Gas Exchange in Plants [EXT]

I extracted this resource from Kimball. This site gives good information on respiration and transpiration.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GasExchange.html#Opening_stomata

Gas Exchange in Plants


In order to carry on photosynthesis, green plants need a supply of carbon dioxide and a means of disposing of oxygen. In order to carry on cellular respiration, plant cells need oxygen and a means of disposing of carbon dioxide (just as animal cells do).

Unlike animals, plants have no specialized organs for gas exchange (with the few inevitable exceptions!). The are several reasons they can get along without them:

• Each part of the plant takes care of its own gas exchange needs. Although plants have an elaborate liquid transport system, it does not participate in gas transport.



• Roots, stems, and leaves respire at rates much lower than are characteristic of animals. Only during photosynthesis are large volumes of gases exchanged and each leaf is well adapted to take care of its own needs.



• The distance that gases must diffuse in even a large plant is not great. Each living cell in the plant is located close to the surface. While obvious for leaves, it is also true for stems. The only living cells in the stem are organized in thin layers just beneath the bark. The cells in the interior are dead and serve only to provide mechanical support.



• Most of the living cells in a plant have at least part of their surface exposed to air. The loose packing of parenchyma cells in leaves, stems, and roots provides an interconnecting system of air spaces. Gases diffuse through air several thousand times faster than through water. Once oxygen and carbon dioxide reach the network of intercellular air spaces (arrows), they diffuse rapidly through them.



• Oxygen and carbon dioxide also pass through the cell wall and plasma membrane of the cell by diffusion. The diffusion of carbon dioxide may be aided by aquaporin channels inserted in the plasma membrane.

Leaves

The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma).

Normally stomata open when the light strikes the leaf in the morning and close during the night.

The immediate cause is a change in the turgor of the guard cells. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes.




The table shows the osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at 150 lb/in2. When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed.


Opening stomata

The increase in osmotic pressure in the guard cells is caused by an uptake of potassium ions (K+). The concentration of K+ in open guard cells far exceeds that in the surrounding cells. This is how it accumulates:

• Blue light is absorbed by phototropin which activates



• a proton pump (an H+-ATPase) in the plasma membrane of the guard cell.



• ATP, generated by the light reactions of photosynthesis, drives the pump.



• As protons (H+) are pumped out of the cell, its interior becomes increasingly negative.



• This attracts additional potassium ions into the cell, raising its osmotic pressure.

Closing stomata

Although open stomata are essential for photosynthesis, they also expose the plant to the risk of losing water through transpiration. Some 90% of the water taken up by a plant is lost in transpiration.

Abscisic acid (ABA) is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration (which often occurs around mid-day).

The mechanism:

• ABA binds to receptors at the surface of the plasma membrane of the guard cells.



• The receptors activate several interconnecting pathways which converge to produce
  a rise in pH in the cytosol

transfer of Ca2+ from the vacuole to the cytosol

• The increased Ca2+ in the cytosol blocks the uptake of K+ into the guard cell while



• the increased pH stimulates the loss of Cl- and organic ions (e.g., malate2-) from the cell.



• The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor.



• The stomata close.

Open stomata also provide an opening through which bacteria can invade the interior of the leaf. However, guard cells have receptors that can detect the presence of molecules associated with bacteria called pathogen-associated molecular patterns (PAMPs). LPS and flagellin are examples. When the guard cells detect these PAMPs, ABA mediates closure of the stoma and thus close the door to bacterial entry.

This system of innate immunity resembles that found in animals. Link to discussion.

Density of stomata

The density of stomata produced on growing leaves varies with such factors as:

• the temperature, humidity, and light intensity around the plant;



• and also, as it turns out, the concentration of carbon dioxide in the air around the leaves. The relationship is inverse; that is, as CO2 goes up, the number of stomata goes down, and vice versa. Some evidence:

• Plants grown in an artificial atmosphere with a high level of CO2 have fewer stomata than normal.



• Herbarium specimens reveal that the number of stomata in a given species has been declining over the last 200 years — the time of the industrial revolution and rising levels of CO2 in the atmosphere [View].

These data can be quantified by determining the stomatal index: the ratio of the number of stomata in a given area divided by the total number of stomata and other epidermal cells in that same area.

How does the plant determine how many stomata to produce?

It turns out that the mature leaves on the plant detect the conditions around them and send a signal (its nature still unknown) that adjusts the number of stomata that will form on the developing leaves.

Two experiments (reported by Lake et al., in Nature, 411:154, 10 May 2001):

• When the mature leaves of the plant (Arabidopsis) are encased in glass tubes filled with high levels (720 ppm) of CO2, the developing leaves have fewer stomata than normal even though they are growing in normal air (360 ppm).



• Conversely, when the mature leaves are given normal air (360 ppm CO2) while the shoot is exposed to high CO2 (720 ppm), the new leaves develop with the normal stomatal index.

Stomata reveal past carbon dioxide levels

Because CO2 levels and stomatal index are inversely related, could fossil leaves tell us about past levels of CO2 in the atmosphere? Yes. As reported by Gregory Retallack (in Nature, 411:287, 17 May 2001), his study of the fossil leaves of the ginkgo and its relatives shows:

• their stomatal indices were high
  late in the Permian period (275–290 million years ago) and again
  in the Pleistocene epoch (1–8 million years ago).



• Both these periods are known from geological evidence to have been times of



• low levels of atmospheric carbon dioxide and
  ice ages (with glaciers).



• Conversely, stomatal indices were low during the Cretaceous period, a time of high CO2 levels and warm climate.

These studies also lend support to the importance of carbon dioxide as a greenhouse gas playing an important role in global warming.

Roots and Stems

Woody stems and mature roots are sheathed in layers of dead cork cells impregnated with suberin — a waxy, waterproof (and airproof) substance. So cork is as impervious to oxygen and carbon dioxide as it is to water.

However, the cork of both mature roots and woody stems is perforated by nonsuberized pores called lenticels. These enable oxygen to reach the intercellular spaces of the interior tissues and carbon dioxide to be released to the atmosphere.

The photo shows the lenticels in the bark of a young stem.

In many annual plants, the stems are green and almost as important for photosynthesis as the leaves. These stems use stomata rather than lenticels for gas exchange.

Transpiration [EXT]

I found this in John Kimball's site. Very good and simplified explanation on Transpiration, evaporation of water from plant leaves.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Transpiration.html

Transpiration

Transpiration is the evaporation of water from plants. It occurs chiefly at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis.

But air that is not fully saturated with water vapor (100% relative humidity) will dry the surfaces of cells with which it comes in contact. So the photosynthesizing leaf loses substantial amount of water by evaporation. This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem of the roots and stem.

Importance

Transpiration is not simply a hazard of plant life. It is the "engine" that pulls water up from the roots to:

• supply photosynthesis (1%-2% of the total)


• bring minerals from the roots for biosynthesis within the leaf


• cool the leaf

Environmental factors that affect the rate of transpiration

1. Light

Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata (mechanism). Light also speeds up transpiration by warming the leaf.

2. Temperature

Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.

3. Humidity

The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases.When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

4. Wind

When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.

5. Soil water

A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.

The volume of water lost in transpiration can be very high. It has been estimated that over the growing season, one acre of corn plants may transpire 400,000 gallons of water. As liquid water, this would cover the field with a lake 15 inches deep. An acre of forest probably does even better.

Under Shaded Light - Miniature Rose

by Richard Chew



In this posting, I wish to share some experiences of growing miniature rose under shaded light.



I purchased this miniature rose in beginning Dec 2008 and placed it at my window sill since then.

As I have mentioned in 'Photosynthesis - How many hours of sunlight does your rose need? ' posting, that a rose plant would require a minimum of 160,000 lux hours of light to survive.



Assuming indirect sunlight produces about 30,000 lux (light intensity) in an hour, easily this rose will get about 270,000 lux hours each day (about 9 hours indirect light per day), which is sufficient to fuel growth.

New flower bulbs started to emerge after about a month since I last pruned off the spent flower. However this time, the size of bloom is noticeably smaller and the new leaves texture is lighter than the earliers ones. I believe if the fertilizing is enhanced, the bloom quality may improve.

Though it may not produce the same bloom size when it was grown at more direct sun light, the important thing to note is there is no ill effect or any serious detrimental effect when its grown under shaded light. Despite lacking direct sunlight for more than a month, the bush looks healthy.

And to add on, this miniature rose was never been treated with any insecticide nor pesticide. This goes to show that miniature rose can aclimitised with minimal human intervention and without chemicals. Based on this observation, I can safely conclude that roses have the capability to adapt to its surroundings. The only difference is that the quality of bloom shall varied due to different environment conditions. The important point to note is that growing miniature rose under shaded light do not cause any detrimental effect.

At this point of time, I am inclining to the idea that miniature rose can be grown under shaded light.

Some questions to be considered as these findings are pursued further ...

1. Why are the blooms noticeably smaller? Was it caused purely by lack of direct sunlight?



2. Or the fertilizing dosage was weak therefore contributed to smaller bloom development?

3. Or perhaps both?

In my next prune, I shall increase the fertilizing dosage and examine the size of next bloom in February.

My Home-Made Compost Tea

by Richard Chew



I like to share how I make compost tea for my roses. At this time of writing, I have already made 3 attempts to fine tune the compost tea to suit my roses. It was successful for some, but for majority cases not so successful.



This posting is just to give you an idea. I suggest you wait for my later postings if you decide to to make your own. My caution to you is that, if its not done correctly it may infect your roses with unfavourable diseases.



Let me share with you my purpose and goal for making this compost tea. It is my dream to grow roses in Malaysia with 'zero' fungicide and pesticide. If I may elaborate further, no chemicals, meaning fully organic; fully natural. In order to achieve this goal, it is critical to identify the right soil organism for growing roses.



The purpose of this experiment is to find the correct ingredients to innoculate the right soil organism that favours roses.



It is quite simple. The following were the tools that I used.




1. Air Pump (similar those used for fish aquarium)

2. Air Stone

3. Pail (5 to 8 litre)

4. Air Tube

5. Air Stone Holder (with rubber suction)








I purchased a battery operated Air Pump but battery failed after running for 12 hours or so. I suggest that you purchase electrical powered ones and with a silencer feature, so that it can run quietly for at 48 hours continously without interuption. This is the amount of time required to complete the brewing process.

I half filled the plastic pail with water. And placed the air stone to the bottom of the pail. I used the rubber suction holder to hold the air stone so that it was locked firmly at the bottom.

Found an odd stocking at my wife's cabinet.

Filled the small pot with soil. The details of the soil is described at the end of this posting.

Poured the soil into the stocking. And inserted the soil to the center of the stocking.

Placed the stocking (with the soil) and tied both ends at the ear of the pail. Adjusted it so that the soil stayed submerged at the surface of the water.

Ensured the air stone was positioned correctly so that the air bubbles worked the entire compost tea.

Added some seaweed solution into the compost tea, to feed the soil organism.

Left it to brew for 48 hours.

After 2 days, removed the 'compost tea bag' (stocking with soil).

The compost tea was ready.

Filled the sprayer with the compost tea.





Sprayed the tea on the soil to innoculate the soil with new soil organism.

After about a week, some of my roses got infected with black spots on the stem. However for the more established ones there were no effect at all.

The positive result is that my chlorotic rose responded well. Initially the leaves turned lighter green (with dark green veins) quite rapidly. Then gradually the leaves turned darker.

Below were the ingredients that I used for my compost tea bag.

At this point of time, my conclusion is that compost tea works better for established roses (healthy ones). If the roses are young or not so healthy, feeding compost tea may weakened the roses. I believe the main reason is due to inability to deal with more variety of soil organism introduced into the soil, thus weakened the rose. If its sprayed on established roses, it seemed to respond better for example my chlorotic rose and my French Lace rose.

Based on my obeservation, AACT compost tea (Active Aerated Compost Teat) provides a noticeable reaction on my roses. However I suspect the soil type used to brew the compost tea wasn't suited for my roses. I believed I may have innoculated the wrong type of soil organism that caused the infection on some of the roses.

The next step is to source the right kind of soil medium to brew the compost tea.

The Color of Soil [EXT]

I extracted this from NRCS. This is good resource to help assess soil quality by understanding the colour of soil. http://soils.usda.gov/education/resources/k_12/lessons/color/


The Color of Soil


The first impression we have when looking at bare earth or soil is of color. Bright colors especially, catch our eye. Geographers are familiar with Red Desert soils in California, Arizona, and Nevada (Arizona State Soil); and Gray Desert soils in Idaho, Utah, and Nevada (Nevada State Soil). We have the White Sands in New Mexico, Green Sands along the Atlantic Coast, and Redbeds in Texas and Oklahoma (Oklahoma State Soil). The Red River between Oklahoma and Texas carries red sediment downstream, particularly in times of flood. The Yellow River (Hwang Ho) in China carries yellow sediment. Surface soils in the Great Plains and Corn Belt are darkened and enriched by organic matter.



Earth materials found in such locations as those mentioned above were used as coloring agents early in the development of most human cultures. As earth material was fashioned into utilitarian vessels, artistic colors inevitably were incorporated into them. Indigenous North American cultures used contrasting earth colors as body paints, and modern American culture uses colored earth in cosmetics and ceramics and as pigments for paints.


Munsell Color System

Red, brown, yellow, yellowish-red, grayish-brown, and pale red are all good descriptive colors of soil, but not very exact. Just as paint stores have pages of color chips, soil scientists use a book of color chips that follow the Munsell System of Color Notation (http://www.munsell.com/). The Munsell System allows for direct comparison of soils anywhere in the world. The system has three components: hue (a specific color), value (lightness and darkness), and chroma (color intensity) that are arranged in books of color chips. Soil is held next to the chips to find a visual match and assigned the corresponding Munsell notation. For example, a brown soil may be noted as: hue value/chroma (10YR 5/3). With a soil color book with Munsell notations, a science student or teacher can visually connect soil colors with natural environments of the area, and students can learn to read and record the color, scientifically. Soil color by Munsell notation is one of many standard methods used to describe soils for soil survey. Munsell color notations can be used to define an archeological site or to make comparisons in a criminal investigation. Even carpet manufacturers use Munsell soil colors to match carpet colors to local soils so that the carpet will not show the dirt (soil) tracked into the house.



Soil Composition and Color

Soil color and other properties including texture, structure, and consistence are used to distinguish and identify soil horizons (layers) and to group soils according to the soil classification system called Soil Taxonomy. Color development and distribution of color within a soil profile are part of weathering. As rocks containing iron or manganese weather, the elements oxidize. Iron forms small crystals with a yellow or red color, organic matter decomposes into black humus, and manganese forms black mineral deposits. These pigments paint the soil (Michigan State Soil). Color is also affected by the environment: aerobic environments produce sweeping vistas of uniform or subtly changing color, and anaerobic ( lacking oxygen), wet environments disrupt color flow with complex, often intriguing patterns and points of accent. With depth below the soil surface, colors usually become lighter, yellower, or redder.

Interpreting Soil Color

Color can be used as a clue to mineral content of a soil. Iron minerals, by far, provide the most and the greatest variety of pigments in earth and soil (see the following table).



Properties of Minerals



























Relatively large crystals of goethite give the ubiquitous yellow pigment of aerobic soils. Smaller goethite crystals produce shades of brown. Hematite (Greek for blood-like) adds rich red tints. Large hematite crystals give a purplish-red color to geologic sediments that, in a soil, may be inherited from the geologic parent material. In general, goethite soil colors occur more frequently in temperate climates, and hematite colors are more prevalent in hot deserts and tropical climates.


Color - or lack of color - can also tell us something about the environment. Anaerobic environments occur when a soil has a high water table or water settles above an impermeable layer. In many soils, the water table rises in the rainy season. When standing water covers soil, any oxygen in the water is used rapidly, and then the aerobic bacteria go dormant. Anaerobic bacteria use ferric iron (Fe3+) in goethite and hematite as an electron acceptor in their metabolism. In the process, iron is reduced to colorless, water-soluble ferrous iron (Fe2+), which is returned to the soil. Other anaerobic bacteria use Mn4+ as an electron acceptor, which is reduced to colorless, soluble Mn2+. The loss of pigment leaves gray colors of the underlying mineral. If water stays high for long periods, the entire zone turns gray.


When the water table edges down in the dry season, oxygen reenters. Soluble iron oxidizes into characteristic orange colored mottles of lepidocrocite (same formula as goethite but different crystal structure) on cracks in the soil. If the soil aerates rapidly, bright red mottles of ferrihydrite form in pores and on cracks. Usually ferrihydrite is not stable and, in time, alters to lepidocrocite.


Along seacoasts, tide waters saturate soils twice daily, bringing soluble sulfate anions. Anaerobic bacteria use the sulfate as an electron acceptor and release sulfide (S2-) which combines with ferrous iron to precipitate black iron sulfide. A little hydrochloric acid (HCl) dropped on this black pigment quickly produces a rotten egg odor of hydrogen sulfide (H2S) gas. Soils that release H2S gas are called sulfidic soils. With time, iron sulfide alters to pyrite (FeS2) and imparts a metallic bluish color. If sulfidic soils are drained and aerated, they quickly become very acid (pH 2.5 to 3.5), and a distinctive pale yellow pigment of jarosite forms. This is the mark of an acid sulfate soil that is quite corrosive and grows few plants.


Galuconitic green sands form in shallow ocean water near a coast. They become part of soils that form after sea level drops. White colors of uncoated calcite, dolomite, and gypsum are common in geologic materials and soils in arid climates. A little carbonate dissolves in water, moves downward, and precipitates in soft white bodies or harder nodules. It also accumulates in root pores as lacy, dendritic (tree-branch) patterns.


Influence of Organic Matter on Soil Color

Soil has living organisms and dead organic matter, which decomposes into black humus. In grassland (prairie) soils the dark color permeates through the surface layers bringing with it nutrients and high fertility (Kansas State Soil). Deeper in the soil, the organic pigment coats surfaces of soil, making them darker than the color inside. Humus color decreases with depth and iron pigments become more apparent. In forested areas, organic matter (leaves, needles, pine cones, dead animals) accumulates on top of the soil. Water-soluble carbon moves down through the soil and scavenges bits of humus and iron that accumulate below in black, humic bands over reddish iron bands. Often, a white layer, mostly quartz occurs between organic matter on the surface where pigments were removed (Wisconsin State Soil).

Organic matter plays an indirect, but crucial role in the removal of iron and manganese pigments in wet soils. All bacteria, including those that reduce iron and manganese, must have a food source. Therefore, anaerobic bacteria thrive in concentrations of organic matter, particularly in dead roots. Here, concentrations of gray mottles develop.


Soil color is a study of various chemical processes acting on soil. These processes include the weathering of geologic material, the chemistry of oxidation-reduction actions upon the various minerals of soil, especially iron and manganese, and the biochemistry of the decomposition of organic matter. Other aspects of Earth science such as climate, physical geography, and geology all influence the rates and conditions under which these chemical reactions occur.


Soil adds beauty to our landscapes. These colors blend with vegetation, sky, and water. For art students and others who may be interested in creating a natural look to their artwork, try to incorporate finely ground colored soils as pigments into your work.


Adapted from: Lynn, W.C. and Pearson, M.J., The Color of Soil, The Science Teacher, May 2000.