Transpiration

Introduction

Most of the water a plant absorbs is not used for a plantís daily functioning. It is instead lost through transpiration, the evaporation of water through the leaf surface and stomata, and through guttation, which is the loss of water from the vascular tissues in the margins of leaves.

There are three levels of transport in plants: uptake and release of water and solutes by individual cells, short distance cell to cell transport at tissue and organ levels, and long distance transport of sap by xylem and phloem at the whole plant level. The transport of water is controlled by water potential. Water will always move from an area of high water potential to an area with low water potential. This water potential is affected by pressure, gravity, and solute concentration.

Water moves into the plant through osmosis and creates a hydrostatic root pressure that forces the water upward for a short distance, however, the main force in moving water is the upward pull due to transpiration. This pull is increased by waterís natural properties such as adhesion and cohesion. Transpiration decreases the water potential in the stele causing water to move in and pull upward into the leaves and other areas of low water potential. Pressure begins to build in the leaves, so to prevent downward movement, guttation occurs. Guttation occurs through leaf openings on the leaf margins called hydrathodes. Loss of water through transpiration can be facilitated by the opening and closing of the stomata depending on environmental conditions.

There are three types of cells in plants: parenchyma, sclerenchyma, and collenchyma. Parenchyma cells are the most abundant and are not specialized. They are found in the mesophyll of leaves, the flesh of fruits, the pith of stems, and the root and stem cortex. Sclerenchyma are elongated cells that make up fibers. They have thick secondary walls and the protoplasts often die as they grow older. They are used for support and are found in vascular tissue. Collenchyma cells are living at maturity and have a thickened secondary wall.

Hypothesis

In Lab 9A, all of the plants in this experiment will lose water through transpiration, but those affected by the heat sink and the fan will lose a larger amount of water due to the environmental conditions. This transpiration will pull water from the potometer into the plant. The structure and cell types of a stem cross-section can be observed under a microscope.

Materials

Exercise 9A: Transpiration

The materials needed for this exercise were a pan of water, timer, a beaker containing water (heat sink), scissors, 1-mL pipette, a plant cutting, ring stand, clamps, clear plastic tubing, petroleum jelly, a fan, lamp, spray bottle, a scale, calculator, and a plastic bag.

Exercise 9B: Structure of the Stem

The materials needed for this exercise were a nut-and-bolt microtome, single-edge razor blade, plant stems, paraffin, 50% ethanol, distilled water, 50% glycerin, toluidine blue O stain, a microscope slide and cover slip, pencil, paper, and a light microscope.

Methods

Exercise 9A: Transpiration

The tip of the pipette was placed in the plastic tubing and they were submerged in a tray of water. Water was drawn into the pipette and tubing until no bubbles were left. The plant stem was cut underwater and inserted into the plastic tubing. Petroleum jelly was immediately placed around the tube edging to form an airtight seal around the stem. The tubing was bent into a "U" shape and two clamps were used on the ring stand to hold the potometer in place. The potometer was allowed to equilibrate for ten minutes.

The plant was exposed to a fan, which was placed one meter away and set on low speed. The time zero reading was recorded and then it was continually recorded every three minutes for 30 minutes. After the experiment, all the leaves were cut off the plant and massed by cutting a one cm2 box and massing it.

Exercise 9B: Structure of the Stem

A nut-and-bolt microtome was obtained and a small cup was formed by unscrewing the bolt. The stem was placed in the microtome and melted paraffin was poured around the stem. The paraffin was allowed to dry and the excess stem was cut off. The bolt was twisted just a little and then cut with the blade. The slice was placed in the 50% ethanol. The slices were left in the ethanol for five minutes. Using the forceps, the slices were moved to a dish of the toluidine blue O stain and left for one minute. The sections were rinsed in distilled water. The section was mounted on the slide with a drop of 50% glycerin. A cover slip was placed over the slide. The cross section was observed under a light microscope and drawn.

Results

Table 9.1: Individual Potometer Readings

Time (min)

Beginning (0)

3

6

9

12

15

18

21

24

27

30

Reading (mL)

.02

.03

.04

.05

.06

.07

.09

.10

.11

.13

.13

Class Potometer Readings

Time (min)

Beginning (0)

3

6

9

12

15

18

21

24

27

30

Room

.53

.54

.55

.56

.57

.58

.59

.60

.61

.62

.63

Mist

.34

.36

.38

.40

.42

.43

.43

.44

.45

.45

.46

Light

.67

.68

.69

.70

.71

.72

.73

.74

.75

.77

.79

Fan

.02

.03

.04

.05

.06

.07

.09

.10

.11

.13

.13

Mass of leaves = 1.1 g
Leaf Surface Area = 0.0044 m2

Table 9.2: Individual Water Loss in mL/m2

Time Interval (minutes)

0-3

3-6

6-9

9-12

12-15

15-18

18-21

21-24

24-27

27-30

Water Loss (mL)

.01

.01

.01

.01

.01

.02

.01

.01

.01

0

Water Loss per m2

2.27

2.27

2.27

2.27

2.27

4.55

2.27

2.27

2.27

0

 

Table 9.3: Class Average Cumulative Water Loss in mL/m2

Time (minutes)

Treatment

0

3

6

9

12

15

18

21

24

27

30

Room

0

5

5

5

5

5

5

5

5

5

5

Light

0