TRANSPIRATION What is Transpiration An evaporation of water

TRANSPIRATION

What is Transpiration? • An evaporation of water in the form of water vapour from the surface of the plant to the atmosphere • Where does transpiration take place?

Transpiration • Transpiration mainly takes place through openings on leaves – STOMATA • If the stomata is opened, then water vapour will be lost into the atmosphere • In some cases, transpiration also takes place through the lenticels and cuticles

Transpiration 1) A thin film of water covers each mesophyll cell

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Transpiration 1) A thin film of water covers each mesophyll cell 2) There are numerous air spaces between the mesophyll cells

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Transpiration 1) A thin film of water covers each mesophyll cell 2) There are numerous air spaces between the mesophyll cells 3) Water evaporates from the film of water surrounding the mesophyll cells into the air spaces (and eventually diffuses out of the stomata into the atmosphere)

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Transpiration 4) As water diffuses out of the cell, the water potential within the cell will decrease. Due to OSMOSIS, water from adjacent cells will be drawn into the cell to replace the water loss These adjacent cells will in turn draw water from other neighboring cells

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Transpiration 5) Water is drawn from the XYLEM vessels (in the veins) into the neighboring mesophyll cells There will be a water potential difference between the xylem vessels and the mesophyll cells. As water is drawn from the xylem vessels, a suction pressure will develop and this pressure will pull water up the xylem vessels from the roots to the leaves

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Transpiration • The pressure that allows water to be pulled from the roots to the leaves in the xylem vessels is called TRANSPIRATION PULL • It allows the transportation of water and minerals in plants

Investigation #1: Studying the loss of water by a plant into the atmosphere

Procedure polythene bag

Discussions 1) What is the purpose of enclosing the pot and lower part of the plants with a polythene bag? Ans: The polythene bag prevents evaporation of water from the soil, and also prevents vapour released by soil microorganisms from affecting the result of the experiment

Discussions 2) What do you think will happen in the two setups after 2 hours? Ans: Water would be found condensed on the bell jar with the leafy plant inside

Discussions 3) How can you show that it is water? Ans: We can use anhydrous cobalt chloride paper to test it. It will turn the paper from blue to pink. Alternatively, we can use anhydrous copper sulphate. Water will turn it from white to blue

Discussions 4) Why is it better to use forceps instead of fingers to hold cobalt chloride paper? Ans: It is to avoid moisture on our fingers from being absorbed by the cobalt chloride paper

Discussions 5) How would you explain the results of this experiment? Ans: Transpiration occurs in plants through their leaves

I have no trouble absorbing sunlight, but I just keep losing and losing water. Is there any way that I can prevent excessive water loss?

Adaptations to Prevent Water Loss 1) Waxy layer of cuticle on the leaf’s outer surface of the epidermis

Adaptations to Prevent Water Loss 1) A waxy layer of cuticle covers the outer surface of the epidermis 2) More stomata are present in the lower epidermis than the upper epidermis 3) Guard cells control the closing and opening of stomata

Stomata • Stomata are pores in the epidermis where gaseous exchange takes place during photosynthesis (or respiration)

Stoma Guard cell Epidermal cell

Opened Stoma Closed Stoma

Guard Cells • Each stoma is surrounded by two guard cells which contain chloroplasts • It is kidney-shaped • The inner wall is thicker than the outer wall Guard cell Stoma

How Guard Cells Control the Size of Stoma • Guard cells contain chloroplasts that carry out photosynthesis in the presence of light. Carbohydrates are formed and thus lower the water potential of the cell • Water enters the guard cells from adjacent cells by osmosis and guard cells become turgid

How Guard Cells Control the Size of Stoma • The inner wall is thicker than the outer wall, so the cell stretches to the outer side and stoma is opened • At night, there is no photosynthesis. Guard cells become flaccid and so they return to the original shape and stoma is closed

Distribution of Stomata in Leaves

Distribution of Stomata in Leaves 1) Normal Plants - Mainly on the lower surface of plants 2) Floating plants - Mainly on the upper surface of plants - Leaves may also have air sacs to keep them afloat. These sacs can be used in gaseous exchange

Distribution of Stomata in Leaves 3) Submerged Aquatic Plants - No stomata (not required since gaseous exchange can be carried out by diffusion though the leave surface) - No cuticle (the primary function of cuticle is to prevent excess water transpiration which is not present in aquatic plants)

Distribution of Stomata in Leaves 4) Plants in dry and hot conditions - usually have much less stomata to reduce the amount of water loss

Investigation #2: Investigating stomatal distribution in a leaf by using cobalt chloride paper

cobalt chloride paper sellotape Which piece of cobalt chloride paper will turn pink first? Ans: The piece of cobalt chloride paper attached to the lower epidermis of the leaf will turn pink first.

Investigation #3: Comparing the abundance of stomata on the upper and lower surfaces of a leaf

Introduction to Investigation • This investigation allows us to compare the amount of stomata present on the upper and lower surfaces of a leaf by putting the leaf in hot water and observing the amount of bubbles appeared

Procedure forceps hot water leaf

Discussions 1) Which surface has more air bubbles coming off? Ans: There should be more air bubbles appearing on the lower surface of the leaf

Discussions 2) Why do air bubbles appear on the leaf surfaces? Ans: Air in the air spaces between the mesophyll cells in leaf expands on heating and passes out through stomata of the leaf, forming air bubbles

Discussions 3) What does the result show? Ans: The result shows that more stomata are present on the lower epidermis of a leaf

Investigation #4: Comparing the abundance of stomata on the upper and lower surfaces of leaves by weighing

Introduction to Investigation • As water is lost by evaporation, the weight of a detached leaf will decrease with time. In this investigation, leaves will be treated differently (with vaseline) and the loss in weight will then be compared among the leaves.

Procedure A Smear with vaseline on both surfaces of the leaf B Smear with vaseline on lower surface of the leaf only C Smear with vaseline on upper surface of the leaf only D Do not apply vaseline on the leaf

Discussions 1) What is the function of vaseline in this experiment? Ans: It is used to block the stomata so as to prevent water being lost by evaporation

Discussions 2) Which leaf would show the greatest/least change in weight? Ans: Leaf D > Leaf C > Leaf B > Leaf A

Discussions 3) What do the results of leaves B and C indicate? Ans: The change in weight of leaf B is less than that in leaf C, which indicates that more water is lost in leaf C. This suggests that there are more stomata present in the lower surface than the upper surface of leaves

Xerophytes Plants living in hot and dry environment

Adaptations of Xerophytes 1) They have numerous epidermal hair - trap moisture

Adaptations of Xerophytes 1) They have numerous epidermal hair trap moisture 2) Some have sunken stomata - also trap moisture -

Adaptations of Xerophytes 1) They have numerous epidermal hair trap moisture 2) Some have sunken stomata - also trap moisture 3) Some have rolling leaves which enclose the stomata – reduce contact between stomata and the environment

Adaptations of Xerophytes 1) They have numerous epidermal hair - trap moisture 2) Some have sunken stomata - also trap moisture 3) Some have rolling leaves which enclose the stomata - reduce contact between stomata and the environment 4) Some have small, needle-like or spiny leaves, which have a small surface area

Adaptations of Xerophytes 1) 2) 3) They have numerous epidermal hair Some have sunken stomata Some have rolling leaves which enclose the stomata 4) Some have small, needle-like or spiny leaves, which have a small surface area 5) Some have fleshy stems or leaves to help store water

Adaptations of Xerophytes • Some trees also shed their leaves during dry seasons (e. g. autumn) in order to reduce the rate of transpiration

Investigation #5: Comparing the rates of transpiration of a leafy shoot under different environmental conditions using a bubble potometer

Introduction to Investigation • Bubble potometer can be used for estimating the rate of water uptake by a plant. That is, it can be used as an indirect method in measuring the rate of transpiration. In this investigation, the transpiration rates of a plant under different conditions will be compared

Procedure graduated capillary tube leafy shoot reservoir tap air/water meniscus

Results Table Condition Normal condition Under sunlight Near a fan Covered with plastic bag Position of Bubble (Start) Position of Bubble (20 mins. ) Distance Traveled by Bubble

Discussions 1) Why is it important to cut the leafy shoot under water? Ans: This prevents air from entering the xylem vessels of the stem and blocking the water uptake

Discussions 2) What is the relationship between transpiration rate and the distance traveled by the bubble? Ans: The rate of movement of the bubble is proportional to the transpiration rate of the plant. Under normal conditions, the rate of water absorption of a plant is equal to the rate of transpiration

Discussions 3) Compare the transpiration rate of the leafy shoot under the different conditions.

Under Sunlight • The plant has faster transpiration rate when it is placed under sunlight. Stomata open wider when the plant is placed under sunlight. Therefore the diffusion rate of water vapour to the atmosphere, and thus the transpiration rate, increases. High light intensity also increases the atmospheric temperature which in turn increases the rate of transpiration

Near a Fan • The plant has faster transpiration rate when it is placed near a fan. Greater air movement carries water vapour away from the surface of the leaf at a faster rate. This keeps the concentration of water vapour around the stomata at a low level. Hence, water vapour diffuses out of the leaf faster and the rate of transpiration is increased

Covered with Plastic Bag • The plant has a slower transpiration rate when it is covered with a plastic bag. High humidity in the surrounding air decreases the concentration gradient of water vapour between the leaf inside and that of the atmosphere. Hence, water vapour diffuses out of the leaf more slowly and the rate of transpiration is decreased

Limitations • The potometer can only measure the rate of water uptake by the leafy shoot but cannot directly measure the rate of transpiration. If a dehydrated plant is used, the rate of water absorption is higher than the transpiration rate. Under dry condition, the transpiration rate of the plant may exceed its rate of water absorption. • Therefore, when using a bubble potometer, assume that the rate of water absorption by the plant is equivalent to the rate of transpiration

Investigation #6: Comparing the rates of transpiration of a leafy shoot under different environmental conditions using a weight potometer

Introduction to Investigation • Another way to measure the a plant’s rate of transpiration is to measure its loss in weight using an electrical balance. This is called a weight potometer. In this investigation, the transpiration rates of a plant under different conditions will be compared

Procedure

Discussions 1) What is the function adding a layer of oil in the set-up? Ans: The layer of oil can prevent evaporation of water in the flask, which can affect the result

Discussions 2) What is the relation between the transpiration rate and the change in weight of the plant? Ans: Since the water loss from the set-up is due to transpiration only, the change in weight of the set-up is directly proportional to the transpiration rate of the plant

Discussions 3) What limitations may lead to inaccurate results in the experiment? Ans: The initial weight of the set-up may not be accurate because water may be present on the wall of the apparatus and also on the leafy shoot

Discussions 4) Compare the weight potometer and the bubble potometer. Which one is easier to use? Which one is more accurate? Ans: A weight potometer is easier to use and is more accurate to measure the transpiration rate of plants. It is because the weight potometer can measure the rate of transpiration directly whereas the bubble potometer can only measure the rate of water uptake of plants

Environmental Factors Affecting the Rate of Transpiration • There are FIVE environmental factors which affect the rate of transpiration. They are: (I) Light Intensity (IV) Wind Speed (II) Temperature (V) Water Supply (III) Humidity

Light Intensity Å More stomata open wider in light, so plants can get enough carbon dioxide from atmosphere for carrying out photosynthesis Å Light will increase temperature Å Rate of diffusion/evaporation of water vapour through stomata will increase Å Rate of transpiration increases

Effect of Light Intensity on Rate of Transpiration Rate of transpiration Maximum rate Light intensity (lux)

Temperature Rate of diffusion of water vapour from intercellular space in leaf to outside Rate of evaporation of water from mesophyll cells Ability of air to hold water vapour Rate of transpiration

Effect of Temperature on Rate of Transpiration Rate of transpiration Temperature (0 C)

Humidity Rate of Outside Transpiration A decrease in humidity makes the diffusion gradient of water vapour from the moist intercellular space of a leaf to the external atmosphere steeper, therefore the rate of diffusion of water vapour increases

Effect of Relative Humidity on Rate of Transpiration Rate of transpiration Relative humidity (%)

Wind Speed Water vapour around Under very windy conditions, stomata will be closed to reduce water loss the leaf is swept away Transpiration rate Wind blows increases Diffusion gradient between intercellular space in leaves and outside becomes steeper

Effect of Wind Speed on Rate of Transpiration Rate of transpiration Wind speed (km/hr)

Availability of Water Lack of water (plants become dehydrated) Soil dries, plant wilts and stomata close Transpiration rate decreases

Wilting – the leaves and stems become flaccid due to dehydration

Enrichment Reading 9. 1 Daily Changes in Transpiration Rate

Plant Tissues • • • Plant tissues fall into 3 fundamental categories: Dermal tissues Ground tissues Vascular tissues ＊These tissues can be found in roots, stems and leaves

Dermal Tissues • Provides a protective layer around the plant • Exists as a single layer of cells called the epidermis • No chloroplasts

Dermal Tissues • In the shoot system the epidermis secretes a waxy layer called the cuticle (a protective barrier to retard water loss and to prevent infection) • Stomata are found on the epidermis to allow gas exchange to occur • Guard cells control the closing and opening of stomata • Guard cells are the only cells in epidermis that contain chloroplasts

Ground Tissues • Make up the bulk of the plant • Ground tissues are needed for storage, mechanical support and energy production • They can be classified into four broad categories – parenchyma, chlorenchyma, collenchyma and sclerenchyma

Parenchyma Cells • • • The most abundant ground tissues Loosely arranged with intracellular spaces Have thin walls and large vacuoles Metabolically active Perform a variety of functions, such as photosynthesis, repair, food storage and secretion

Parenchyma Cells

Vascular Tissues • • • Continuous throughout the plant Usually embedded in ground tissues Composed of two complex conducting tissues which form the vascular bundles • Xylem + Phloem = Vascular bundles

Xylem • Conduct water and minerals (one way only: from roots to shoot) • Provide support to plant • The cell walls of xylem cells derive most of their strength from lignin, a chemical compound produced only by plants • It is composed of tracheids, vessel elements, fibers, and parenchyma cells

Xylem 1) Tracheids – long, thin cells with closed ends and are dead at maturity. Contain numerous pits through which water moves 2) Vessel elements – similar to tracheids but contain holes at each end are joined end -to-end forming vessels. They are thickwalled and non-living (no cytoplasm and no nuclei)

Phloem • Transport organic materials (glucose) synthesized by the plant from leaves to the rest of the plant • It is composed of sieve tube members, companion cells, fibers and parenchyma cells

Phloem 1) Sieve tube members - a sieve tube, like xylem vessels, is a series of cells (sieve elements) joined end to end. The cross walls between successive sieve elements are perforated, forming sieve plates. The cell walls are thin. Although the cells are living, they lack a nucleus. Unlike xylem vessels, the cells walls are not thickened by lignin

Phloem 2) Companion cells – specialized parenchyma cells that develop alongside a sieve tube member. They are elongated, thin-walled and possess a nucleus. Companion cells are linked with the sieve tubes by small canals filled with cytoplasm, which are smaller than pits. Companion cells help to regulate the metabolic activities of sieve tube elements, and help to load and unload the food for transport

Different Parts of a Dicot Plant • Let’s examine the distribution of different tissues in various parts of a dicotyledonous plant: 1) Root 2) Stem 3) Leaf

Cambium • Layer of thin-walled cells between xylem and phloem • The cambium produces new layers of phloem on the outside and new layers of xylem on the inside, thus increasing the diameter of the stem

Structure of Dicot Root • Outermost layer: epidermis – no cuticle • Cortex – thin walled parenchyma allow movement of water and minerals • Vascular tissues in centre of root • Tip: root cap

Root Cap • A protective layer at the very tip of root • To protect the delicate cells of root from being damaged as the root grows down through the soil • Growing point is behind root cap – by active cell division

Functions of Roots 1) Water and minerals absorption 2) Anchorage

Adaptations for Absorption of Water and Minerals • Extensive branching system – what is advantage? • Outermost layer consists of epidermal cells that lack cuticle – what is advantage? • Some epidermal cells near the root tip have root hairs – what is advantage?

Adsorption of Soil Water by Root Hairs • Soil water is a dilute solution of salts – it is more dilute than the cell sap and cytoplasm of root hair • Water will pass by OSMOSIS into root hair through cell wall and cell membrane

Absorption of Minerals by Root Hairs • Concentration of minerals in soil is usually lower than that inside the root epidermal cells • Can minerals be taken up by osmosis? ? ?

Water and Minerals Transportation • Can you design an experiment to show that water and minerals are transported along the xylem vessels only?

Capillary Action • • • Adhesion Cohesion Against gravity

Root Pressure • Minerals are actively transported from root to xylem • If transpiration rate is low, salts accumulate in xylem and thus water potential is lowered • Therefore, water enters xylem by osmosis • Pressure builds up, pushing the content in the xylem upwards • If stem is cut, water can be seen gushing out (“stem-bleeding”)

Transportation of Organic Substances • Mesophyll cells in leaves (high [ ] of carbohydrates) -> sieve tubes of phloem (low [ ] of carbohydrates) -> active growing areas (e. g. root tips) /storage areas (e. g. fruits)

Why do plants need a support system?

Why do plants need a support system? 1) Prevent leaves from shaded by other plants 2) Allow leaves to receive maximum amount of sunlight for photosynthesis 3) Display flowers as to facilitate dispersal of pollen or seeds

Support in Young Dicot Plants Support in young plants or non-woody parts of plants is contributed mainly by turgidity of the thin-walled cells (parenchyma) in the cortex and pith

Support in Young Dicot Plants • Thin-walled cells in cortex and pith with large central vacuole • Water enters the cell vacuole by osmosis • Pressure exerted on the tough epidermis • Stem becomes hard and upright

Support in Young Dicot Plants Water absorbed by osmosis turgid Water lost by osmosis flaccid Wilting

Support in Young Dicot Plants • Xylem vessels contain cells walls with lignin – thick and rigid • Vascular bundles arranged in a ring near the epidermis – prevent bending • In roots – vascular bundles in centre to give roots more penetrating power and more resistance to stretching (prevent uprooting)

Support in Old Dicot Plants • Majority of xylem is thick-walled cells (e. g. xylem vessels) – make stem rigid • Plant no longer rely on turgidity for support • Support is now contributed mainly be rigidity of the lignified cells in xylem

Support in Old Dicot Plants • Thin-walled cells, cambium, found between xylem and phloem • As plant matures, cambium cells divide to form new cells -- inner side -> new xylem -- outer side -> new phloem • Accumulation of xylem tissues -> stem becomes woody and increases in diameter

Support in Old Dicot Plants