Senescence and Post-Harvest Storage

 

MONOCARPIC SENESCENCE

Monocarpic senescence literally means “flower once then die”.  During the reproductive phase, the “sink” demand of the developing flowers, fruit then seed can drain the vegetative “sources” to the point that senescence occurs.

 

Fig. 1.   Dry matter accumulation patterns for Kent and D66-5566, 1971. (from Egli and Leggert 1973)

 

Fig. 3. Seasonal uptake and accumulation of N, P, K, Ca, and Mg by soybeans at weekly intervals¹ from field hydroponic gravel culture systems.(from Harper 1971)

RESPIRATION AND SENESCENCE

 

All living organisms must conduct respiration in every living cell and at all times.  Sometimes respiration is very fast, for example if the organ if actively growing, and sometimes it barely perceptible, for example if the organ is dormant.  Respiration breaks down glucose and uses the energy that was in the carbon-carbon bond to make metabolic energy (mainly a compound called adenosine triphosphate or ATP).  Carbon dioxide is given off as a by-product.  If there is no oxygen around, then only partial respiration occurs in the form of anaerobic fermentation.  This produces ethanol as a by-product and is the basis of wine making and all fermentation (yogurt, cheese, etc.).

 

One process involving respiration that is particularly important to horticulturist is ripening of fruit.  In climacteric fruit, the respiration rises very rapidly during ripening, then decreases as the fruit senesces.  If you prevent or decrease the rise of respiration, then you can prolong post-harvest storage life.  Ethylene is what causes the increase in respiration, so decreasing ethylene is also a strategy used to increase post-harvest storage life.

 

What are other ways to decrease respiration and prolong the storage life of fruit and vegetable produce or cut flowers?  Look at the equation for respiration.  We can make the reaction go slower by either decreasing things on the left side of the arrow or increasing things on the right side of the arrow.  Practically, we can decrease respiration by either increasing carbon dioxide or decreasing oxygen.  You want to increase carbon dioxide to about 2-5% (up from about 350 ppm in the ambient air) and/or decrease oxygen to about 3% (down from 21% in the ambient air).  You never want to decrease oxygen to near zero, because anaerobic fermentation would occur and anaerobic bacteria might start growing.

 

Of course the easiest way to decrease respiration is to decrease temperature.  You may not have thought about it, but the refrigerator in your house is nothing more than a respiration inhibitor chamber.

 

All of the above is the basis of controlled-atmosphere storage.

 

If in addition to the above, if you store produce or flowers under a light vacuum, you will pull the ethylene out of the inside of the plant and the atmosphere around the plant.  This will dramatically decrease respiration.  This is called hypobaric storage.

 

Summary, we can decrease respiration by doing the following:

§         decrease temperature

§         decrease oxygen

§         decrease pressure, e.g. light vacuum

§         decrease ethylene

§         increase carbon dioxide

NET CHEMICAL EQUATION FOR RESPIRATION

 

 

 

 

 

BIOCHEMICAL REACTIONS OF RESPIRATION

 

Ethylene, Respiration and Senescence Relations

 

 

1)      Climacteric Fruit –    a fruit  where ethylene triggers an increase in respiration and the ripening process..

 

Climacteric Fruit Ripening and Climacteric Rise

 

Climacteric Fruit		Non-Climacteric Fruit
apple apricot avocado banana cantaloupe fig guava ? mango	olive pawpaw peach pear plum persimmon tomato	bell pepper blueberry cherry grape pineapple strawberry citrus watermelon

 

2)      Non-Climacteric Fruit color – used to cause degreening of citrus

 

3)      Flower Senescence

 

a)    Flower fading – flower fade after pollination

 

b)   Flower longevity - causes senescence (death) of cut flowers

 

4)      Leaf Senescence

 

a)    Leaf  epinasty (curling and contortion or leaves) - causes in some plants

 

b)   Leaf abscission (leaf drop) - causes in some plants

 

 

Manipulating Ethylene, Respiration and Senescence

 

1)   Ethylene

 

Biosynthetic Pathway of Ethylene Synthesis

methionine ® s-adenosylmethionine ® 1-aminocyclopropane-1-carboxylic acid ® ethylene

                           (SAM)                                                 (ACC)

 

Ethylene inhibitors - chemicals that inhibit the synthesis or action of ethylene

 

Ethylene Synthesis Inhibitors (block synthesis of SAM ® ACC)

·        AVG - aminoethoxyvinyl glycine

·        MVG - methoxyvinyl glycine

·        AOA - aminoacetic acid

 

Ethylene Action Blockers (ethylene ® block action)

·        STS - silver thiosulfate

·        CO₂ - carbon dioxide

·        Ni - nickel

·        Co – cobalt

·        MCP – 1-mehtylcyclopropane

o          it is a gas that can saturate the receptor sites, and block action for several days

o          EthylBloc – commercial compound

 

2)   Temperature

• respiration decreases when temperature decreases.
• respiration ceases at about freezing temperatures (32 ^oF)
• increasing temperature increases respiration, until temperature gets too high, then respiration decreases when tissue deteriorates

 

3)   Oxygen

• respiration decreases when oxygen decreases
• under very low to no oxygen, anaerobic respiration occurs.

 

 

4)   Carbon Dioxide

• respiration decreases when carbon dioxide increases

Post-Harvest Storage to Extend Shelf Life

 

1)         Refrigeration

 

• decrease temperature – mid-30’s ^oF

 

 

2)   Controlled Atmosphere Storage – CA Storage

 

How Apples are Packaged and Stored Apples are stored in cold storage warehouses. Inside a regular warehouse, apples can be stored for about 5 months because it is cooled to 30-32 degrees Fahrenheit. Inside a special controlled atmosphere warehouse, apples can be stored for almost 12 months because the temperature, humidity, oxygen and carbon dioxide are constantly monitored and controlled to prevent the fruit from ripening too quickly. (from Dole: www.dole5aday.com/ReferenceCenter/ Encyclopedia/Apples/apple_transported2.jsp

 

• low temperature - mid-30’s ^oF

·        high CO_{2 -} approx. 2-5%)

• low 0₂ - approx. 3%)
• high humidity  -approx. 90%)
• ethylene removed -scrubbed

 

 

3)   Hypobaric Storage        low pressure storage, i.e. a light vacuum.

 

Hypobaric Storage Shipping Container

(from http://www.refrigeratedvehicles.com/)

 

·        same as above, plus

·        low pressure

o       decreases 0₂

o       _{decreases ethylene}

 

4)   Modified Atmosphere Packaging – MAP

 

 

MAP broccoli

(from http://www.packagingdigest.com/articles/200203/32.php)

 

MAP uses selectively permeable bags and the fruit or vegetable’s own respiration to maintain an increased level of carbon dioxide and decreased level of oxygen, but avoiding low enough oxygen to avoid anaerobic respiration.

 

Fruits and vegetables continue to respire after harvest.  If you seal them in a plastic bag, the produce will deplete the atmosphere in the bag of oxygen and will cause the produce to undergo anaerobic respiration.  This will causes ethanol and off-flavors to form and may allow anaerobic bacteria to grow and cause spoilage.

 

In MAP, the produce is place in a selectively permeable bag that allows oxygen, carbon dioxide and ethylene to diffuse in and out so equilibrium is set-up between the inside of the bag to the outside of the bag.  The goal is to use a bag that allows some oxygen to diffuse in to avoid anaerobic fermentation, but allow excessive carbon dioxide and ethylene to escape..

 

Permeability of Various Films

Film	Thickness (micron)	Permeability (l/m2/d/atm)
		O2	CO2
polyvenylchloride	14-18	20	120
ethylenevenlyacetate	10-25	32	134
low density polyethylene	25-50	6	20
p0lystyrene	50	4	13

 

The bag must be designed for each fruit and vegetable.  Produce with very high rates of respiration require a bag that allows more oxygen to diffuse in to avoid anaerobic respiration.

Respiration Rates of Vegetables

 

 

Two Types of MAP

 

1)  Passive MAP

 

The produce is put in a bag.  If the permeability of the bag is properly matched with the respiration of the produce, the ideal atmosphere will evolve inside the sealed bag.  Absorbers may be added to scavenge ethylene. 

 

2)  Active MAP

 

The produce is put in a bag, and the air in the bag is replaced with air that has the proper mixture of oxygen and carbon dioxide.  Absorbers may be added to scavenge ethylene. 

Maximum Storage Time with Various Storage Methods

(from http://atlasuhv.com/products/hypobaric_storage/hypobaric_storage.php

 

Commodity	Maximum Storage Time (days)			Hypobaric Benefit Factor
	Standard Refrigeration	Control Atmosphere	Hypobaric Advanced Atmosphere
spinach	14-Oct	slight benefit	50	5 x
avocado (Lula)	30	42-60	>102	3.5 x
banana	14-21	42-56	150	11 x
cherry (sweet)	14-21	28-35	56-70	4 x
lime (Persian)	14-28	juice loss, peel thickens	90+	6.5 x
mango (Fla. varieties)	14-21	little or no benefit	>50	3.5 x
papaya (Solo)	12	12+ (slight benefit)	28	2.3 x
pear (Bartlett)	60	100	200	3.3 x
strawberry	7	7+ (off-flavor)	21	3 x
asparagus	14-21	slight benefit - off odors	28-42	2 x
cucumber	14-Sep	14+ (slight benefit)	49	3.5 x
green pepper	14-21	no benefit	50	3.5 x
mushroom	5	6	21	4.2 x
apples (various)	200	300	300+	1.5 x
carnation (flower)	21-42	no benefit	140	6.6 x
protea (flower)	<7	no benefit	30+	4.2 x
rose (flower)	14-Jul	no benefit	42	6 x

The above data from S.P. Burg in Postharvest Physiology and Hypobaric Storage of Fresh Produce, CABI Publishing, 2004, ISBN 0 85199 801 1