Phytohormones

• E. Starling coined the term “hormone” in connection with the discovery of Secretin.

• Hormones are organic compounds secreted in small quantities that regulate the plant’s activities after being translocated to the organ or site of reaction where they show their specific effect.

• Five major kinds of plant hormones are :

Auxins

Cytokinins

Gibberellins

Ethylene

Abscisic acid

Auxins

• 1898 - proposed by Charles Darwin in “The power of movements in plants” while working on Canary grass (Phalaris cannariensis).

• 1928 - W. Went discovered auxins by Avena curvature test.

• 1931 - Kogl and Haagen Smit isolated Auxin-A (Auxintriolic acid)

• 1934 - Kogl and Haagen Smit isolated Auxin-B (Auxinolonic acid)

• 1935 – isolated heteroauxins.

Natural auxin : Indole-3-acetic acid (IAA)

Synthetic auxins : Nephthalene acid (NAA), indole butyric acid (IBA), indole propionic acid (IPA), 2,4-Dichlorophenoxyacetic acid (2,4-D), 2,4,5-Trichlorobenzoic acid (2,4,5-T) etc.

Funtions of Auxins

• Elongation of the cell.

• Apical dominance

• Root initiation (IAA, NAA, IBA, IPA )

• Herbicides ( 2,4-D, 2,4,5-T)

• inhibit sprouting of potato

• induce flowering (NAA,  2,4-D)

• abscission of fruits.

• stimulate cell wall synthesis and parthenocarpy.

Cytokinins ( Kinetins )

• 1940s - extracted from coconut milk.

Adenine-type cytokinis : kinetin, zeatin, 6-benzylaminopurine (BA )

Phenylurea-type cytokinis : diphenylurea, thidiazuron (TDZ)

Functions of Cytokinins

• Activates cell division.

• Acts in concert with Auxins :  Formation of callus , cell elongation in roots, embryos & fruit

• Acts in reverse with Auxins :  promote axillary bud, inhibits branch roots

• Prevents ageing and senescence in plants.

Gibberellins

•1926 -  isolated from a fungus Gibberella fugikuroi in the “bakanae disease”infected rice plant.

• > 100 naturally occurring gibberellins

Functions of Gibberellins

• induces seed germination

• stimulate stem elongation and leaf expansion.

• break dormancy in potato tubers and tree buds in winter.

• Induces flowering in LD plants

• causes parthenocarpy in pome fruits

• increases the number of male flowers  in moneecious plants

Ethylene

• gaseous hormone.

• synthesized from methionine.

• released from roots, shoot meristems and ripening fruits.

Functions of Ethtylene:

• induces ripening of fruits.

• accelerates abscission of leaves, stems, flowers and fruits.

• induces flowering in pineapple.

• Induces epinasty in leaves.

Abscisic acid (ABA)

•  produced in mature leaves, stem, fruits and seeds.

Functions of Abscisic acid

• accelerates abscission.

• inhibits seed germination

• also accelerates senescence in leaves.

• causes the closing of stomata

• Inhibits fruit ripening 

Climacteric Fruits

Fruits can be classified based on their physiology and ripening behaviour : climacteric and non-climacteric.

Climacteric fruits have high respiration rate during the fruit’s ripening.  During ripening, the production of ethylene increase dramatically. 

In a nutshell, climacteric fruits are able to ripen after harvested.

Non-climacteric fruits do not undergo a rapid ripening phase.  They mature slowly whist attached to the parent plant, and their eating quality cannot improve after harvest.  Non-climacteric fruits have relatively low respiration rates that decline slowly after harvest.  They produce ethylene at low rates.  Application of ethylene has little effect other than the hastening of senescence changes.

In short, non-climacteric fruits are not able to ripen after harvested.  

Climacteric	Non-Climacteric
able to ripen after harvest	NOT able to ripen after harvest
increase respiration rate, peaked, then decline at senesce	NO drastic respiration rate, decline after harvest
lots of biosynthetic activity	LESS biosynthetic activity
lots of ATP consumed	LESS ATP consumed
sharp ethylene production	NO sharp ethylene production
exogenous ethylene trigger endogenous ethylene production	exogenous ethylene DOES NOT trigger endogenous ethylene production

Climacteric is the final physiological process that marks the end of fruit maturation and the beginning of fruit senescence.  It is also defined as a period in the ontogeny of certain fruits in which a series of biochemical changes initiated by the autocatalytic production of ethylene making the changes from growth to senescence involving increased respiration leading to ripening.

Several major changes take place during ripening of climacteric fruits :

1.. production of ethylene

2. rise in respiration rate

3. colour change

4. flesh softening

5. formation of volatiles & flavours

1. Production of ethylene

Ethylene is a natural plant growth regulator that is synthesized by all plants.  It has many biological functions in growing plants.  In fruit, ethylene plays an equally  major role in ripening of climacteric fruits.  Ethylene initiates the ripening of climacteric fruits, hastens their senescence and abscission.

Climacteric fruits show increase of ethylene production as respiration rate peak.  Ethylene is both a promoter and a product of fruit ripening, and an accelerant of senescence in vegetables and cut flowers.  Ethylene exposure brings about an autocatalytic production of ethylene, and thus accelerate ripening.

Ethylene as a ripening trigger is used commercially with banana, avocado and kiwifruit.  Conversely, if fruits are to be stored a long time, then the ambient ethylene must be remove.

2.  Rise in respiration rate

All living organisms must conduct respiration.  Respiration breaks down glucose and produce ATPs.  ATPs were used in conversion of starch to sucrose, which contributes to the sweetness of the fruit.

In climacteric fruits, respiration rate rises rapidly during ripening, then decreases as the fruit senesces.

By decreasing the rate of respiration, post-harvest storage life of climacteric fruits can be prolonged.  So, ethylene is removed using a scrubber.  CO₂ content can also be increased, while O₂ level, temperature and pressure are decreased.

3. Colour change

Colour change by fruits involves chlorophyll loss and increase in production of yellow, orange, red or purple pigments. 

However, non-climacteric fruits also exhibit change of colour.

           

4. Flesh softening

Softening of fruit as hemicellulose & middle lamella degraded.  Degradation of polysaccharides results in senescence and abscission of fruits, as well as development of volatiles and flavours.

5. Formation of volatiles & flavours

Different cell compartments degrade into amino acids, esters, aldehydes, alcohols, terpenoids, etc. to create attractive volatiles and flovours.

Polysaccharides are reduce to monosaccharides which contribute to the sweetness flavour.

These volatiles and flavour join with sugars and acids in creating unique flavour of every fruit.  Mango’s ocimene, myrcene & dimethylstyrene ; muskmelon’s ethyl-2-methyl butyrate, 3-methyl butyl acetate, ethyl butyrate, ethyl hexanoate, hexyl acetate, benzyl aceate ; apple’s butyl ethanoate, 2-metnyl butyl ethanoate, hexyl ethanoate ; strawberry’s furaneol.

	Climacteric	Non-Climacteric
Actinidiaceae	Kiwifruit
Annonaceae	Soursop Sugarapple
Anarcardiaceae	Mango
Arecales	Dates
Bromeliaceae		Pineapple
Bombacaceae	Durian
Caricaceae	Papaya
Cucurbitaceae	Cantaloupes	Cucumber Pumpkin Squash Watermelon
Ebenaceae	Persimmon
Ericaceae		Blueberry
Lauraceae	Avocado
Lythraceae		Pomegranate
Moraceae	Breadfruit     Fig Jackfruit
Musaceae	Banana
Myrtaceae	Guava	Jambu Air
Oleaceae		Olive
Oxaladaceae		Starfruit
Rhamnaceae		Jujube
Passifloraceae	Passionfruit
Rosaceae	Apple     Apricot Nectarines Peach Pear Plum	Blackberry Cherry   Raspberry Strawberry
Rutaceae		Grapefruit     Lime Pamelo Orange Tangerine
Sapindaceae		Lychee        Rambutan
Sapotaceae	Ciku
Solanaceae	Eggplant Tomato	Peppers
Vitaceae		Grape