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Seed Development
Generic seed definition - Ripened ovule that consists of an embryo, its stored food supply and protective coverings.
PARTS OF A SEED The parts of a seed are derived from the fruit. Sometimes parts of the fruit fuse and it is difficult to distinguish them. Important parts of a seed are as follows: Embryo (will become the plant) Embryo axis - shoot tip, root tip Cotyledons - first leaves Plumule - first true leaves Food storage tissues (provide energy source for germination) Cotyledons Endosperm Perisperm Female gametophyte of conifers Seed coverings (protect embryo and/or seed) Seed coats Inner - membranous Outer - thicker, often colored Fruit tissues Endocarp - pit or stone Hardened fruit may dry and coalesce to seed
TYPES OF SEEDS Endospermic - endosperm constitute a major portion of seed Non - endospermic - embryo constitutes major portion of seed Processes of seed development that can affect ability to germinate 1. Morphological development (differentiation of the embryo) 2. Acquisition of ability to germinate 3. Accumulation of reserve foods 4. Development of primary dormancy 5. Seed dispersal, handling, storage
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Three stages of seed development – 1. Morphological development – formation of the embryo 2. Cell expansion – food reserves are
Histodifferentiation
Cell expansion
Maturation drying
Fresh wt
produced Water wt
3. Maturation drying – drying of the seed
(in
orthodox seeds)
Dry wt
Stage I
Stage II
Stage III
I.MORPHOLOGICAL DEVELOPMENT Stage I - Ovary (fruit) and Ovule (seed) increase in size. Embryo is at the proembryo stage. Nucellus increases, endosperm small.
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Stage II - Endosperm enlarges - digests nucellus which disappears NON-ENDOSPERMIC SEEDS - Embryo enlarges, digests the endosperm ENDOSPERMIC SEEDS - Embryo does not develop much. Remains small. Stage III - Fleshy fruits continue to grow (peach, apple). Variable among families. Can get final growth of embryo although it is usually complete by Stage II. Get increase in seed dry weight rather than size at this . Now we have the mature seed. Cannot always separate fruit seeds in grains because they fuse during development. Differences in seed morphology are described often by size of embryo and endosperm in relation to each other and to the seed.
and
total
Note that environmental factors can also affect seed development
Gymnosperms During fertilization: Male - Microsporangia – each microspore mother cell gives rise haploid microspores. Each of these develop into a pollen cell containing 2 cells – generative cell and tube cell.
to 4
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Female –Single cell in megasporangium undergoes meiosis to give rise to 4 haploid megaspores. Three of these degenerate and the fourth undergoes continues mitosis not followed by cell division. No cell division occurs with
the
result that the
megaspore has free nuclei (ranging from
256-
8000 depending on species). Cell walls
form
around each nucleus, after which 2 or
more
archegonia develop at the micropylar
end
of the ovule. Each archegonium will contain a single egg, all eggs may be fertilized.
APOMIXIS When embryo is not produced as a result of meiosis and fertilization. 1. Adventitious apomixis embryos developing from tissue other than zygote. Often from nucellar
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tissue, tissue is 2 N = mother plant. Outside embryo sac and in addition to zygote. Common in citrus. 2. Recurrent - Get development of the egg, but not complete meiosis. Thus get 2N , similar to mother. May be parthenocarpic. Onion, raspberry, some apple. 3. Nonrecurrent - (rare). Egg develops into embryo directly; thus is l N and haploid. No fertilization occurs. Other forms of multiple embryo formation 1. Vegetative - in grasses mostly. Buds or bulbils in the inflorescence instead of flowers. 2. Polyembryony in conifers - More than 1 embryo develops from the original embryo.
Significance of apomixis 1. Good for producing genetically uniform seedling population. Note - this is 2. Better for rootstocks than for fruit-bearing scions, because apomictic seedlings go through the juvenile-adult phase and thus bearing is very delayed compared to grafted or budded material. 3. Also good for avoiding viruses that are not seed-transmitted. Significance of morphological development to propagation Sometimes the embryo or endosperm does not develop properly or is missing. Often occurs in collections of seeds from trees and shrubs in the wild. Conduct a CUT test to see whether the embryo is there. Can also do an X-ray. Obviously this would impact germination stand if there is a large percentage of empty seed.
Some reasons for lack of embryo 1. Genetic sterility as in a. self-pollination of woody crops, many of which do better if cross pollinated. b. interspecific hybrids that abort 2. Growth cracks - physiological conditions also known as split pit. Associated with light crops, excessive vigor (vegetative over reproductive); often in fruit crops . Seen in cherry 3. Environmental causes such as frost, insects
ACQUISITION OF ABILITY TO GERMINATE In general embryo cannot germinate at earlier stages of development.There are internal controls that inhibit germination of seed within the fruit. As it gets older, it acquires ability to germinate. Can check out when the embryo can germinate by removing it as it develops and placing it on a suitable medium in sterile culture. Interestingly, can often get it to germinate in culture, when it does not occur naturally. This has been developed into techniques that enable us to "rescue" embryos from non-natural interspecific hybrids and get them to grow into plants. Done in Lilium, , peach and other plants. Normally the embryo would abort.
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2. CELL EXPANSION - ACCUMULATION OF FOOD RESERVES During development get large
Formation of mRNAs during seed development
quantity of stored food Constitutive
reserves in the form of fats,
Embryo specific
oils, proteins, or starch or
heavy, well-filled seed. This
Seed storage proteins
mRNA sets
combination. These produce a
Early embryogenesis
Late embryogenesis Lea mRNAs Late embryogenesis to early germination
generally occurs during Stage III
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and terminates with ripening
Germination specific
Germination
3. INDUCTION OF PRIMARY DORMANCY Related to ability to germinate. Not good if embryo germinates while on the plant. Often is not favorable time and place for germination when still on the seed. Season, weather, etc. Primary dormancy is a condition that develops in the seed while it is still on the plant. Vivipary - premature germination on the plant. Examples: citrus, mangrove. On mangrove the long-javelin-shaped root of the embryo enables it to stick in the mud when it falls. In most seeds have mechanisms which control germination so that it occurs at favorable time and place. 1. Control of moisture content 2. Dormancy mechanisms
Control of moisture level - different kinds of seeds dry to different extents 1. Seeds "dry�during maturation. Moisture level too low for germination. Usually moisture content drops to about 30% or less on the plant. Is dried further during harvest and for storage. Orthodox seed. 2. Seeds have to be kept moist. Many of these seeds cannot be dried out without damage to the seeds. Recalcitrant seed.
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a. Examples are seeds produced in tropical regions or under conditions of high humidity such as citrus, palms, tropical fruits, sugar cane. b. Spring ripening seeds such as some maple, elm, poplar. Seeds ripen in spring, drop to ground and germinate immediately without dehydration. c. Fall ripening such as oaks which overwinter moist and then germinate in the spring. 3. Seeds are dry but are inside fleshy fruit and are thus inhibited. Often must remain moist to retain viability. Orange is an example.
5. RIPENING, SEED DISPERSAL AND STORAGE Storage of seeds very important. DESICCATION TOLERANT Moisture For example, seeds that have to be kept moist will either die or get a secondary dormancy if allowed to dry out. For storage - most seeds are desiccation tolerant and are sold with a moisture content around 4-6%. Temperature At lower temperatures, get longer storage life. Usually seeds are kept near freezing. For long-term storage go to below freezing (OoF) For very long storage liquid nitrogen at -196 oC is used. DESICCATION SENSITIVE Temperature kept low, 4째C Moisture content about 25 - 30% Fungicide control for disease particularly important