DEBBRA MARCEL JP/8544/13 ASSIGNMENT 1

JIB 224 Plant Physiology: Assignment 1

Chapter 2: Whole Plant Water Relations

Question: The transpiration rate tends to be greatest under conditions of low

humidity, bright sunlight, and moderate winds. Explain why.

The evaporation of water from plants is a biological process called transpiration,

where all plant consists of almost 90% water. Approximately 10% of all evaporating

water on this planet came from plant. Plant will lose a lot of water by evaporation due to

the reason that air that less saturated with water vapor will dry the surfaces of plant cells

with which it comes in contact with. Therefore, it is crucial for plant to transport more

water from the soil to the leaves through the xylem to replace the transpired water.

Generally, the tiny openings of undersurface of a leaf called stomata opened for the

passage of carbon dioxide gas and oxygen gas during photosynthesis and control the

occurrence of transpiration of the leaves. It is because the opening of stomata allows

transpiration because the water pressure in the guard cells is much more than the water

pressure of surrounding. There are several factors that naturally influence the rate of

transpiration in plants, internally as well as externally. Internally, transpiration is

influenced by the size of leaf, the presence or absence of wax cuticle, the thickness of

leaf, the numbers of stomata on the leaf surface, the shape and size of stomata and

many more (Salisbury and Ross, 1992). Meanwhile, the external factors are including

the low humidity, bright sunlight, and moderate winds.

Low humidity or more commonly known as relative humidity (RH) is the ratio of

the actual water content of air to the maximum amount of water that can be held by air

at a given temperature. Transpiration occurs when water vapor moves outward from

higher to lower water potential or from less negative to more negative water potential

values. When the leaves have sufficient amount of water and the open stomata, the

differences between the water vapor molecules concentration (in the cavities between

cells in the leaf) with water vapor molecules concentration in the air will influence the

rate of transpiration. The regulating stomata movement and atmospheric demand

affects the transpiration rate environmentally. At high RH or moist air, the stomata tend

to close and thus limit the exit of water vapor from the plant. This is because the

atmosphere contains more water and has low atmospheric demand when RH is high,

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JIB 224 Plant Physiology: Assignment 1

thus reducing the driving force for transpiration. As the RH of the air surrounding the

plant increases, the transpiration rates reduce. Due to the stronger atmospheric

demand, low RH causes faster transpiration. There is an occurrence of water gradient

from the leaf to the atmosphere when the reduction of water happened in the air. The

lower the RH, the lesser the moisture will be in the atmosphere, thus the driving force

for transpiration become greater. In fact, water is easier to evaporate in dryer air than

into more saturated air. This environmental factor affects transpiration by regulating

stomatal movement and atmospheric demand. Nevertheless, transpiration still occurs

as long as the stomata are open, even at saturated condition of 100% RH (as seen in

hydrated leaf) because the expelled water vapor readily condenses (Hopkins, 2008).

Besides RH, the light intensity is also one of the environmental factors affecting

transpiration in plants. There are two-way of light that influence the rate of transpiration.

The first way is by affecting the temperature of the leaf while the second way is by

effecting on the stomata open-lid. Commonly, transpiration rate is high during daytime,

especially when under the bright sunlight compare to the transpiration during night time.

This is because the stomata triggered to open during daytime so that the carbon dioxide

will be available for the light-dependent process of photosynthesis. This is mainly

because the light controls the opening of the stomata which is the path where water

mainly escapes from the leaf surface in gaseous state as water vapor. On the other

hand, stomata are closed in most plants (except in CAM plants) during the dark

especially between sunset to sunrise. Furthermore, stomata are most sensitive to the

light predominating at sunrise (blue light) and also can open at extremely low levels of

light at dawn in order for them to access carbon dioxide for photosynthesis once the sun

hits their leaves.

Last but not least, moderate winds also affect greatly to the rate of transpiration

because it modifies the effective length of the diffusion path for exiting water molecules

by removing the thin moist layer of air that can be found next to the surface of a leaf.

Besides increase the length of the diffusion path, this moist air called boundary layer

also reduce the light penetration into the leaf causing lesser water potential gradient

from the leaf and hence decrease the rate of transpiration. Besides the thickness of the

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JIB 224 Plant Physiology: Assignment 1

epidermal layer, water vapor molecules exiting in the leaf must also diffuse through the

boundary layer. However, wind can alter the rate of transpiration by replacing the

boundary layer that has more saturated air close to the leaf with the drier air thus

increasing water potential gradient and enhancing transpiration. By removing the

boundary layer, wind can also increase the movement of the air around a plant resulting

a higher transpiration as the path for water to the atmosphere become shorter.

However, strong wind may cause excessive loss of water from leaves leading to the

closure of stomata. Without the presence of wind, the air around the leave may have

very minimum movement, causing the humidity of the air around the leaf to increase.

Chapter 2: Whole Plant Water Relations

Question: Describe the anatomy of xylem tissue and explain why it is an efficient

system for the transport of water through the plant.

Xylem is a highly specialized conductive tissue of vascular plants, which mainly

transports water and soluble mineral nutrients from the roots throughout the plant as

well as provides mechanical support to the plants. Xylem also important in replacing

water lost during transpiration and photosynthesis. Anatomically, the structures of xylem

are mainly consists of tracheary elements, which are the most distinctive components in

xylem and known as the principal water conducting cells in plant. Tracheary elements

consist of tracheids and vessel elements and can be distinguished by their shape

(Figure 1). Tracheids are usually very long tracheary elements and posses thickened

secondary walls that composed of cellulose, hemicellulose, and lignin. These

components allow tracheids to provide the structural support of the plant. The long-

tapered ends of tracheid also allow the maximum pit-pairs between consecutive cells.

Meanwhile, vessels elements are much shorter than tracheids, which are elongated

type of single cells arranged end-to-end in longitudinal series. At maturity, the openings

called perforation plates formed when the end walls of the vessel members have

dissolved away.

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JIB 224 Plant Physiology: Assignment 1

Figure 1: The structure of tracheid and vessel elements

(Source: Taiz and Zeiger, 2006)

The transport mechanism in xylem is passive, which is no energy involved

because the tracheary elements themselves are dead by maturity and no longer have

living contents. However, the organized structures of xylem tissues allow efficient

system for the transport of water due to the water potential gradient that exists between

the water surrounding the roots at one end and the air that surrounds the leaves at

another. These two extremes that connected by the xylem, support the water column

that extend from the roots to the leaves. The water potential of air usually has very

negative value even during extreme high humidity and tends to be more negative inside

of the leaf cells especially when the leaves of the plant lose water to the air. Therefore,

water will gradually moves out from xylem cells to leaf cells. Xylem sap consists mainly

of water and inorganic ions, although some organic chemicals can be found as well.

There are two phenomena that cause xylem sap to flow; the capillary action and the

transpirational pull. The capillary action is the primary force of water movement upwards

in plants, which is caused by the adhesion between the water and the surface of the

xylem conduits. This force is crucial to create an equilibrium configuration to balance the

gravity especially when transpiration removes water at the top. Meanwhile, the

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JIB 224 Plant Physiology: Assignment 1

transpiration pull required for the small diameter vessels to allow the transportation of

water, otherwise the water column will be broken down the cavitation.

More water is drawn up through the plant to replace the water that evaporates

from the leaves. When this happened, the water molecules inside the water column of

the capillary xylem elements are pulled upwards by a cohesion force, commonly known

as the cohesion tension theory of sap ascent. This theory is most widely accepted for

water movement in plants and the force mainly happen when there is continuous

column of water molecules from the tips of the roots through the stem and into the aerial

parts of the plants. The three forces that responsible for the upward movement of the

water column are the water column weight, the water adhesion to the cell walls of

tracheary elements and also the water adhesion to the soil particles. The upward

movement of water molecules in each tracheary element will create tension in the water

column, and therefore making it to become narrower. The presence of negative

pressure inside the tracheary elements are very strong during high transpiration,

causing these cells to collapse inward. Therefore, the presence of secondary thickened

walls of vessel elements and tracheids are very important to reinforce the walls and

prevent inward collapse to happen when tremendous forces produced inside the

tracheary element.

Meanwhile, tracheids and vessel elements are also adapted for optimal

conductance. Conductance is known as the ability of tracheary elements to allow

movement of water, meaning that a slight increase in diameter of the element will

increase the conductance. The conductance of tracheary element is the fourth power of

the radius of the element, which is related to the Hagen Poiseuille Law. Under certain

circumstances, tracheary elements with small diameters could also be beneficial to the

plants. For example, the adhesion force (hydrogen bonding of water molecules to the

wall) will reinforce and strengthen the water column. If the tension of water column is

too strong, cavitation or the break of water column will happen and the ‘air bubble’ or

embolism will form in the element. In this case, small-diameter elements will have fewer

problems than the large-diameter elements. The surface tension of water will prevent

the embolism in tracheids from passing through the pit membrane so that it will not

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JIB 224 Plant Physiology: Assignment 1

expand to the whole cell. The embolisms in vessels elements also can cause the whole

vessels in plants to become dysfunctional for water transport if it spread throughout the

elements in plants by the perforations that link consecutive vessel elements.

Chapter 3: Roots, Soils, and Nutrient Uptake

Question: Describe the colloidal properties of soil. How do the properties of

colloids help to ensure the availability of nutrient elements in the soil?

Soil is the major components of Earth’s ecosystem, and known as the end

product of the biotic activities, relief, influence of the climate, and parent materials

interacting over the times. It is actually the mixture of minerals, organic matters, gases,

liquids, and the myriad of organisms that together support the life of plant. Soil are

important as; a medium for plant growth; for water storage, supply and purification; a

modifier of the atmosphere; as well as a habitat for certain organisms such as

decomposers. Due to the numerous physical, chemical and biological processes

happen in the atmosphere, soil continually undergoes development, which include

weathering with associated erosion. Soils will add carbon dioxide to the atmosphere

when the atmosphere became warmer due to increased biological activity at higher

temperature.

The soil colloidal is known as one of the most influential factor in stabilizing soil

fertility. These elements act as repositories of moisture and nutrients and also function

as buffer to the variations of solution moisture and ions in the soil. The soil colloids are

also the most active ingredients of the soil because it has a major function to determine

the physical and chemical properties of soil. As soils are formed during the weathering

process, some minerals and organic matter are broken down into extremely small

particles. The changes of chemical properties further reduce these particles size until

they become microscopic, thus the very smallest particles are considered as colloids.

Colloids is very important to keep nutrient locked in the soil so that there will not be

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leached from the soil. It may also release those stored ions in response to the changes

of soil pH in order to make them beneficial to the plant.

Clay minerals or hydrous oxides are the examples of inorganic colloids which

usually make up the bulk of soil colloids. The size of soil colloids is less than 0.001 mm,

and the fraction of clay includes particles also very tiny sizes less than 0.002 mm. That

is the reason why all clay minerals are not totally colloidal. Meanwhile, highly

decomposed organic matters generally called humus are the example of organic

colloids, which are chemically more reactive and usually have greater influence on the

properties of soil per unit weight than the inorganic colloids. Humus is amorphous and

its chemical and physical characteristics are not well defined. On the other hand, clay

mineral are usually in plate-like structure and crystalline in nature as well as having

characteristic of physical and chemical configuration.

The soil colloids are mainly responsible for the reactivity of chemical in soils.

Inorganic colloids much more than the organic colloids in most soils. The type of clays

exist in the soil are determined by the type of parent material and the degree of

surrounding weathering. Colloids generally have a net negative charge, which

developed during their formation process. Balanced and surrounded by thousands the

loosely held of cations, the negative charge of colloids viewed as huge anions. The

colloid surfaces also absorb the water molecules, which are part of the hydrated

components of the cations. The effective radius of the cations are greatly influenced by

the amount of hydration. That is why the amount of water related to a particular cation

is crucial. Moreover, each type of the soil colloids can attract and hold particles with

positive charge, just like how the poles of a magnet attract each other. Likewise, colloids

repel other particles that have negative charge, as like how the poles of a magnet repel

each other.

As a conclusion, the soil colloids are very important and provide many benefits to

the soils because the colloids maintain the availability of the nutrient contents in the

soils by adsorbing, holding, and releasing the ions in soils necessarily. As both

inorganic and organic colloids are mixed well with other solids found in soil, the bulk of

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JIB 224 Plant Physiology: Assignment 1

the solids in soil are significantly inert, making the presence of colloids determine the

soil's physical and chemical characters.

Chapter 4: Plant and Inorganic Nutrients

Question: Explain the difference between autotrophic and heterotrophic nutrition.

There are two main mode of nutrition in living organism, namely autotrophic and

heterotrophic. This criteria is one of the major characteristics required for organism

classification. The main producers in the food chains of the world ecosystems is the

autotrophs. This is because most ecosystems are supported by the primary production

performed by autotrophic organisms.

Autotrophic nutrition is a process where organisms manufacture complex organic

compounds for biosynthesis by taking energy from the environment in the form of

sunlight or inorganic chemicals to create energy-rich molecules such as carbohydrates.

The best examples of autotrophic organisms are plants on land and algae in water.

Autotrophic organisms prepare food from carbon dioxide, water and sunlight.

Chlorophyll is required to allow photons that released by nuclear fusion reactions in the

sun captured by these organisms, enable them to produce their own food by reducing

carbon dioxide as well as create a storage for chemical energy through photosynthesis.

Photosynthesis is a process of splitting a water molecule, oxygen release into the

atmosphere, and carbon dioxide reduction so that the hydrogen atoms can be released

to fuel the metabolic process of primary production. Water is known as the reducing

agent used by most autotrophs, but hydrogen compounds such as hydrogen sulfide can

also be used by some autotroph.

Basically, autotrophs can be further divided into photoautotrophs,

chemoautotrophs and lithotrophs. By depending on physical energy from sunlight,

photoautotrophic organisms able to manufacture chemical energy in reduced carbon

form. Meanwhile, the chemoautotrophic organisms make use of electron donors as a

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JIB 224 Plant Physiology: Assignment 1

source of energy, which mainly come from inorganic chemical sources. Last but not

least, lithotrophic organisms utilize the inorganic compounds, such as ammonium,

elemental sulfur, ferrous iron and hydrogen sulfide as their reducing agents for

biosynthesis process and storage of chemical energy. Moreover, photoautotrophs and

lithoautotrophs utilizing a portion of the ATP energy that synthesized during

photosynthesis or after the process of oxidation on the inorganic compounds by

reducing the NADP+ into NADPH to produce the organic compounds.

In contrast to autotrophs, heterotrophic organisms require organic compounds as

their source of living energy that have been made by other organisms. Heterotrophic

organism is also known as the consumer of the autotrophic organism. This type of

organisms carry out functions necessary for their life by take in autotrophs as their

foods. When the heterotrophs consume the autotrophs, the carbohydrates, fats, and

proteins stored in autotrophs will be used as the energy sources for the heterotrophs. All

animals, most bacteria and protozoa as well as almost all fungi are heterotrophs.

Heterotrophs normally obtain their energy by breaking down organic molecules from

food source. However, some fungi may also obtain energy from radiation, such as

radiotrophic fungi which can be found inside a nuclear power plant reactor of Chernobyl.

Moreover, carnivorous type of organisms rely on autotrophs indirectly, because the

nutrients obtained by them came from their heterotroph prey that have consumed the

autotrophs.

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JIB 224 Plant Physiology: Assignment 1

References

1. Campbell, N.A., & Reece, J.B. 2008. Biology. 6th Edition. San Francisco (CA):

Benjamin Cummings. p. 1247

2. Hopkins W.G. and Huner, N.P.A. Introduction to Plant Physiology, 2008, 4th

Edition, Wiley and Sons.

3. Moore, R., Clark, W.D., Vodopich, D.S. 2003. Botany. 2nd Edition. New York, NY:

McGraw-Hill Companies, Inc. p. 496-520.

4. Salisbury, F.B. and Ross, C.W. 1992. Plant Physiology. 4th Edition. Wadsworth

Inc. p. 682.

5. Taiz, L., and Zeiger, E. 2006. Plant Physiology. 4th Edition. Sinauer Associates

Inc. Publishers. Sunderland, Massachusetts.