Class 12  >  Reproduction in Plant (Part - 9) - Notes, Botany, Class 12

Reproduction in Plant (Part - 9) - Notes, Botany, Class 12 - Notes - Class 12

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Reproduction in Plant

III.. FACTORS AFFECTING SEED GERMINATION :

The seed germination takes place in the presence of favourable condition such as temperature , moisture , and air . This is a very delicated stage in the life of the plant. Any minute change in the environment have harmful effect on growing embryo. So favourable environment should be essential for germination. The following factors are essential for germination of seed.

1. Moisture :

The moisture or water is the most important factor for germination of seed. Generally, the cells of embryo contain about 10 -1 5% water in the dormancy period. The vital activities like growth and development is unable to continue in this less amount of water. For active life processes, water must be pre sent about 75-90% . The seed absorbs  water and swell up to increase in size before the germination. Water absorbs through seed coat and micropyle . The following effects have been observed in seed due to absorption of water -

. The seed coat becomes soft and absorbed water reaches to the cells of embryo. When the ratio of  water increases in the cells, the protoplasmic contents become more active.

II . The size of embryo increased due to water absorption due to this, seed coat breaks up and radicle and plumule easily comes out. Seed coat absorbs water, it becomes soft and increase its permeability. Resulting it increases rate of exchange of oxygen and CO2. The rate of respiration increases in active cells of embryo. Thus it requires more oxygen.

III . The enzymes are present in the cells of endosperm or cotyledons which are active only in the presence of water. The stored food changed into dissolved form by the activity of enzymes and it reaches to the active embryo.

2. Oxygen [O2 ]

The process like cell division, cell elongation etc. of the embryo require energy . This energy is released by the oxidation of organic substances. Oxygen is essential for oxidation process. The upper surface of soil contains sufficient amount of O2. The healthy germination does not take place in the absence or scarcity of oxygen in deep soil so crop seed sown in the soil by the farmers upto 5-7 c.m. deep.

3. Temperature :

The suitable temperature is essential for germination of seed. The protoplasm of the cell remains active at certain range of temperature. Most of seed do not germinate in between the range of 00 C to 50and above the 45 0 C. The favourable range of temperature is 20-250 for germination of seed.

4. Food or Nutrition : The growing embryo requires [needs] nutrition during germination. The embryo depends upon stored food materials in cotyledons or endosperm in the germination period upto the formation of primary root from the radicle and first leaf from the plumule.

5. Light :

Light produces different effect in different variety of seeds. Most of the plants do not require light up to the formation of first leaf. The rate of germination very fast in the absence of light. But for some plants light is very essential for germination. They will not germinate in the dark. For example seed of Orchids, Tobacco , Mistletoe etc.

After the development of newly leaves on shoot light becomes very essential factor. The chlorophyll is not formed in the absence of light and this new formed plant [juvenile] will die in the absence of photosynthesis and deficiency of food.

IV. SPECIAL POINTS

1 . Highest amount of fat is found in endosperm of Coconut. 

2 . Embryonic development of Capsella is endoscopic because it is developed towards chalazal region of the zygote.

3. Single celled suspensor is found in Triticum [ wheat] and Caryophyllum . Their seeds are caryopsis fruit.

4 . Suspensor is absent in Sunflower and Marigold. 

5. 125 meiotic divisions are essential for development of 100 grains of Wheat .

6. Two generation and three genotypic cells are present in Angiospermic seed . 

7 . Heaviest seed (6 k.g.) is found in Lodoshia. Its fruit is 1 meter in length and wt. of fruit is 18kg .

8. Edible part of Betelnut is endosperm. 

9. Abscisic acid is the natural germination inhibitor of seed.

10 . Anatomical transition from root to stem occurs in hypocotyl region.

11 . Seed of the Ruella dispersed by jaculat or technique.

12. Smallest or minute seeds are found in Orchids which are lightest in plant kingdom and are called " Dust seeds " [w t. 20.33 mg].

13. Xenia (by Focke) - Effect of pollens inside Embryosac on Endosperm (except Embryo) e.g Maize. 

14. Metaxenia - Effect of pollen out side the Embryosac on seed coat or pericarp. In Datepalm, maturity time as well as size of fruits can be changed by using different pollens.

15 . The pollen grains of insect pollinated flower bears oily layer is called pollen-kitt. It is made up of lipid and carotenoids.

16. The longest pollen tube is found in Maize .

17. Seeds  of a large number of species live for several years. Some seeds can remain alive for hunderds of years. There are several records of very old yet viable seeds. The oldest is that of a lupine, Lupinus arctic us excavated from Arctic Tundra. The seed germinated and flowered after an est imated record of 10,000 years of dormancy. A recent record of 2000 years old viable seed is of the date plam, Phoenix dactylifera discovered during the archeological excavation at King Herod's palace near the Dead Sea.

18. The endosperm is formed after the fertilization in Angiosperms . It is triploid while in Gymnosperm , endosperm is formed before the fertilization and haploid in nature .

19 . Normally , Dicotyledon embryonic development is Cruifer-type which is also known as onagrad-types. 

20 . The ovules of some parasitic plants donot have integument e. g. Sandal [Santalum] and Viscum 

21 . The polysiphonous condition is found in families of Malvaceae and cucurbitaceae .

 22 . Those plants which live immortal through the seeds are known as therophytes .

23 . The synergids are absent in the embryosac of Plumbago .

24. The pollin ation takes place at two ce lled structure of pollen grains in most of Angiosperms.

25. Monospor icem bryo sac extensively found in Angiosperm. It is discovered by Strasburger in Polygon umdivarictum . It develops only from one functional me gaspore.

2 6 . The seed of Cuscuta and Santalum lacks of cotyledon

2 7 . The middle layer is absent in Wolffia 28. Costum and Nico dia plants have mult ilayere d tape tum.

2 9 . The dormancy is absent in the seed of most of Crop plants and M an g roove plants like Rhi zop hora .

3 0 . The fru it fo rmed from the ovary wit hout fert iliza tion is c alle d "P arthen oca rpic fruits " .

31. Lotus, Nymphaea and Alis ma like aquatic plants are entomophilous.

3 2 . All the microspores of anther of Calotropis are joined together to form a special, structure called Pollinium .
These staminal structures are called tr a nslat or ap par at us .

3 3 . In some pla nt s like Typh a, Dros era, Cr yptoste gi a al l fou r microspore s joi ned toget he r a nd t he y are cal led Compound pollen.

3 4 . For the formation of mature pollen grain, One meiotic and one mitotic divisions required. For the formata ti o n of mature male gametophyte, one meiotic and two mitotic divisions required.

35. In orc his macu lata and Ru mex , T-s hape d tetrad of megas pore is f ormed (In mos t angi osperms g enera lly linear letrads are formed. Isobilateral, tetrahedral and decussate type tetrad of megaspores are absent). 36. Balanoph o ra and Oe nothera plants one mi cropy lar meg aspore of tetrad bec omes func tional res t of three which are present towards the chalaza degenerate.

3 7 . When entry of male gametes into the embryosac are not participate in fusion is called Semigamy .

3 8 . When two pollen tube enter into an ovule and release their contents. It is poss ible to that the one male gamete of one pollen tube fertilize with egg cell and one male gamete of another pollen tube participate in triple fusion (with secondary nucleus). It is called heterofertilization . Because of this xenia effect found in Zea mays .

3 9 . The white fluid is found in green coconut is nuclear endosperm which is known as "liquid sync ytiumonlyonlyonly

40. In Cerea ls [Monocots] outer layer of endosperm is thick walled with dense cytoplasm, which is mainly filled with ale urone g rai n s [h ighly protei n rich ] a nd hence cal le d Aleuro ne L aye r . T h is l ayer secre te so me hy droly t ic enzymes like pr ot e ases a nd amylases .
  I. DEFINITION OF DORMANCY OF SEED

The presence of dormancy in a seed is the mos t important characteristic feature of plants. Because of this character, seeds remain v iable for ma ny years. The seeds are dispersed ver y far place s through water, air or insects. Most of the seeds are unable to germinate just after dispersal. They germinate after sometime. The time between the maturation and germination of seed is known as "Dormancy period". The state of inhibited germination as a result of internal causes is usually called 'dormancy' . This seed dormancy is of considerable advantage to the plant which helps to pass on adverse environmental conditions. The em bryo remains in ac tive in this perio d an d all the growt h pro cess es su spend ed te mpora rily . The re are three mai n ba sic reas ons of dormancy of seed :-

II. REASONS OF SEED DORMANCY

1 . Impermeability of s eed c oat The seed coats of many species of Leguminosae and Convolulaceae families are c ompletely impermeable to mois ture [water] and oxygen at the time of their maturity. S eed co ats, in these are t hick and ha rd . Their cell wall is covered by a layer of lignin which is water proof coating. Such seeds  take more time for germination.
Under natural c onditions seed dormancy is gradually over c ome by the action of microbes [bac teria] in soil, in the alimentary canal of fruit eater birds and due to the presence of low or high temperature.
 

SEED - DORMANCY

2 . Dormant Emb ryo In many spec ies, although, the embryo completely not matured when the seed is ripe, even than it fails to germinates w hen ever the envi ronmental conditions are favoura ble or even seed coat is removed. This is known as "Emb ryo " d or manc y" . Normally embryo dormancy occurs in many fr u its y ielding p lant s of forest. This condition is achieved due to physiological action of seed. Such type of seed must complete their enzym atic and chemi cal reac tion before the g ermination of seed. In t he lack of these reactio ns, seeds are una ble t o ge rmina te. s uch s eed a re k ept i n low t emp erature an d in de sir able m ois ture . Due to this s eed dormancy can be broken. The low temperature is the main reason for germination of seeds of Apple, Peaches, Pea rs , Map ple an d P ine etc.

3. Germination Inh ibitors Germination of some seeds is sometimes checked or preven ted by the presence of some chemical compounds are called "Germi nat ion Inh ibitors " such as Feru li c ac id pre sent in Tomato juic e , Caumarin, Abscisic Ac id, Dormi n and Para ascorb ic acid etc . T his c ondit ion is gene rally foun d in x er o phytic pla n ts . The se germination inhibitors washed away with water is known as "Leaching".
 

 III. METHODS OF BREAKING OF SEED DORMANCY

(i) Scarification :- The hard seed coat is broken in this method so that water and oxygen enter into the seed coat. The dormancy of seeds can be broken by different artificial methods -
(i) By making minute hole/pores with help of pointed sharp apparatus on the seed coat.

(ii) The seeds are rubbed on hard object so that seed coat become thin.

(iii) The partial degeneration of seed coat is carried by the action of sulphuric acid.

(ii) Stratific ation :- The seed kept at low tempr a ture and in the presence of oxygen and water for some time, so that embryo can completes its maturation.

(iii) Lig ht Requirement :- The seeds which are affected by the lig ht are known as " Photoblastic seeds ".
The seeds which germinate in the presence of light are called "positive  photoblastic seeds", such as Lettuce , Capsella , Lepidium and Tobacco etc. T he seeds i n which germinat ion take place in the absence of light are called "negative photoblastic seeds" , such as Ni g ella and Silene etc. Some seeds are not affec ted by light are called "nonphotoblastic seeds".
The dormanc y of photoblastic seeds can be broken by the treatment of red light . The phytochrome red , absorbs red light and convert into ph ytoc hrome far red which increase the germination of seed.

Seed + R [red light] → Germination Seed + R + FR → No germination Seed + R + FR + R → Germination Seed + R + FR + R + FR → No germination

(iv) Alte rnat ive Te mp erat ure :- Seeds are treated with high and low temperature alternatively at high pressure to increase the germination of seed. e.g.The seed of Malilotus (Sweet clover) and Alfa - alfa are treated at 2000 atm os pheric pres sure and 18 C temperature.

(v) Viabi lity :- This is called existanc e of l ife in a s eed . The viability of seed can be tested out by 2, 3 , 5, triph enyl te tr a zo li um chl or id e . The embryonal ax is of living seed becomes pinkin colourin the solution of T.T.C.

(vi) Seed dormancy can also be broken by the treatment of G.A.

IV. SPECIAL POINTS

(1) Hard and imperme ab le se ed coat is found in most of legume plantshich is the ma in reason of seed dormancy. A hard lignin water resistant layer [seed coat] is present on seed of some Leguminoseae plants.

(2) Growth inhibitor is pres ent in the endo sperm of Ir is , in seed coat of Cucurb ita and in the embryo of X ant hium .

(3) The seed dormancy in Eranthis he ema lis and Gi nk go is p re se nt d ue to imm at ure emb ryo .

(4) Some chemic alshave been used success fully to in terruptseed dormancy caused by resistan torhard impermeable seed coat. such as potassium nitrate , Ethylene , Chlrohy drineand Thiourea etc.

( 5 ) Atriplex seed contai ns high concentration of salt of the time of dispersal, whi ch do not allow ge rmination.

BUD DORMANCY
Before showing their respective growth, the bud of many plants undergoes  a dormant phase. This is known as bud dormancy. This period starts in late summer and terminate in the sprin g sea so n.  

CAUSES OF BUD DORMANCY -
According to Hemberg in woody plant, the bud dormancy is caused by Abscisic ac id. The level of endog enous A BA increases with the onset of dormant period and decreases when it is broken. In non w oody plant like Pota to , the bud dorma nc y is agai n d ue to the pre sen ce of a n i nh ibitor. B a n ne t Cl ark a nd Keffo rd (19 53) ident ified t h is subs ta nce as Inh ibito r-b . It p re sent i n t he peel of dorma nt Pota to tub er . T he i nh ibitor-b is re sponsible for checki ng t he sprout i ng of buds located i n t he 'eye s' of Potato tube r.
Accordi ng to Wareing. The bud dormancy, at least in woody plant, is caused by shor t day length, wh ile it is broken by long day.

 BREAKING OF BUD DORMANCY

( i ) Chilling : The bud dormanc y of some plant can be broken, if they are given cold temperature treatment for specific duration.

( ii ) Alternating temperature treatment : The dormancy of buds of some plants can be broken, if they are subjec ted to low temperature (0°-10°C) for a brief d uration a nd then gi ven w a r m t rea t m en t .

( iii ) High temperature treatment : If dry P ota to tub e rs are stored at 35° C or moist at 20°C, the dormancy of tubers buds is broken.

( iv ) Chemic als : Some c hemicals l ike 2-c hlor o et hano l , Gib bere llin and T h io u rea are capable of breaking of dormanc y of buds . Of these 2-chloro ehtan ol is very effective in breaking the dormancy of P otato tu be r's buds . End ogen ous G ibberelli n play s a very signifi cant ro le in co ntrolli ng the d ormancy of P ota to t ube r s .

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