Seedling

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Seedling

A seedling is a young plant sporophyte

Sporophyte

All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or alga that has a double set of chromosomes....
developing out of a plant embryo from a seed

Seed

A seed is a small Plant embryogenesis plant enclosed in a covering called the seed coat, usually with some Food storage. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant....
. Seedling development starts with germination

Germination

Germination is the process whereby growth emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an flowering plant or gymnosperm....
of the seed. A typical young seedling consists of three main parts: the radicle

Radicle

In botany, the radicle is the first part of a seedling to emerge from the seed during the process of germination. The radicle is the embryonic root of the plant, and grows downward in the soil....
(embryonic root), the hypocotyl

Hypocotyl

Hypocotyl is a Botany term for a part of a germination seedling of a seed plant. As the plant embryo grows at germination, it sends out a shoot called a radicle that becomes the primary root and penetrates down into the soil....
(embryonic shoot), and the cotyledon

Cotyledon

A cotyledon is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaf of a seedling....
s (seed leaves).

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Encyclopedia

A seedling is a young plant sporophyte

Sporophyte

Seed

Germination

Radicle

Hypocotyl

Cotyledon

A cotyledon is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaf of a seedling....
s (seed leaves). The two classes of flowering plant

Flowering plant

The flowering plants or angiosperms are the most widespread group of Embryophytes. The flowering plants and the gymnosperms are the only extant groups of Spermatophyte....
s are distinguished by their numbers of seed leaves: Monocotyledon

Monocotyledon

Monocotyledons or monocots are one of two major groups of flowering plants that are traditionally recognised, the other being dicotyledons or dicots....
s (monocots) have one blade-shaped cotyledon, whereas dicotyledon

Dicotyledon

Dicotyledons, or "dicots", is a name for a group of flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group....
s (dicots) have two round cotyledons. Gymnosperm

Gymnosperm

Gymnosperm is a group of spermatophyte seed-bearing plants with ovules on scales, which are usually arranged in cone-like structures. The other major group of seed-bearing plants, the angiosperms, [from the Greek, 'angion' - container] have ovules enclosed in a carpel, a sporophyll with fused margins....
s are more varied. For example, pine

Pine

Pines are Pinophyta trees in the genus Pinus, in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species....
seedlings have up to eight cotyledons. The seedlings of some flowering plants have no cotyledons at all. These are said to be acotyledon

Acotyledon

Acotyledon is used to refer to seed plants or spermatophytes that lack cotyledons, such as orchids and dodder. Orchid seeds are tiny with underdeveloped embryos....
s.

Germination and early seedling development

During germination, the young plant emerges from its protective seed coat with its radicle first, followed by the cotyledons. The radicle orients towards gravity, while the hypocotyl orients away from gravity and elongates through cell expansion to push the cotyledons out of the ground.

Photomorphogenesis and skotomorphogenesis

Dicot seedlings grown in the light develop short hypocotyls and open cotyledons exposing the epicotyl

Epicotyl

In plant physiology, the epicotyl is the embryonic shoot above the cotyledons. In most plants the epicotyl will eventually develop into the stem and the leaves of the plant....
. This is also referred to as photomorphogenesis

Photomorphogenesis

PhotomorphogenesisLight has profound effects on the development of plants. The light-mediated changes in plant growth and development are called photomorphogenesis....
. In contrast, seedlings grown in the dark develop long hypocotyl

Hypocotyl

Etiolation

Etiolation occurs when plants are grown in either partial or complete absence of light, and is characterized by long, weak stems; smaller, sparser leaves due to longer internodes; and a pale yellow color ....
. Etiolated seedlings are yellowish in color as chlorophyll

Chlorophyll

Chlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from Greek language: ?????? and f????? ....
synthesis and chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
development depend on light. They will open their cotyledons and turn green when treated with light.

In a natural situation, seedling development starts with skotomorphogenesis while the seedling is growing through the soil and attempting to reach the light as fast as possible. During this phase, the cotyledons are tightly closed and form the apical hook to protect the shoot apical meristem from damage while pushing through the soil. In many plants, the seed coat still covers the cotyledons for extra protection.

Upon breaking the surface and reaching the light, the seedling's developmental program is switched to photomorphogenesis. The cotyledons open upon contact with light (splitting the seed coat open, if still present) and become green, forming the first photosynthetic organs of the young plant. Until this stage, the seedling lives off the energy reserves stored in the seed. The opening of the cotyledons exposes the shoot apical meristem and the plumule consisting of the first true leaves of the young plant.

The seedlings sense light through the light receptors phytochrome

Phytochrome

Phytochrome is a photoreceptor protein, a pigment that plants use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum....
(red and far-red light) and cryptochrome

Cryptochrome

File:RDB 2IJG.pngCryptochromes are a class of blue light photoreceptors of plants and animals. They form a family of flavoproteins that regulate germination, elongation, photoperiodism, and other responses in higher plants....
(blue light). Mutation

Mutation

In biology, mutations are changes to the nucleotide sequence of the genetic material of an organism. Mutations can be caused by copying errors in the genetic material during cell division, by exposure to ultraviolet or ionizing radiation, chemical mutagens, or virus , or can be induced by the organism, itself, by cellular processes such as s...
s in these photo receptors and their signal transduction

Signal transduction

In biology, 'signal transduction' refers to any process by which a cell converts one kind of signal or stimulus into another. Most processes of signal transduction involve ordered sequences of biochemistry chemical reaction inside the cell, which are carried out by enzymes, activated by Second messenger systems, resulting in a signal tran...
components lead to seedling development that is at odds with light conditions, for example seedlings that show photomorphogenesis when grown in the dark.

Seedling growth and maturation

Once the seedling starts to photosynthesize

Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
, it is no longer dependent on the seed's energy reserves. The apical meristems start growing and give rise to the root

Root

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial root or aerating ....
and shoot

Shoot

Shoots are new plant growth, they can include plant stem, flowering stems with flower buds, leaves. The new growth from seed germination that grows upward is a shoot where leaves will develop....
. The first "true" leaves

Leaf

In botany, a leaf is an above-ground plant Organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin, to expose the cells containing chloroplast to light over a broad area, and to allow light to penetrate fully into the tissues....
expand and can often be distinguished from the round cotyledons through their species-dependent distinct shapes. While the plant is growing and developing additional leaves, the cotyledons eventually senesce

Senescence

Senescence encompasses all of the biological processes of a living organism's approaching an advanced age . The word senescence is derived from the Latin word senex, meaning "old man" or "old age" or "advanced in age"....
and fall off. Seedling growth is also affected by mechanical stimulation, such as by wind or other forms of physical contact, through a process called thigmomorphogenesis

Thigmomorphogenesis

Thigmomorphogenesis is the response by plants to mechanical sensation by altering their growth patterns. In the wild, these patterns can be evinced by wind, raindrops, and rubbing by passing animals....
.

Consumption of seedlings

Seedlings are commonly eaten as a health food. These seedlings are usually labeled sprout

Sprouting

Sprouting is the practice of soaking, draining and then rinsing seeds at regular intervals until they germinate, or sprout. This can be a semi-automated or fully automated process when done on a large scale for commercial use....
s, but in a botanical sense are actually seedlings. There is controversy in whether seedlings, or sprouts, are really worth eating. These seedlings are used many times in organic foods and are eaten for their concentrations of certain vitamins the seedling naturally contains. This can be viewed out of proportion as the seedling is usually too small to contain enough vitamins or minerals to be in line with the amount they are claimed to obtain.

Bibliography

P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2