Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Gas Exchange:  Stomata & Transpiration

I. Photosynthesis/Transpiration Paradox (or perhaps more accurately, a "Compromise" or "Dilemma")
    Recall the equation for photosynthesis: CO₂ + H₂O � (CH₂O) _n + O₂. This equation tells us that:

1. Gases are important for the overall energy metabolism of plants;
2. Plants must exchange gases with the environment; and
3. In order to obtain carbon dioxide plants will necessarily loose water (transpire) or in short, transpiration is a necessary evil of photosynthesis.

II. Theoretical considerations.

1. A large surface area is required for efficient gas exchange (e.g., animals have lungs and gills; plants have leaves and within the leaf - spongy layer).
   
   
2. A large surface area for exchanging gases offers a large surface area for desiccation. Animals solve this problem by placing the absorptive surface inside a humid cavity (lung) opened with a small exit pore(s). Plants put the absorptive surface (spongy mesophyll) inside the leaf and cover it with a water impermeable layer (cuticle) peppered with a series of pores (stomata). The cuticle is comprised of waxes that minimize desiccation.
   
   
3. Placing the absorptive surfaces inside the organism to reduce desiccation presents a problem - getting the gases to the absorptive surface. Animals use an active pumping mechanism (lungs/diaphragm) to move gases inside the organism by bulk flow. The gases are circulated by another pumping system (heart). The distances needed to move the gases are too great to be accounted for by simple diffusion. Plants do not have a pumping mechanism for moving gases. They rely primarily on diffusion (and bulk flow). In either case, plants do not actively move gases. This is one reason why leaves must be thin - diffusion is not efficient over long distances (i.e., diffusion is inversely related to the square of distance).
   
   
4. The Compromise Revisited - In order to obtain carbon dioxide for photosynthesis plants needed to evolve a large, thin absorptive surface (leaves with spongy layer) and then protect it from desiccation. Not only is water loss a "necessary evil" of photosynthesis, but to make matters worse, the tendency to loose water is greater than the tendency of carbon dioxide to diffuse into the plant. As evidence, let's calculate the ratio of water loss to the amount of carbon fixed. If carbon dioxide uptake and water loss are equal, this ratio should be close to one. In reality, experiments show that this ratio is closer to 200! In other words for every 200 kg of water transpired, 1 kg of dry matter is fixed by a plant. Fortunately, plants have devised an ingenious compromise – stomata.
    
5. For more information on gas exchange theory, click here.

III. Stomatal Structure & Function

1. Types of guard cells: (1) elliptical or kidney-shaped. These are characteristic of dicots; and (2) dumb-bell or dog-bone shaped - characteristic of grasses.  For images of stomata from a variety of plants, click here.  In addition, you will see many examples in lab.
   
   
2. Common features - (1) thickened inner walls; (2) bands of cellulose fibers that radiate out around the circumference of the pore; and (3) chloroplasts (in fact, guard cells are the only epidermal cells with chloroplasts)
   
   
3. Mechanics of Guard Cell Action
        Guard cells open because of the osmotic entry of entry of water into the GC. In turn, this increases the turgidity (water pressure) in the GC and causes them to elongate. The radial orientation of cellulose microfibrils prevents increase in girth. Since GC are attached at the ends and because the inner wall is thicker, the guard cells belly out with the outer wall moving more and pulling open the guard cell. Guard cell closure essential involves reversing this process. Water entry into the guard cells is controlled by increasing the solute concentration (osmotic concentration) in the guard cells. This occurs by: (a) transporting potassium (and chloride) ions into the guard cells from surrounding (subsidiary) cells. This process is mediated by a proton pump; and (b) by sugars produced during photosynthesis or from starch breakdown (recall that guard cells are the only epidermal cells with chloroplasts). We can summarize the mechanics of GC action as follows:

stoma closed (GC flaccid) � add solute � lower water potential �  water uptake (osmosis) � increase pressure � stoma open (GC turgid)
 

4. Environmental Control of GC Action.
        Guard cells respond to their environment, especially any factors that impact the photosynthesis/transpiration compromise. Thus, we expect any factor important in photosynthesis to exert regulatory control on GC. And, we also expect water, a major player in photosynthesis, to have the final word on control since if a plant dries out too much it's as good as dead!

• Light - exerts strong control. In general: light = open; dark = closed. (reverse in CAM plants). What kind of light is important? Red & blue light – these are important for photosynthesis which (a) produces sugars (sucrose and glucose) for osmotic regulation; (b) produces ATP (via photophosphorylation) to power ion pumps; (c) reduces internal CO₂ levels which stimulates opening (see below). Blue light is also important - There is an additional effect of blue light on stomatal activity that is irrespective of its role in photosynthesis. What is blue light doing? Blue light: (a) activates a H⁺-ATPase in the membrane; and (b) stimulates starch breakdown.

• Carbon dioxide - intracellular level is most critical. This is an important regulatory control.

lo CO₂ (i.e., during the day, used by photosynthesis) = open

hi CO₂ (i.e., at night, produced during respiration) = closed

• Water - protects against excessive water loss. This is the prevailing and overriding control mechanism. There are two mechanisms by which water loss regulates stomatal closure, one is active and the other passive.

Hydropassive Control - simply put, as the plant looses water, the turgidity of the leaf cells, including guard cells, decreases and this results in stomatal closure. The plant is not "intentionally" closing the stoma, it is simply the consequence of drying out.

Hydroactive Control - this mechanism is one in which the plant actually seems to monitor its water status. When the water potential drops below some critical level, it engages a cascade of events that close the stomata. Presumably the plant is measuring pressure (turgor) and then synthesizes or releases an anti-transpirant that is translocated (moved) to the GC to cause closure. The anti-transpirant is abscisic acid (ABA), one of the major plant growth regulators. It is active in very low concentration (10^-6 M) and appears very rapidly after water stress (within 7 minutes).

• Temperature - increased temperatures usually increase stomatal action, presumably to open them for evaporative cooling. If the temperature becomes too high the stomata close due to water stress and increased CO₂ that results from respiration.

• Wind - often causes closure because it: (a) brings CO₂ enriched air; and (b) increases the rate of transpiration that causes water stress which causes the stomata to close. In some cases, wind causes stomatal opening to increase transpiration for cooling.

IV.  Anti-transpirants

• ABA
• other chemicals

V. Why does transpiration occur?

1. Transport in plants. This is important to a small degree. Transpiration is certainly not a necessity.
2. Heat loss (latent heat of vaporization)
3. Carry nutrients in the soil to the plant