Transpiration stomatal resistance

Plant water status is affected by environmental pollution and consequently influences plant function at every level of biological organisation. It can be characterized by measurements of the relative water content (RWC), the water deficit, the water potential ( P ) and the osmotic potential ( Fq), along with transpiration rate and stomatal resistance. Since for the latter four parameters, tissue samples are removed from the plant, they are usually determined in the end of an experiment. If several sampling times are needed, then additional plants/replicates must be included. 

Transpiration rate and stomatal resistance are the most commonly measured plant gas exchange parameters using IRGA. Their behaviour in A. thaliana plants exposed to heavy metal ion stress is demonstrated in Figure 2 B and C. 

For our applications to transpiration and photosynthesis, we will define a stomatal conductance, and a stomatal resistance, r for the diffusion of species j using Equation 8.4 ... 

Fig. 5.21. Diurnal patterns of stomatal resistance for an experimental Kalanchoe daigremontiana plant grown in a pot. The pattern is similar to Opuntia basilaris except for opening at the end of the light period. Data were obtained by calculation from transpiration estimates by Osmond. The time along the abscissa is separately experimental...

In their study of corn and soybeans, Bazzaz et al. [3] found a decrease in net photosynthesis and transpiration with increasing lead treatment levels from PbCl2. At lower lead concentrations corn, a C4 plant, was more sensitive. At higher lead concentrations soybeans, a C3 plant, was more sensitive, and was found to have lower photosynthesis and transpiration rates. The rate changes of the two processes are related to changes in the leaf stomatal resistance to CO2, and water vapor diffusion [3]. It is believed that the difference in storage areas of photosynthesis products in C3 and C4 plants causes this difference. 

The pathway of least resistance for gases to cross an epidermis—and thus to enter or to exit from a leaf—is through the adjustable space between a pair of guard cells (Fig. 1-2). This pore, and its two surrounding guard cells, is called a stoma or stomate (plural stomata and stomates, respectively). When they are open, the stomatal pores allow for the entry of CO2 into the leaf and for the exit of photosynthetically produced O2. The inevitable loss of water vapor by transpiration also occurs mainly through the stomatal pores, as we will discuss in Chapter 8 (Section 8.1B). Stomata thus serve... 

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