L5 - Transpiration

Question 1

What determines transpiration?

Answer 1

( Transpiration = Loss of water
through the gas phase)

1) Vapor pressure is the pressure
exerted by a vapor in equilibrium
with solids or liquids. (ex high VP at normal T
-> volatile)

2)Driving force: difference in vapor
pressure (VPD) between water in
the gas phase of the plant and the
outside air

Question 2

Role of transpiration

Answer 2

▪ Plants/trees: driving force for uptake of minerals
▪ Plants/trees: Way to regulate the temperature of leaves
(A full grown oak may transpire up to 400 liter per day )

Question 3

What determines VP difference?

Answer 3

• Humidity in surrouding air (humidity inside of the product =100%)
• temperature of both air and products

Question 4

1. If VP in plant is higher the VP in surrounding air product will _____ water through _____.
2. Higher VP means ____ water loss.

Answer 4

1. lose - diffusion

2. more

Question 5

What is relative humidity ?

Answer 5

RH= current vapor pressure/maximal vapor pressure

-influenced strongly by temperature

Question 6

When you increase temperature what happens to the RH ?

Answer 6

RH increases

Question 7

Intercellular spaces contain SATURATED water vapor.

• Vapor pressure is maximal at the given temperature
• inside the product = 100% RH

Answer 7

Storage atomosphere is less saturated with water vapor

Question 8

Calculate water flux

Ohms law

Answer 8

Ohms law: V=I x R or
VPdiff = weight loss x resistance
▪ Resistance= VPdiff / weight loss (assumed constant)
-> see slide 16 for calculations

Question 9

What happens during cooling
increase of Temperature difference results to :
a. water loss
b. VP diff

Answer 9

a. increase of water loss
b. increase of VP diff
* often slow cooling is used but faster is better in principle

Question 10

What is the natural fruit wax on apples

Answer 10

Ursolic acid and is highly water repellent

It increases during preharvest and postharvest (feels fat)

Question 11

What is cuticula used for ?

Answer 11

Cuticula: transpirations pathway
-> protective, hydrophobic, waxy coverings produced by the epidermal cells to minimize water loss and effectively
reduce pathogen entry…

Question 12

Transpiration pathways

Answer 12

▪ Stem scars….until wound healing (they start to shrivel 5% water loss)
▪ Lenticels formed from stomata
▪ Stomata
▪ injuries/lesions (mechanical or diseases)

Question 13

Stomata regulate transpiration

Answer 13

• mainly on leaves and stems
• most fruts and flower petals do not have stomata -> unripe younf fruits have stomata
• leafy vegetabels have stomata

*Transpiration rate depends on the opening of stomata,number of stomata, area or leaf.

Question 14

Functional stomata

When do they close?

Answer 14

• in the dark
• when internal CO2 concentrations rise (no photosynthesis)
• water stress (synthesis of ABA)

Question 15

Transpiration in flowers

Answer 15

• during dry storage may lose water rapidly due to large area of leaves
• are often transported in water (or preservative solutions)
• during the vase life the water balance often becomes negative (more transpiration than water uptake)

Question 16

Xylem blockage

a) prevention for bacteria
b) prevention for air embolisms
c) prevention of enzymatic wounding reactions

Answer 16

a) antibacterial compounds (not allowed)
- chlorine compounds (CLO2
- silver thiosulfate, ethylene inhibitor
b) cut under water
c) at least 4h of cold water (inactivates enzymes)

Question 17

Prevention of water loss

*up to 10% water loss does not affect quality

Answer 17

1. cool products down
2. low T high H
3. low air movement (boundary layer)
4. minimize temperature fluctuations
5. waxes, coatings, packaging
6. prevent skin damage
7. for leaves: place in the dark (stomata close)
8. Decrease surface area (remove leaves)
9. anti transpirants (chemicals that lead to stomatal closure ex ABA)