Seed and Fruits Chapter 6 PDF

CHAPTER 6 SEED AND FRUITS By Hamizah Othman At the end of this chapter, students should be able to: Recognize the seed structure Explain the seed germination Explain the seed dormancy Describe the fruit development SEED STRUCTURE OVERVIEW All flowering plants produce seeds. Differe...

CHAPTER 6 SEED AND FRUITS By Hamizah Othman At the end of this chapter, students should be able to: Recognize the seed structure Explain the seed germination Explain the seed dormancy Describe the fruit development SEED STRUCTURE OVERVIEW All flowering plants produce seeds. Different seed is found in different shapes, sizes, and colors. It can be a pip (like an apple), a nut (in walnut), or a bean (pea, pulses) A seed is a SMALL EMBRYONIC PLANT enclosed in a covering called SEED COAT, usually with some stored food. Develop from the fertilized ovules Generally , a seed has three parts: Seed Coat Endosperm Embryo SEED COAT It is the outermost covering of a seed. It develops from the integuments of the ovule. The seed coat usually consists of two layers. A hard and leathery outer layer called testa. A thin and papery inner layer called tegmen. The seed coat protects the seed from environmental factors. A thick seed coat protects the seed from sunlight and water. It prevents the loss of water and entry of parasites within the seeds. The hard seed coats prevent germination during unfavourable environmental conditions. An opening in the integument of the ovule is known as the micropyle and is visible on some seed coats. The hilum is also visible which is equivalent to the naval in humans where the umbilical cord is attached. COTYLEDON Cotyledon is the embryonic seed leaf and is also called seed leaf. It stores food for the growing embryo. After the seed germinates, the cotyledons emerge, enlarge, and become green. Based on the number of cotyledons in a seed, seeds can be of two types - Monocotyledonous seeds - Possess a single cotyledon, e.g, maize, wheat, etc. Dicotyledonous seeds - Possess two cotyledons, e.g, beans, gram, etc. EMBRYO The embryo consists of an embryonal axis which has a radicle at the lower end, which eventually develops into the root system, and a plumule at the other end, which eventually develops into the shoot system. ENDOSPERM It is a triploid nutritive tissue which has stored food for the developing embryo. It is formed by the fusion of one of the haploid male gametes with the diploid central cell within the embryo sac.  Based on whether the mature seed possesses an endosperm, seeds can be of two types- Endospermic or Non-endospermic or albuminous seeds exalbuminous seeds Endospermic or albuminous seeds Mature seeds in which the endosperm persists  and is not completely used up during embryo development Example of dicot endospermic seed - castor. Example of monocot endospermic seed - rice, maize. Non-endospermic or Exalbuminous Seeds Mature seeds in which the endosperm does not persist  and is completely used up during embryo development Example of dicot exalbuminous seed - beans, grams, peas, etc. Example of monocot exalbuminous seed - orchid seeds. STRUCTURE OF MONOCOTYLEDON SEED A monocotyledon seed is characterized by having a single cotyledon and typically, a prominent endosperm that stores food. In a typical monocot seed the seed coat is membranous and fused with the fruit wall. Endosperm is bulky and stores food in the form of starch. Examples of Monocotyledon Seeds Cereals: Rice, wheat, maize. Grasses: Barley, oats. Others: Palm seeds, bamboo. The outer layer of the endosperm is proteinaceous and is known as the aleurone layer. Embryo is small and situated in a groove at one end of the seed. It has one large and shield shaped cotyledon which is in a reduced state and is known as scutellum. Short embryonal axis has a plumule and a radicle. The plumule is covered by a sheath called the coleoptile. The radicle is covered by a sheath called coleorhiza. STRUCTURE OF DICOTYLEDON SEED A typical dicot seed such as a bean seed has the following parts Hilum - It is a scar which marks the site of attachment of the seed to the ovary wall. Micropyle - It is a pore that lies just below the hilum.  Absorption of water and exchange of respiratory gases occurs through the micropyle. Seed coat - Hard protective covering of the seed which consists of an outer testa and inner tegmen.  Hilum Micropy le **Label in red box are Embryo Opening up the seed exposes two massive and fleshy cotyledons which store food for the growing embryo. The cotyledons are laterally attached to an embryonal axis  which consists of a radicle and a plumule. The region between the radicle and the point of attachment of the cotyledons  is known as hypocotyl. The region between the plumule and the point of attachment of the cotyledons  is known as epicotyl Difference Between Monocot and Dicot Seeds DICOT SEED MONOCOT SEED There are two cotyledons in the There is a single cotyledon in the seed seed Endosperm is absent in most of Endosperm is present in most of them but present in a few of them them and absent in a few of them There is no protective sheath for The radicle is protected radicle and plumule by coleorhiza and the plumule is protected by coleoptile Why seeds advantages for plant?  Maintain dormancy until better environmental condition arise  Contain enough food supply until photosynthesis possible  Dispersal of plants  Afford protection to young plant at vulnerable developmental stage SEED GERMINATION SEED GERMINATION The process by which a dormant embryo becomes active, grows out of the seed coat and develops into a seedling is called seed germination. The embryo uses the water from outside and the food stored from the cotyledons or endosperm to grow. Seeds of most plants germinate when they are provided with favorable environmental conditions such as: Moisture/ Water :  To germinate, seeds must to take in water (imbibition) Air (Oxygen) :  Oxygen in the presence of enough moisture causes respiration to start metabolism;  This creates energy for the germination process Suitable temperature:  Most common annual vegetables have optimal germination temperatures between 24- 32 0C Proper sequence of light and darkness:  Generally, seeds require darkness to germinate but sometime it requires light. STAGES OF SEED GERMINATION IMBIBITIO ACTIVATION GROWTH RESPIRATIO EMERGENC N OF OF N OF E WATER ENZYMES EMBRYO STAGES OF SEED GERMINATION 1. Imbibition:  The seed absorbs water through the micropyle, causing it to swell.  This activates hydrolytic enzymes, initiating metabolic activities. 2. Activation of Enzymes:  Enzymes such as amylase, protease, and lipase are activated.  Stored food in seed (starch, proteins, and fats) in the cotyledons or endosperm is broken down into soluble forms like sugars and amino acids.  leading to the release of nutrients required for growth. 3. Respiration:  The seed switches from anaerobic to aerobic respiration as oxygen becomes available.  ATP is produced for cellular activities and growth. 4. Growth of the Embryo:  As a result, the embryo expands, and the seed coat breaks,  allowing the emergence of the primary root and the shoot.  The radicle grows downward to form the primary root.  The plumule grows upward to form the shoot. 5. Emergence:  The seedling emerges from the soil,  marking the end of germination  and the start of independent growth. Types of seed germination Radicle is the first part which comes out during seed germination. It comes out through the micropyle and develops into the root system. This helps the young plant to fix in the soil and to absorb water, nutrients etc. after this the epicotyl and hypocotyl develop and form the shoot system.  Hypogeal germination Types of Germination Epigeal Germination HYPOGEAL GERMINATION The hypogeal germination of the seeds is when:  the epicotyl grows first and  cotyledons remain underground The radicle grows deep into the soil and a root system is formed. Then the epicotyl elongates and forms an arch called epicotyl bent.  This pushes the plumule upwards out of the soil.  Hypocotyl growth is restricted Plumule grows fast leaving the cotyledons in the soil.  Cotyledons never become green and  gradually they dry out and fall.  Hypogeal germination is common in monocotyledons like rice, wheat, maize, palms etc.  Some dicots like bengal gram, black gram, pea, mango etc also show this kind of germination.  Hypogeal germination Process:  Seed absorb water caused seed coat to swell  Radicle grows down  1st internode and epicotyl grow up  new leaves form & start food production  New root system develops above the 1st internode just beneath the soil  Radicle dies after permanent roots form https://makeagif.com/gif/runner-bean-hypogeal-germination-time-lapse-filmed-over-24-days-OIR-nS EPIGEAL GERMINATION The epigeal germination of the seeds in where:  the hypocotyl grows first,  pushing up the cotyledons to the soil surface Hypocotyl elongates rapidly and pushes the cotyledons upwards above the soil.  These cotyledons act as the first leaves of the plant. In most cases cotyledons become green, flat and leaf like.  The green cotyledons can make food for a while.  The growing stem utilises this food.  This will finally dry up and fall off and seedling becomes an independent plant.  Dicot plants like castor, cotton, tamarind etc. show epigeal germination.  It can also be observed in a few monocots like onions. SEED DORMANCY SEED DORMANCY Seed dormancy is defined as a state in which seeds are prevented from germinating even under environmental conditions normally favourable for germination. OR Dormancy is the inability of certain seeds to germinate because of the thick seed coat or because of the existing environmental factors such as temperature and moisture. Dormancy is caused by conditions within the seed that prevent germination under normally ideal conditions. Three kinds of SEED COAT Coat is too hard or too thick. DORMANCY Air and water cannot penetrate. dormancy RUDIMENTARY Primitive or Immature embryos EMBRYOS Embryo requires some metabolic EMBRYONIC change to stimulate germination. DORMANCY Germination held in check by chemical mechanisms. Seed dormancy is a survival mechanism or evolutionary adaptation of plants. It is a natural process of preservation. Dormancy follows the storage of seeds for later use by animals and man. In temperate zones, the dormancy of seeds helps the plants to tide over severe colds which may be injurious to their vegetative and reproducing growth. In tropical regions, the dormancy of seeds resulting from their impermeable seed coats ensures good chances of survival. Helps in the dispersal of the seeds through unfavorable environmental conditions. Importance of Seed Dormancy Important for plant survival during drought and cold temperature Seed can remain alive for several years that can provide a continuous source of new plants even when all the mature plants of the area have died. Helps seed to get dispersed over long distances through unfavorable environment Give additional time for seed dispersal over distances Helpful in creation of a seed bank Postpone germination and ensures that seed will germinate only under optimal light, temperature and moisture CAUSES OF SEED DORMANCY 1. Hard Seed Coat  Seeds of many species possess hard seed coat (leguminoceae and malvaceae)  Such seeds remain dormant.  It is due to the seed coat that covering the embryo are too hard and impermeable to water, and oxygen which prevents germination.  Hard seed coat prevents germination due to following reasons: i. Hard seed coat prevents the entry of water into the seed. ii. Hard coat obstruct exchange of gases, especially oxygen. Oxygen is necessary for respiration. iii. Hard seed coat causes mechanical resistance, thus radicle does not come out CAUSES OF SEED DORMANCY 2. Rudimentary/ Immature Embryo The embryo is not fully developed inside the seed at dispersal. Seeds remain dormant and fail to germinate. A dormancy that is inherent in the embryo and is not due to any influence of the seed coat. E.g. Pinus sp. 3. Excessive Salts In some seeds contain a high concentration of solutes which do not allow the embryo to resume its growth. CAUSES OF SEED DORMANCY 4. Light Sensitive Seeds  There two type of seeds for light sensitivity: i. Positive photoblastic seeds:  Seeds of most of the species germinate equally well both in the dark and in light.  Therefore light is not necessary for germination.  However, seeds of some species like lettuce remain dormant in the dark.  They do not germinate till these are exposed to light. These are called positive photoblastic seeds.  Therefore dark inhibits germination in these seeds. ii. Negative photoblastic seeds:  Seeds of some species like phlox.  Nemophila and Silene remain dormant when exposed to light.  They germinate only in the dark.  These are called negative photoblastic seeds. CAUSES OF SEED DORMANCY 5. Chilling Requirement  Seeds of many temperate trees remain dormant and do not germinate after harvest.  Eg: apple, walnut and pinus.  Such seeds need chilling temperature (1 - 5 degrees) for a few weeks.  This requirement is met in nature.  These seeds lie buried in the soil during winter.  Therefore they get chilling temperature.  But some seed become dormant in nature.  They need artificial chilling. CAUSES OF SEED DORMANCY 6. Dormancy due to growth inhibitory chemicals There are several naturally occurring compounds which inhibit germination of seeds. Seeds of many species contain several phenolic compounds. These phenolic compounds are ferulic acid, parasorbic acid and coumarin. These compounds are present in the embryo or in the seed coats. It is also due to the presence of inhibitors like abscisic acid (ABA) and the absence of growth promoters such as giberellins acid (GAs). Methods to overcome dormancy Seed coat dormancy 1. Mechanical scarification – rub seed to rough surface 2. Heat water scarification– soak in hot water 3. Acid scarification– soaks seed in acid Embryo dormancy 1. Stratification – expose seed to low temperature Scarification  The process of breaking, scratching, mechanically altering or softening the seed coats to make permeable to water and gases. 1. Mechanical Scarification  Example by rubbing them against rough surface using sand paper or puncturing the seed coat with pointed needle or knife. 2. Hot water scarification Drop into hot water (77 to 100oC). Remove from heat, allow to cool and soak for 24 hours in cold water which make the seed coat to soften @ wash off the incubators. 3. Acid scarification  Soaking the seed for a few seconds (15-60sec) in concentration Hydrochloric acid or sulphuric acid.  At the end of treatment, the seed are washes to remove the remnant acid. Stratification Refrigerated Stratification Stratification is accomplished by placing seeds in moist planting medium in a cold environment (2 weeks to 3 months). Seeds require moisture, cold and oxygen. In general, this period of time allows an undeveloped embryo to mature. FRUIT DEVELOPMENT Fruit Development Fruit is a mature of ripen ovary formed after fertilization After double fertilization ovary developed into fruit and ovule form seeds. A fruit may contain one or more seeds. Fruit provide protection for the enclosed seeds. During fruit development, the wall of ovary becomes pericarp, the thickened wall of fruit. The fruit usually ripens about the same as its seed are completing their development. Parthenocarpic fruit is the formation without fertilization ovary and it is seedless. This occurs in plant species that have many ovules in their ovary such as banana. From ovary to fruit The ovary of the flower contains the ovules. As fertilized ovules develop into seeds, the ovary wall develops into the fruit. The term “fruit” refers to a mature ovary that contains seeds. Parts of fruit The three fruit layers are:  Exocarp: the outermost layer often consisting of only the epidermis  Mesocarp: or middle layer, which varies in thickness  Endocarp: which shows considerable variation from one species to another  TYPES OF FRUIT Types of fruits Simple fruit Compound fruit Aggregate fruit Multiple fruit Accessory fruit Types of fruits A. SIMPLE FRUIT  develops from a single ovary of a single flower;  may be either dry or fleshy when mature B. COMPOUND FRUIT  Develops from several ovaries in either a single or multiple flowers 1) Aggregate fruit  develops from one flower with many ovaries  Has many small fruits each with a seed  Eg: blackberry, raspberry, strawberry 2) Multiple fruit  develops from ovaries of several flowers borne together on same stalk  Separate flowers merged or packed closely together  Eg: pineapple, jackfruit, mulberry, breadfruit 3) Accessory fruit  Develop from the tissue surrounding the ovary  They develop from flowers that have inferior ovaries and the receptacle of hypanthium become part of the fruit  in which the edible part is primarily tissue other than ovary tissue  edible flesh from the receptacle  Eg: apple,