Lecture - 16 Seed dormancy and germination, Fruit ripening

Seed dormancy and germination, Fruit ripening:-

Seed dormancy:- A condition in which seeds are prevented from germinating even under the favourable environmental conditions for germination. Seed require a period of rest before being capable of germination.

Causes of seed dormancy:-

> Light

> Temperature

> Hard Seed Coat

> Period after ripening

> Germination inhibitors

> Immaturity of the seed embryo

> Impermeability of seed coat to water

> Impermeability of seed coat to oxygen

> Mechanically resistant seed coat

> Presence of high concentrate solutes

Types of seed dormancy:-

i. Primary dormancy:- The seed is released from the plant, which is already in a dormant state.

ii. Secondary dormancy:- The released seed from a plant becomes dormant due to environmental conditions.

Treatment to break dormancy in seeds:-

a. Seed coat treatment:- These treatments make a hard seed coat permeable to water or gases either by softening or cracking. This process is called scarification. The treatment can be either chemical or physical in nature.

b. Embryo treatments:-

i. Stratification:- The incubation of seeds at an appropriate low temperature over a moist layer before transferring to a temperature suitable for germination.

ii. High-temperature treatment:- Incubation at 40-50 °C for a few hours to a few days may have an effect in overcoming dormancy in some species. For instance, rice seeds treated with hot water at 40°C for at least 4 hours.

iii. Chemical treatments:- Plant growth regulators or other chemicals can be used to induce seed germination.

Advantages of seed dormancy:-

i. In temperate zones, the dormancy of seeds helps the plants to tide over severe cold which may be injurious for their vegetative and reproductive growth.

ii. In tropical regions, the dormancy of seeds resulting from their impermeable seed coats ensures good chances of survival during water stress.

Disadvantages of seed dormancy:-

i. prevents prompt and uniform emergence of seedlings.

ii. interferes with planting schedules.

iii. contributes to "volunteering" of crops.

iv. causes problems to the seed analyst.

Importance of Seed Dormancy:-

> It follows the storage of seeds for later use by animals and man.

> It helps in the dispersal of the seeds through the unfavourable environment.

> Dormancy induced by the inhibitors present in the seed coats is highly useful to desert plants.

> Allows the seeds to continue to be in suspended animation without any harm during cold or high summer temperature and even under drought conditions.

> Dormancy helps seeds to remain alive in the soil for several years and provides a continuous source of new plants, even when all the mature plants of the area have died down due to natural disasters.

Regulation of seed dormancy and germination:- Plant hormones, mainly abscisic acid (ABA) and gibberellin (GA), are the major endogenous factors that act antagonistically in the control of seed dormancy and germination; ABA positively regulates the induction and maintenance of dormancy, while GA enhances germination.

Seed Germination (बीज अंकुरण):- Seed germination is the process by which a seed develops into a new plant. It involves several stages, each requiring specific environmental conditions such as moisture, temperature, and sometimes light.

(बीज अंकुरण वह प्रक्रिया है जिसके द्वारा एक बीज एक नए पौधे में विकसित होता है। इसमें कई चरण शामिल होते हैं, जिनमें प्रत्येक को विशिष्ट पर्यावरणीय परिस्थितियों जैसे नमी, तापमान, और कभी-कभी प्रकाश की आवश्यकता होती है।)

Stages of Germination (अंकुरण के चरण):-

i. Imbibition (Water Uptake) [अंत:शोषण (जल ग्रहण)]:-

Process (प्रक्रिया):- The seed absorbs water, causing it to swell and the seed coat to soften. This stage activates enzymes and metabolic processes necessary for germination.

(बीज पानी को अवशोषित करता है, जिससे यह फूलता है और बीज चोल नरम हो जाता है। यह चरण एंजाइमों और अंकुरण के लिए आवश्यक उपापचय प्रक्रियाओं को सक्रिय करता है।)

Importance (महत्व):- Water is essential for breaking down stored nutrients and preparing the seed for growth.

(पानी संग्रहीत पोषक तत्वों को तोड़ने और बीज को विकास के लिए तैयार करने के लिए आवश्यक है।)

ii. Activation (Metabolic Processes) [सक्रियण (उपापचय प्रक्रियाएँ)]:-

Process (प्रक्रिया):- Enzymes break down stored nutrients in the endosperm or cotyledons to provide energy for growth. Respiration rates increase, and the embryo begins to grow.

(एंजाइम भ्रूणपोष या बीजपत्र में संग्रहीत पोषक तत्वों को तोड़ते हैं ताकि विकास के लिए ऊर्जा प्रदान कर सकें। श्वसन दरें बढ़ जाती हैं, और भ्रूण वृद्धि करने लगता है।)

Importance (महत्व):- Provides the energy and building blocks required for cell division and growth.

(कोशिका विभाजन और वृद्धि के लिए आवश्यक ऊर्जा और निर्माण ब्लॉक प्रदान करता है।)

iii. Radicle Emergence (मूलांकुर उद्भव):-

Process (प्रक्रिया):- The radicle (primary root) emerges first, anchoring the seedling and beginning the absorption of water and nutrients from the soil.

(मूलांकुर (प्राथमिक जड़) सबसे पहले उभरती है, जिससे अंकुर को स्थिरता मिलती है और मिट्टी से पानी और पोषक तत्वों का अवशोषण शुरू होता है।)

Importance (महत्व):- Establishes the root system and supports the seedling's further growth.

(जड़ प्रणाली की स्थापना करता है और अंकुर की आगे की वृद्धि को समर्थन देता है।)

iv. Shoot Emergence (प्ररोह उद्भव):-

Process (प्रक्रिया):- The shoot (stem and leaves) pushes through the soil surface, beginning photosynthesis and further growth.

[प्ररोह (तना और पत्तियाँ) मिट्टी की सतह के माध्यम से उभरता है, प्रकाश संश्लेषण और आगे की वृद्धि को शुरू करता है।]

Importance (महत्व):- Allows the seedling to start producing its own food through photosynthesis.

(अंकुर को प्रकाश संश्लेषण के माध्यम से अपना भोजन उत्पन्न करने की अनुमति देता है।)

v. Seedling Development (अंकुर विकास):-

Process (प्रक्रिया):- The seedling develops true leaves and begins to establish a more extensive root system.

(अंकुर सत्य पर्ण विकसित करता है और एक अधिक व्यापक जड़ प्रणाली स्थापित करता है।)

Importance (महत्व):- Ensures the plant’s ability to grow, reproduce, and mature.

(पौधे की वृद्धि, प्रजनन, और परिपक्वता की क्षमता सुनिश्चित करता है।)

Factors Affecting Seed Germination (बीज अंकुरण को प्रभावित करने वाले कारक):-

i. Water (जल):- Essential for imbibition and enzyme activation. Too much or too little water can hinder germination.

(अंत:शोषण और एंजाइम सक्रियता के लिए आवश्यक। बहुत अधिक या बहुत कम जल अंकुरण को बाधित कर सकता है।)

ii. Temperature (तापमान):- Each plant species has an optimal temperature range for germination. Extreme temperatures can inhibit the process.

(प्रत्येक पौधे की जाति के लिए अंकुरण के लिए एक इष्टतम तापमान सीमा होती है। अत्यधिक तापमान इस प्रक्रिया को रोक सकते हैं।)

iii. Light (प्रकाश):- Some seeds require light to germinate, while others need darkness. Light-sensitive seeds may have specialized mechanisms for light detection.

(कुछ बीजों को अंकुरण के लिए प्रकाश की आवश्यकता होती है, जबकि अन्य को अंधकार की। प्रकाश-संवेदनशील बीजों में प्रकाश पहचान के लिए विशेष तंत्र हो सकते हैं।)

iv. Oxygen (ऑक्सीजन):- Germination requires adequate oxygen for respiration. Compacted soil or waterlogged conditions can limit oxygen availability.

(अंकुरण के लिए पर्याप्त ऑक्सीजन की आवश्यकता होती है। संकुचित मिट्टी या जल-जमाव वाली परिस्थितियाँ ऑक्सीजन की उपलब्धता को सीमित कर सकती हैं।)

v. pH Levels (pH स्तर):- The acidity or alkalinity of the soil can affect enzyme activity and seedling health.

(मिट्टी की अम्लता या क्षारीयता एंजाइम गतिविधि और अंकुर स्वास्थ्य को प्रभावित कर सकती है।)

Special Germination Strategies (विशेष अंकुरण रणनीतियाँ):-

i. Dormancy (सुप्तावस्था):- Some seeds have built-in dormancy mechanisms that prevent germination until conditions are favorable. This can include physical barriers, physiological processes, or requirements for specific environmental cues.

(कुछ बीजों में अंतर्निर्मित सुप्तावस्था तंत्र होते हैं जो अनुकूल परिस्थितियों के आने तक अंकुरण को रोकते हैं। इसमें भौतिक अवरोध, शारीरिक प्रक्रियाएँ, या विशिष्ट पर्यावरणीय संकेतों की आवश्यकताएँ शामिल हो सकती हैं।)

ii. Scarification (अपरिष्करण):- Some seeds need physical or chemical abrasion to break down their tough seed coat and promote germination.

(कुछ बीजों को उनके कठोर बीज कोट को तोड़ने और अंकुरण को बढ़ावा देने के लिए भौतिक या रासायनिक घर्षण की आवश्यकता होती है।)

iii. Stratification (स्तरीकरण):- Certain seeds require a period of cold temperatures to simulate winter conditions before they will germinate.

(कुछ बीजों को अंकुरण से पहले सर्दियों की स्थिति का अनुकरण करने के लिए ठंडे तापमान की अवधि की आवश्यकता होती है।)

Fruit Ripening:-

> Fruit ripening is a genetically programmed stage of development overlapping with senescence.

> The fruit is said to be ripe when it attains its full flavour and aroma and other characteristics of the best fruit of that particular cultivar.

> The words “mature “and “ripe” are essentially synonymous when used to describe these fruits that ripe on the plants known as non-climacteric.

> However, in case of climacteric fruits a mature fruit require period before attaining a desirable stage of edibility.

Table 1. List of climacteric and non-climacteric fruits

Changes during Fruit Ripening:-

1. Cell Wall Changes:-

- Cell wall consists of pectic substances and cellulose as the main components alongwith sma1amounts of hemicellulose and non-cellulosic polysaccharides.

- In cell wall, the changes particularly in the middle lamella which is rich in pectic polysaccharides are degraded and solubilised during ripening.

- During this softening, there is a loss of neutral sugars (galactose and arabinose-major components of neutral protein) and acidic pectin (rhamnogalacturonan) of all cell wall.

- The major enzymes implicated in the softening of fruits are pectine1asterase, polygalacturonase cellulase and β- galactosidase.

2. Starch:-

- During fruit ripening sugar levels within fruit tend to increase due to either increased sugar importation from the plant or to the mobilization of starch reserves within the fruit, depending on the fruit type and whether it is ripened on or off the plant.

- With the advancement of maturity, the accumulated starch is hydrolysed into sugars (glucose, fructose or sugars) which are known as a characteristic event for fruit ripening.

- Further breakdown of sucrose into glucose and fructose is probably mediated by the action of invertase.

- In vegetables like potato and peas on the other hand, the higher sucrose content which remains high at fresh immature stage, converts into starch with the approach of maturity.

3. Organic Acids:-

- With the onset of fruit ripening there is downward trend in the levels of organic acids.

- The decline in the content of organic acids during fruit ripening might be the result of an increase in membrane permeability which allows acids to be stored in the respiring cells, formation of salts of malic acid, reduction in the amounts of acid translocated from the leaves, reduced ability of fruits to synthesize organic acids with fruit maturity, translocation into sugars and dilution effect due to the increase in the volume of fruit.

4. Colour:-

- With the approach of maturation, the most obvious change which take place is the degradation of chlorophyll and is accompanied by the synthesis of other pigments usually either anthocyanins or carotenoids.

- They can give rise to a wide range of colours (from red to blue).

- The chloroplasts in green immature fruit generally lose chlorophyll on ripening and change into chromoplasts which contain carotenoid pigments.

- Carotenoids are normally synthesized in green plant tissue a major product being 3-carotene.

- However, in many fruits additional - carotene and lycopene is synthesized during ripening.

5. Flavouring Compounds:-

- Although fruit flavour depends on the complex interaction of sugars, organic acids, phenolics and volatile compounds but the characteristic flavour of an individual fruit or vegetable is derived from the production of specific flavouring volatile.

- These compounds are mainly esters, alcohols, aldehydes, acids and ketones. At least 230 and 330 different compounds in apple and orange fruits have been indicated respectively.

6. Ascorbic Acid:-

- L-ascorbic acid (Vitamin C) is the naturally occurring ascorbic acid in fruits.

- A reduced amount of ascorbic acid is noticed in pome, stone and berry fruits at the time of harvest.

- An increase in ascorbic acid content with the increase in fruit growth has been and the levels declined with the advancement of maturity and onset of fruit ripening in pear, sweet potatoes, potato, asparagus and okra during the course of post harvest handling.

7. Phenolics:- The phenolic content of most fruits declines from high levels during early growth to low levels when the fruit is considered to be physiologically mature and thereafter susceptible to the induction of ripening.

8. Amino Acids and Proteins:-

- Decrease in free amino acid which often reflects an increase in protein synthesis.

- During senescence the level of free amino acids increases reflecting a breakdown enzymes and decreased metabolic activity.

9. Ethylene Production and Respiration:- Physiological events responsible to ripening process are as follows:

a. Ethylene production

b. Rise in respiration

a. Ethylene production:- In climacteric fruits such as mango, banana, ethylene production increase and causes:

i. Rise in respiration

ii. Rise in temperature

iii. Rise in activity of hydrolytic enzymes

- Ethylene is produced from an essential amino acid — methionine.

Following the steps as below:

b. Rise in respiration:-

- Respiration is required for releasing energy and the substrate for synthesis of several organic compounds required in the ripening process.

- During ripening in climacteric fruits, there is rise in respiration called climacteric.

- The clirnacteric peak is obtained very fast when temperature is relatively high.

- Respiration is a most deteriorating process of the harvested fruits and vegetables which leads to the oxidative breakdown of the complex materials (carbohydrates or acids) of cell into simpler molecules (CO2 and water) with the concurrent production of energy required by the cell for the completion of chemical reactions.

- In brief, the process of respiration can be summed up with the following reaction:

C6H1206 +6O2 6 CO2 + 6 H20 + energy

Use of Chemicals for increasing shelf life of fruits and vegetable:-

a. Ethylene absorbent:-

- Ethylene is responsible for decreasing shelf life.

- Putting KMNO4 @ 100 ppm soaked filter paper can minimized ripening and increase shelf life.

- In Banana this method is very useful.

b. Antifungal Agents:-

- SOPP: Sodium orthophenylphenate

- Diphenyl wraps protection against moulds, stem-end rot.

- Dibromoletrachloroethane and esters give better flavour.

c. Use of Inhibitors:-