Tuesday 29 May 2012

APOMIXIS


CHAPTER 2                                         SEXUAL REPRODUCTION IN FLOWERING PLANTS

APOMIXIS AND POLYEMBRYONY

[1]  Although seeds, in general are the products of fertilisation, a few flowering plants such as some species of Asteraceae and grasses, have evolved a special mechanism, to produce seeds without fertilisation, called apomixis.

[2]  Apomixis is a form of asexual reproduction that mimics sexual reproduction.

[3]  There are several ways of development of apomictic seeds.

[4]  In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation.

[5]  More often, as in many Citrus and Mango varieties some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos. In such species each ovule contains many embryos. Occurrence of more than one embryo in a seed is referred to as polyembryony.

[6]  Hybrid varieties of several of our food and vegetable crops are being extensively cultivated.

[7]  Cultivation of hybrids has tremendously increased productivity.

[8]  One of the problems of hybrids is that hybrid seeds have to be produced every year.

[9]  If the seeds collected from hybrids are sown, the plants in the progeny will segregate and do not maintain hybrid characters.

[10]  Production of hybrid seeds is costly and hence the cost of hybrid seeds become too expensive for the farmers.

[11]  If these hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crop year after year and he does not have to buy hybrid seeds every year.

[12]  Because of the importance of apomixis in hybrid seed industry, active research is going on in many laboratories around the world to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.

SEED


CHAPTER 2                                          SEXUAL REPRODUCTION IN FLOWERING PLANTS

Seed

[1]  In angiosperms, the seed is the final product of sexual reproduction.

 [2]  It is often described as a fertilised ovule.

[3]  Seeds are formed inside fruits.

[4]  A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.

[5]  The cotyledons of the embryo are simple structures, generally thick and swollen due to storage of food reserves (as in legumes).

[6]  Mature seeds may be non-albuminous or albuminous.

[7]  Non-albuminous seeds have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut).

[8]  Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor, sunflower).

[9]  Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm.

[10]  Integuments of ovules harden as tough protective seed coats. The micropyle remains as a small pore in the seed coat. This facilitates entry of oxygen and water into the seed during germination.



[11]  As the seed matures, its water content is reduced and seeds become relatively dry (10-15 per cent moisture by mass).  The general metabolic activity of the embryo slows down. The embryo may enter a state of inactivity called dormancy, or if favourable conditions are available (adequate moisture, oxygen and suitable temperature), they germinate.

[12]  As ovules mature into seeds, the ovary develops into a fruit, i.e., the transformation of ovules into seeds and ovary into fruit proceeds simultaneously.

[13]  The wall of the ovary develops into the wall of fruit called pericarp.

[14]  The fruits may be fleshy as in guava, orange, mango, etc., or may be dry, as in groundnut, and mustard, etc.

[15]  Many fruits have evolved mechanisms for dispersal of seeds.

[16]  In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off.

[17]  In a few species such as apple, strawberry, cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits.

[18]  Most fruits however develop only from the ovary and are called true fruits.

[19]  Although in most of the species, fruits are the results of fertilisation, there are a few species in which fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. Banana is one such example.

[20]  Parthenocarpy can be induced through the application of growth hormones and such fruits are seedless.


[21]  Seeds offer several advantages to angiosperms

·                   Firstly, since reproductive processes such as pollination and fertilisation are independent of water, seed formation is more dependable.

·                   Also seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.

·                   As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own.

·                   The hard seed coat provides protection to the young embryo.

·                   Being products of sexual reproduction, they generate new genetic combinations leading to variations.

[22]  Seed is the basis of our agriculture.

[23]  Dehydration and dormancy of mature seeds are crucial for storage of seeds which can be used as food throughout the year and also to raise crop in the next season.

[24]  Can you imagine agriculture in the absence of seeds, or in the presence of seeds which germinate straight away soon after formation and cannot be stored ?

[25]  How long do the seeds remain alive after they are dispersed

·       This period again varies greatly. In a few species the seeds lose viability within a few months.

·       Seeds of a large number of species live for several years.

·       Some seeds can remain alive for hundreds of years.

·       There are several records of very old yet viable seeds.

·       The oldest is that of a lupine, Lupinus arcticus excavated from Arctic Tundra.

·       The seed germinated and flowered after an estimated record of 10,000 years of dormancy.

·       A recent record of 2000 years old viable seed is of the date palm, Phoenix dactylifera discovered during the archeological excavation at King Herod’s palace near the Dead Sea.