15-18 mg dry weight increase per day dm leaf showed a ratio of about 50 for actual water loss to dry matter accumulated. 

Neales et al. (1968) estimated Transpiration Ratios for Aeonium hawarthii. Agave americana, and pineapple in comparison with sunflower and tobacco. Over the day/night cycle under their experimental conditions, they estimated TR of 154 (Aeonium), 47 (Agave), and 116 (pineapple) for the CAM succulents and 166 (sunflower) and 162 (tobacco) for the mesophytes. Erler (1969) also reported a low TR for Agave americana of 70, and Sideris and Krauss (1955) estimated 55 for pineapple. 

Fig. 6.3. Growth of Echeveria pumila correlated with transpiration ratio. Greenhouse-grown plants (from Meinzer and Rundel, 1973, by permission)...

The transpiration ratio of O. basilaris varied significantly during the year (see Fig. 6.7), although mean TR values remained within the values reported for the three optional types of photosynthesizing plants, i.e., 450-600 for C3-plants, 250-350 for C4-plants, and 25-150 for CAM plants. 

Similarly to Opuntia basilaris, F. acanthodes has shallow roots and its water potential recovers within 24 h after rainfall. The optimal temperature for dark CO2 fixation was 12.6° C, remarkably low. The transpiration ratio was about 70 for an entire year, which is in good agreement with values of other CAM plants (see Chap. 6.2.1). As indicated earlier (Chap. 6.2.1), during the dry season the TR values observed in F. acanthodes range from 18 to 105. 

Joshi,M.C., Boyer, J.S., Kramer, P.J. Growth, carbon dioxide exchange, transpiration and transpiration ratio of pineapple. Bot. Gaz. 126,174-179 (1965)... 

The amount of growth occurring when rainfall is limited depends on the ratio of assimilation rate to transpiration rate. In a leaf the instantaneous transpiration efficiency, A E, is given approximately by... 

Data compiled by Hanks (1983) show that, contrary to earlier notions, the m coefficient in Equation 1 may be affected by the genotype. A relatively greater net gain of carbon for the same rate of transpiration under stress may be reflected in the m coefficient (Equation 1), in the ratio between assimilation and transpiration (assimilation ratio) or in the agronomic index, WUE (Equation 2). 

Jones, H.G. (1976). Crop characteristics and the ratio between assimilation and transpiration. Journal of Applied Ecology, 13, 605-22. 

For understanding these tendencies, we will consider the values of the biogeo-chemical coefficient of aqueous migration. This coefficient Cw is the ratio between the content of an element in the sum of water-soluble salts and in geological rocks. The values of Cw for certain chemical species are smaller in Arid ecosystems than those in Forest ecosystems. We can suggest two explanations. First, soils of Forest ecosystems are enriched in water-soluble metal-organic complexes (see Chapter 7). Second, most chemical species are trapped in the transpiration barrier of upper soil layers of Arid ecosystems. 

Reaction dynamics as opposed to reaction kinetics strives to unravel the fundamentals of reactions—just how they transpire, how intramolecular vibrational energy redistributions provide energy to the modes most involved along the reaction coordinate, how specihc reaction states progress to specihc product states, why product energy distributions and ratios of alternative products are as they are, and, of course, how fast the basic processes on an atomic scale and relevant timeframe occur. 

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