L14 - Ripening and Senescence

Question 1

Ripening:

• Definition
• Changes that happen
• When does it take place

Answer 1

Ripening: Biochemical and structural changes that make the fruit attractive for us (animals) to eat (and distribute the seeds)

● Changes in color&raquo_space; attractiveness
● Volatile production&raquo_space; aroma
● Softening&raquo_space; texture, juiciness
● Changes in nutrient composition&raquo_space; taste
● Ripening may take place plant attached, or after harvest

Takes place both when attached to plant or after harvest

Dependent on the temperature

Question 2

Climacteric fruits

Answer 2

Climacteric: ethylene plays an important role in the ripening. Often an increase in respiration is observed

So fruits that can be harvested and ripened off the plant. Fruits undergo significant physiological changes.

Large increase in sugar during ripening because they have starch

Question 3

Non climacteric fruits

Answer 3

Fruits that are not capable of continuing the ripening process (physiological changes) once removed from the plant

No increase in sugar content

Decrease in respiration after harvest

Changes in firmness, external color and aroma

Question 4

List of Climacteric/Non climacteric fruit

Answer 4

Slides 4-5

Question 5

Color change. Example in fruits

Answer 5

The chlorophyll is broken down and other compounds may be produced during ripening

• Tomato: lycopene produced

• Banana: the yellow compounds
were already there

• Apple: anthocyanin
produced

 Avocado: specific anthocyanins produced (cyanidine-3-O glucoside)

 Mango flesh :
carotenoids produced

 Mango peel : anthocyanin produced

Question 6

Firmness loss, softening

Answer 6

Cell walls loosen due to the action of several
enzymes (e.g. cellulase , pectate lyase , pectin
methyl esterase, poly galacturonase , glucosidase, xylosidase, expansin, XET) -> Fruit softens

Cell wall properties determine the textural features of the product (Crunchy, mealy, juicy)

Question 7

Nutritional changes during ripening

Answer 7

In many (but not all) fruit (apple, mango, banana, pear, ..), starch (or another carbohydrate storage form) is converted into sugar
(sweet) and/or acids are broken down (may also be converted into sugar.

In some products (strawberry) sucrose is converted into gl+fr which may make the fruit more sweet

Several enzymes are involved. Sugars may stimulate respiration (ATP production)

Question 8

Starch and ripening

Answer 8

Note that starch is a poly glucose molecule: many conversions take place during ripening

Amount of starch = ripeness indicator

Question 9

Volatiles and ripening

Answer 9

Volatiles produced during breakdown of:

• Protein
• Fatty acids
• Phenolic compounds
• Carotenoids
• Amino acids

Volatile production reflects the ongoing ripening processes

Question 10

Senescence or Aging. What is it?

Answer 10

An ordered process that leads to the death of a cell, not just an accidental event. Programmed Cell Death

Senescence is an active process which is slowed down at lower temperature

Ethylene may stimulate the process

Question 11

PCD types in animals

Answer 11

 Apoptosis:
Cell contents of dying cell is degraded in other cells (type 1)

Autophagic cell death: Large scale autophagy (self eating) prior to or during cell death (type 2)

 Programmed necrosis: No self eating, no degradation of cell contents in other cells. Swelling and breakdown of cell membrane (type 3)

All forms of programmed cell death may be accompanied
by “apoptotic features”

Question 12

More about apoptosis

Answer 12

Most cells die in an ordered way showing recognizable morphological features

Apoptotic bodies are taken up and content is degraded in lysosome (vacuole) of other (living) cell

Question 13

Morphological features of Autophagic PCD

Answer 13

 Large scale autophagy is observed ( Cytoplasmic components
are degraded in lytic compartments of the same cell)

 Also some of the apoptosis features are observed

 It is not clear if autophagy is the reason that the cell dies

 It may be that when all components have been eaten, the cell enters the apoptotic pathway

Question 14

Necrosis

Answer 14

 Happens in response to severe stress

 Gives rise to inflammatory response

 The process can be non programmed, but also be executed in a controlled way (called programmed necrosis, type 3)

Question 15

Senescence: General information

Answer 15

 Cells die in an ordered way, they follow a program

 Often, at first most cell content is degraded in the
vacuole ( may also include essential vitamins

 Degradation products are redistributed and re used

 The type of cell death observed during over ripening and
senescence in fruit, leaves and flowers is most similar the animal Autophagic Cell Death (self eating)

 In plants this type of cell death has now been termed Vacuolar Cell Death

Question 16

Determination of PCD, tools?

Answer 16

● Microscopic observations (nuclear and cellular changes)

● DNA laddering (suggest the involvement of a cell death specific endo nuclease)

● TUNEL staining (detects double strands
DNA breakdown in the nucleus)

Question 17

What happens during senescence

Answer 17

 Increasing number of cells die

 Cell content floods into the intercellular spaces

 Degrading enzymes break down the tissue structure

 Result: softening, loss of crispness, slimy, glassiness browning and finally desiccation

Question 18

Postharvest stresses can lead to cell death

Answer 18

 Handling (bruising)
 Treatments (hot water, Ozon, Low oxygen, high CO2..)
 Chilling injury

Question 19

Senescence. How does it work? (Summary)

Answer 19

Schematic slide 60

Question 20

Summary of senescence

Answer 20

● Cells often die in an ordered manner , with the aim
to degrade the cell contents and redistribute and
re use the building blocks (in the intact plant)

● Many deteriorative processes postharvest are
accompanied by PCD

● Ethylene is a stimulator of PCD

● The PCD machinery involves oxidative stress (ROS, H2O2) and a class of specific proteases caspase
like proteases ) that cleave specific substrates leading to cell death with typical morphological
features