Download Laboratory 9: Transpiration

Laboratory 9: Transpiration YOU MUST KNOW
• The role of water potential and transpiration in the movement of water
from roots to leaves .
• The effects of various environmental conditions on the rate of transpiration.
• How to identify xylem and phloem and relate their structure to their
function.
Overview of the Lab
Transpiration is the major mechanism that drives the movement of water
through a plant. In the first section of this laboratory, you investigate, facto r
that influence the rate of transpiration. In the second section, you study plan
anatomy as it relates to transport.
Hints and Review
I
I
I
I
I
346
Review hydrogen bonding! (Topic 1, Chapter 3) In water, a hydrogen bond is
weak bond between the hydrogen of one water molecule and the oxygen 0 .
another, and it accounts for the unique properties of water, including adhesion
and cohesion.
Water enters a plant through the root hairs, passes through the tissues of the
root into the xylem, and travels up through the xylem vessels into the leaves.
Transpiration is the evaporation of water from the leaves through the stom­
ates. It is the major factor that pulls the water up through the plant.
When water enters the roots, hydrogen bonds link each water molecule to the
next (cohesion) so the molecules of water are pulled up the thin xylem vessels
like beads on a string. The water molecules also cling to the thin walls of the
xylem cells (adhesion). The water moves up the plant, enters the leaves, moves
into air spaces in the leaf, and then evaporates (transpires) through the stomata
(singular, stoma).
Stomata are the pores in the epidermis of a leaf. There are hundreds of stomata
in the epidermis of a leaf. Most are located in the lower epidermis. This reduces
water loss because the lower surface receives less solar radiation than the upper
surface. Each stoma allows the carbon dioxide necessary for photosynthesis to
enter, while water evaporates through each one in transpiration.
PART III: THE LABORATORY
~
Guard cells are cells surrounding each stoma. They help to regulate the rate of
transpiration by opening and closing the stomata. To understand how they
function, study the following figures. As you look at the figures, keep in mind
that an increase in solute concentration lowers the water potential of the solu ­
tion, and that water moves from a region with higher water potential to a
region of lower wa ter poten tial.
Mesophyll ce ll s
Xylem cell wall
Xylem vessel
-.--,....+--
Air space
Water molecule
Guard cell
Adhesion
.-
"""-:;1-- Cohesion
Stoma
Figure 9.la
Figure 9.lb
I Notice that in Figure 9.2a the guard cells are turgid, or swollen, and the stom­
atal opening is large. This turgidity is caused by the accumulation of K+
(potassium ions) in the guard cells. As K+ levels increase in the guard cells, the
water potential of the guard cells drops, and water enters the guard cells.
I In Figure 9.2b, the guard cells have lost water, which causes the cells to become
flaccid and the stomatal opening to close. This may occur when the plant has
lost an excessive amount of water. In addition, it generally occurs daily as light
levels drop and the use of CO 2 in photosynthesis decreases.
Potassium ions
' - --1""­
Turgid guard cell
Vacuole filled
with water
1----...,.- Flacc id guard cell
Stoma c losed
Stoma open
Figure 9.2a
Figure 9.2b
I A leaf needs carbon dioxide and water for photosynthesis. For carbon dioxide
to enter, the stomata on the surface of the leaf must be open. Transpiration
draws water from the roots into the leaf mesophyll. However, the plant must
not lose so much water during transpiration that it wilts. The plant must strike
a balance between conserving water and bringing in sufficient amounts of CO 2
for photosynthesis.
LABORATOR Y 9: TRAN SPIR ATION
347
Air-tight
seal
Calibrated
pipette
Water-filled
tube
Potomete r
Figure 9.3
l One way to measure water loss from a plant is to use a potometer, a device
measures the rate at which a plant draws up water. Since the plant dray.
water as it loses it by transpiration, you are able to measure the rate of tran ­
ration. The basic elements of a potometer are
I A plant cutting I A calibrated pipette to measure water loss I A length of clear plastic tubing I An air-tight seal between the plant and the water-filled tubing I Figure 9.4 shows a typical setup for this exercise_ The rate of water loss in ea ­
condition is measured.
control is the potometer in room conditions.
I The potometer in bright light generally shows a higher rate of water loss, in'''­
I The
cating more photosynthesis than the control.
I The potometer in the fan shows increased water loss compared to the co" ­
trol. The reason is that the air movement results in greater evaporation, 10\- ­
ering water potential outside the plant surface, and more water is pulled fro
the roots.
I The plant cutting inside the plastic bag is in a situation of high humidit
Because of high water potential in the area surrounding the leaves, the rate 0
transpiration will be low.
I Go to Topic 8, Chapter 35, to review the plant tissue types. Refer to Figure 9.
you read this section. You should be able to recognize and know the function o·
the following cell and tissue types:
1. Parenchyma is the most abundant cell type, and cells are relatively unspe­
cialized. The mesophyll of leaves (where most photosynthesis occurs), and
root cortex (starch storage) are parenchyma.
348
PART III : THE LABORATORY
OAssemble 4 potometers .
f) Place each
potom eter in a different environment : room conditions, mist , wind , and bright light.
a. Control:
c. Wind
b. Mist
d. Bright light
room conditions
e Measure water loss in each potameter every 3 minutes for 30 minutes.
Figure 9.4
Phloem
Xylem
Epidermis
1 mm
Figure 9.5
Dieot stem
349
2. Sclerenchym a cells make up fibers, have th ick secondary cell walls, and
3.
4.
5.
6.
often serve a support function.
Collenchyma cells have thickened cell waLls and function in support.
Xylem cells (tracheids and vessel elements) are dead at maturity. Their
function is water transport.
Phloem (sieve tubes and companion cells) transports solutes.
Epidermal cells are outerm ost and serve a protective function.
Questions
1. All of the following enhan ce water transport in terrestrial plants EXCEPT
(A) Hydrogen bonds linking water molecules
(B) Capillary action due to adhesion of water molecules to the walls 0
xylem
(C) Evaporation of water from the leaves
(D) K+ being transported out of the guard cells
-
2. Under conditions of bright light, in which part of a transpiring plant
would water potential be lowest?
(A) Xylem vessels in the leaves
(B) Xylem vessels in the roots
(C) Root hairs
(D) Spongy mesophyll of the leaves
f~
:
m
c - - --'
_W!
D
~
Figure 9.6
Identify each of the structures in the micrograph of a monocot stem abo\' ­
by answering the following questions using the appropriate choices froIT!
the list.
Choices for Tissue Type: xylem, phloem, parenchyma, epidermis
Choices for Function: solute transport, water transport, food storage, pro ­
tection
350
PART III: THE LABORATORY