The cellular and metabolic events triggered by water uptake during seed germination.
A rapid imbibitions phase (phase 1) launches the resumption of basic metabolism. During this phase, known as ‘physical’ imbibitions, a step-by-step activation of metabolic pathways results from the gradual increase in hydration (arrows).
When the level of hydration exceeds 60%, the rate of hydration slows (phase 2) and new physiological mechanisms prepare cell expansion in the embryonic axes, culminating in the start of cell elongation . Osmotically active substances (solutes, such as sugars, amino acids, and potassium ions) are accumulated and acidification of the cell wall leads to a loosening of the bonds between cell-wall polymers. These events coincide with the activation of the H+ ATPase in the plasmalemma, which results in a further increase in water uptake that may coincide with weakening of the surrounding tissues (the endosperm) as the embryonic axes elongate and germination is completed.
Completion of seed germination can be temporarily blocked by dormancy, which is in turn released by antagonistic interactions between the endogenous plant growth factors abscisic acid (ABA) and gibberellins (GAs). Storage nutrients (lipids, proteins or starch) accumulated in the embryo’s cotyledon and/or endosperm start to be mobilized before completion of germination and are used in the post-germination steps to sustain the young plant in its early growth stages, before it becomes autotrophic. If the cell cycle resumes during germination, the first cell division (mitosis) occurs in the postgerminative phase. The arrows indicate the particular hydration levels that are known to correlate with individual metabolic events.
is present within the seed when it reaches physiological maturity It is imposed upon the seed by the mother plant and remains for some time after the seed is shed.
The seed need only go through an after-ripening period for it to disappear. But, it may also be combined in the seed with other types of dormancy so that the seed remains dormant after the innate dormancy mechanism is removed.
Enforced dormancy is literally "forced" upon the seed by some limitation of the germination environment. Seeds requiring light, alternating temperaturess, or light/dark conditions fall into this category. One or more of these conditions need to be satisfied for the seed to begin germination.
This type of dormancy disappears once the missing condition(s) are supplied. The environment supplies the strict requirements that the seed needs to soften membranes, cause physiological shifts in inhibitors and promotor chemicals, cause different metabolic pathways to activate, or a combination of these and the seed begins to germinate and grow.
Certain seed species are more sensitive to environmental conditions and controls. You'll find that freshly harvested seed is more sensitive to the environmental parameters that cause this type of dormancy. As the seed ages, the narrow environmental conditions that must be met to break this dormancy widen and the seed becomes less sensitive to them.
Induced dormancy has also been referred to as secondary dormancy. In this case, the dormancy is induced upon the seed after conditions of innate and enforced dormancy have been broken or lost from the seed.
Induced dormancy occurs when the seed has imbibed water but has been placed under extremely unfavorable conditions for germination. When later placed under more favorable conditions the seed fails to germinate while still remaining viable. It is often very hard to entice seeds displaying induced dormancy to germinate.
Even seed species that don't normally display dormancy may fall into a state of induced dormancy under the right conditions.
After-ripening describes the loss of the dormant state in a seed over some period of time. In the strictest sense, after-ripening refers to the loss of dormancy mechanisms imposed by the Mother Plant.
Seeds maintained in dry storage or imbibed in the soil seed bank tend to lose this maternal control over dormancy and germination without any applied dormancy breaking methods over a period of days to years depending upon the species. After-ripening is a period of :quiescence that the seed must go through to finalize the separation from the Mother Plant and become autonomous and on its own.
Another type of after-ripening involves seeds with rudimentary embryos. With this after-ripening, the rudimentary embryo must develop into a full embryonic axis before germination can occur. This is a physiological process that may occur over a period of time or may have to be stimulated by certain environmental factors to proceed.
In some species (such as members of the Apiaceae, carrot family), there is a dependence upon where the seed is positioned with the inflorescence as to how rudimentary or developed the embryo is at physiological maturity. Those seeds with a well developed embryo germinate readily while those with rudimentary embryos must develop fully over time to germinate.
Hardseededness is where the seedcoat or pericarp surrounding the seed present a physical barrier to the uptake of water. This type of dormancy is common in the Fabaceae, Convolvulaceae, Geraniaceae, Malvaceae, lamiaceae, and the Poaceae plant family members. Physical abrasion (scarification), or freezing and thawing may be needed to allow water uptake and germination to proceed.
Hardseededness (in its broadest sense), may also prevent germination in many trees and shrubs where the seed unit is a nut or a drupe. With this type of hardseededness, the seedcoat or pericarp presents a physical barrier to the expansion of the germinating seed.
membranes within the pericarp, seedcoat, or sometimes in the endosperm of the seed form a barrier that is permeable to the imbibition of water but impermeable to the uptake of oxygen. Generally, cool temperature between 10C and 15C allow oxygen to make its way into the seed while warm temperatures prevent oxygen uptake.
This type of seed dormancy is often the basis of why certain species need alternating temperatures in order to germinate. Since germination is fueled by the respiration of stored food within the seed, without oxygen, germination cannot occur.
Another variation of impermeable membranes inhibiting germination is found in some species where a membrane layer around the radicle inhibits the maximum uptake of water imbibition into the radicle and presents a physical barrier to the expansion and protrusion of the radicle during germination. These layers often have light dormancy associated with them that needs to be overcome to allow the radicle to emerge.
Physiological Seed Dormancy: Inhibitors and Promotors
Dormancy due to inhibitors is based upon the fact that germination and growth promoting enzymes and hormones can be inhibited, thus preventing germination. Inhibitors, such as absissic acid (ABO) may be at sufficient level to counteract growth promoting enzymes, such as gibberellins (GA).
Usually it is the balance or ratio between inhibitors and promotors that needs to be tipped in the favor of those that will allow germination to proceed. These inhibitors are found in the endosperm, cotyledons, or other food storage tissue. Sometimes these chemicals are found in the outer coverings of the seed or fruit.
Many of these chemicals are water soluble and can be leached from the seed, thus shifting the balance towards the growth promoting chemicals and allowing it to germinate. Others must be degraded into other forms or chemicals to reduce their concentration. With inhibitors that are found within the embryonic axis, it is temperature (and sometimes light) that generally controls this shift.
Temperature may also favor the production of growth promoting hormones and enzymes in the embryonic axis. Cool temperatures generally shift the balance of promotors and inhibitors towards promoting germination.
This model ambient environmental factors (e.g. temperature) affect the ABA/GA balance and the sensitivity to these hormones. ABA synthesis and signaling (GA catabolism) dominates the dormant state, whereas, GA synthesis and signaling (ABA catabolism) dominates the transition to germination. The complex interplay between hormone synthesis, degradation and sensitivities in response to ambient environmental conditions can result in dormancy cycling. Change in the depth of dormancy alters the requirements for germination (sensitivity to the germination environment); when these overlap with changing ambient conditions, germination will proceed to completion Model for the regulation of dormancy and germination by ABA and GA in response to the environment.
Physiological Seed Dormancy: The Role of Temperature Control
As noted earlier when discussing membrane permeability , cool temperatures make certain membranes allow oxygen to be uptaken into the seed. At the same time, cool temperatures may lower the demand for oxygen in the respiration process, thus making it more readily available for other uses.
Oxygen then can be used to oxidate and degrade germination inhibitors while at the same time be used to activate germination promotors. This shifts the balance towards promoting the seed to germinate if the conditions are right.
There are other processes within the seed that are affected by temperature. Cool temperatures trigger shifts in metabolic pathways when the seed is in moist, imbibed conditions. Nucleic acid synthesis is aided by these cool-moist conditions that are found naturally in the spring after snow melt.
Cool temperatures also aid in the digestion of some food reserve components, thus allowing for an increase in germination energy. One other, but very important factor affected by cool temperatures is that they aid in the softening of the endosperm structure. This is particularly true for the area of the endosperm that surrounds the radicle of the embryo. This softening diminishes the physical barrier that impedes the protrusion of the radicle during the germination process.
Light has an important role in the dormancy of some seed species. This light sensitivity may also be connected to the temperature events that we saw above. And, there has been shown that light may also have and interaction with KNO3 and other chemicals that have been shown to promote germination in some species.
It isn't just light but the quality of light reaching the seed. In the diurnal (night to day) shift, the quality of light changes in the red spectrum. During the day, red light in the spectrum promotes shifts within the seed that allow it to germinate.
During the night, the main light comes from the sun reflecting off the moon. This light is strong in the far-red spectrum and light in that spectrum inhibits the processes that favor germination.
Many seed species with small seeds need light to germinate. When buried deeply in the soil, they lack the light and go dormant. This can be seen in your garden where you think you have gotten rid of the weeds and then you disturb the soil. This disturbance brings seeds to the surface where they are stimulated by the light and begin to germinate.
is the phenotypic effect of interactions between genes and the environment.
Gene–environment interaction is exploited by plant and animal breeders to benefit agriculture. For example, plants can be bred to have tolerance for specific environments, such as high or low water availability.
The way that trait expression varies across a range of environments for a given genotype is called its norm of reaction .
In genetic epidemiology it is frequently observed that diseases cluster in families, but family members may not inherit disease as such. Often, they inherit sensitivity to the effects of various environmental risk factors . Individuals may be differently affected by exposure to the same environment in medically significant ways. For example, sunlight exposure has a much stronger influence on skin cancer risk in fair-skinned humans than in individuals with an inherited tendency to darker skin. Gene–environment interaction
Naive nature versus nurture debates assume that variation in a given trait is primarily due to either genetic variability or exposure to environmental experiences.
The current scientific view is that neither genetics nor environment are solely responsible for producing individual variation, and that virtually all traits show gene–environment interaction.
Evidence of statistical interaction between genetic and environmental risk factors is often used as evidence for the existence of an underlying mechanistic interaction.
In some combinations of environments and genotypic ranges, heritability can be 100% even while group differences are completely environmental. For heritability to be 100%, random variation in expression must not occur.
Algae reproduce in astoundingly diverse ways. Some reproduce asexually, others use sexual reproduction, and many use both. Certain environmental conditions, such as lack of nutrients or moisture, may trigger the haploid daughter cells to undergo sexual reproduction. Instead of forming into spores, the haploid daughter cells form gametes that have two different mating strains. These two strains are structurally similar and are called plus and minus strains. Opposite mating strains fuse in a process known as isogamy to form a diploid zygote, which contains two sets of chromosomes. After a period of dormancy, the zygote undergoes meiosis, a type of cell division that reduces the genetic content of a cell by half. This cell division produces four genetically unique haploid cells that eventually grow into mature cells.
is the process by which a living organisms able to produce more of its own kind.
What is Reproduction?
is the biological process by which new "offspring" individual organisms are produced from their "parents". Reproduction is a fundamental feature of all known life ; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: sexual and asexual .
Determines its genetic composition, which, in turn, is the deciding factor to develop suitable breeding and selection methods. Knowledge of mode of reproduction is also essential for its artificial manipulation to breed improved types. Only those breeding and selection methods are suitable for a crop which does not interfere with its natural state or ensure the maintenance of such a state. It is due to such reasons that imposition of self-fertilization on cross-pollinating crops leads to drastic reduction in their performance.
Binary fission - It is a common method of asexual reproduction when the conditions of food, water and tem perature are favorable. In binary fission the parent cell divides to form two identical sister cells. During this process, the chromosome duplicates itself. Along with DNA duplication, the cytoplasm divides into two halves, each having its own nuclear material. This led to the formation of two daughter cells. The two daughter cells grow fully and divide again.
Multiple fission- In this type of fission many individuals are formed from the single parent. This type of reproduction occurs during unfavorable condition. During unfavorable condition the cell forms a protective layer called cysts. The parent cell continues to divide within the cysts. On the approach of the favorable condition it raptures and all the daughter cells come out.
Rhizome- It is a short, swollen underground stem that bears buds and adventitious roots. A new plant develops from the roots of rhizomes.
Layering- Layering involves getting roots to grow from the stem.
Cutting – Cutting is done using a short section of plant stems for propagation.
Budding- Taking a bud from one plant and moving it to another.
Grafting- Grafting is a process in which a section of a stem of one plant is placed onto another plant
2) Vegetative propagation- is often referred to asexual reproduction in plants since no seed is involved in the formation of the new plant.
Sexual Reproduction in Flowering Plants (Angiosperms)
Sexual reproduction is the formation of a new individual following the union of two gametes. In plants, the gametes are egg and sperm and the structures that produce these gametes are located within the flower
Mitosis, process in which a cell’s nucleus replicates and divides in preparation for division of the cell. Mitosis results in two cells that are genetically identical, a necessary condition for the normal functioning of virtually all cells. Mitosis is vital for growth; for repair and replacement of damaged or worn out cells; and for asexual reproduction, or reproduction without eggs and sperm.
Meiosis, process of cell division in which the cell’s genetic information, contained in chromosomes, is mixed and divided into sex cells with half the normal number of chromosomes. The sex cells can later combine to form offspring with the full number of chromosomes. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance.
Meiosis differs from normal cell division, or mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell