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Aseedling is a youngsporophyte developing out of a plantembryo from aseed. Seedling development starts withgermination of the seed. A typical young seedling consists of three main parts: theradicle (embryonic root), thehypocotyl (embryonicshoot), and thecotyledons (seed leaves). The two classes offlowering plants (angiosperms) are distinguished by their numbers of seed leaves:monocotyledons (monocots) have one blade-shaped cotyledon, whereasdicotyledons (dicots) possess two round cotyledons.Gymnosperms are more varied. For example,pine seedlings have up to eight cotyledons.[citation needed] The seedlings of some flowering plants have no cotyledons at all. These are said to beacotyledons.
Theplumule is the part of a seed embryo that develops into the shoot bearing the firsttrue leaves of a plant. In most seeds, for example thesunflower, the plumule is a small conical structure without any leaf structure. Growth of the plumule does not occur until the cotyledons have grown above ground. This isepigeal germination. However, in seeds such as thebroad bean, a leaf structure is visible on the plumule in the seed. These seeds develop by the plumule growing up through the soil with the cotyledons remaining below the surface. This is known ashypogeal germination.
Dicot seedlings grown in the light develop short hypocotyls and open cotyledons exposing theepicotyl. This is also referred to asphotomorphogenesis. In contrast, seedlings grown in the dark develop longhypocotyls and their cotyledons remain closed around the epicotyl in anapical hook. This is referred to as skotomorphogenesis oretiolation. Etiolated seedlings are yellowish in color aschlorophyll synthesis andchloroplast 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 theapical hook to protect the shootapical 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 theplumule consisting of the firsttrue leaves of the young plant.
The seedlings sense light through the light receptorsphytochrome (red and far-red light) andcryptochrome (blue light).Mutations in these photo receptors and theirsignal transduction components lead to seedling development that is at odds with light conditions, for example seedlings that show photomorphogenesis when grown in the dark..
Once the seedling starts tophotosynthesize, it is no longer dependent on the seed's energy reserves. The apical meristems start growing and give rise to theroot andshoot. The first "true"leaves expand and can often be distinguished from the round cotyledons through their species-dependent distinct shapes.[1] While the plant is growing and developing additional leaves, the cotyledons eventuallysenesce and fall off. Seedling growth is also affected by mechanical stimulation, such as by wind or other forms of physical contact, through a process calledthigmomorphogenesis.
Temperature and light intensity interact as they affect seedling growth; at low light levels about 40 lumens/m2 a day/night temperature regime of 28 °C/13 °C is effective (Brix 1972).[2] Aphotoperiod shorter than 14 hours causes growth to stop, whereas a photoperiod extended with low light intensities to 16 h or more brings about continuous (free) growth. Little is gained by using more than 16 h of low light intensity once seedlings are in the free growth mode. Long photoperiods using high light intensities from 10,000 to 20,000 lumens/m2 increase dry matter production, and increasing the photoperiod from 15 to 24 hours may double dry matter growth (Pollard and Logan 1976, Carlson 1979).[3][4]
The effects of carbon dioxide enrichment and nitrogen supply on the growth of white spruce andtrembling aspen were investigated by Brown and Higginbotham (1986).[5] Seedlings were grown in controlled environments with ambient or enriched atmospheric CO2 (350 or 750f1/L, respectively) and with nutrient solutions with high, medium, and low N content (15.5, 1.55, and 0.16 mM). Seedlings were harvested, weighed, and measured at intervals of less than 100 days. N supply strongly affected biomass accumulation, height, and leaf area of both species. In white spruce only, the root weight ratio (RWR) was significantly increased with the low-nitrogen regime. CO2 enrichment for 100 days significantly increased the leaf and total biomass of white spruce seedlings in the high-N regime, RWR of seedlings in the medium-N regime, and root biomass of seedlings in the low-N regime.
First-year seedlings typically have high mortality rates, drought being the principal cause, with roots having been unable to develop enough to maintain contact with soil sufficiently moist to prevent the development of lethal seedling water stress. Somewhat paradoxically, however, Eis (1967a)[6] observed that on both mineral and litter seedbeds, seedling mortality was greater in moist habitats (alluvium andAralia–Dryopteris) than in dry habitats (Cornus–Moss). He commented that in dry habitats after the first growing season surviving seedlings appeared to have a much better chance of continued survival than those in moist or wet habitats, in which frost heave and competition from lesser vegetation became major factors in later years. The annual mortality documented by Eis (1967a)[6] is instructive.
Seedlings are particularly vulnerable to attack by pests and diseases[7] and can consequently experience high mortality rates. Diseases which are especially damaging to seedlings includedamping off. Pests which are especially damaging to seedlings includecutworms,pillbugs,slugs andsnails.[8]
Seedlings are generally transplanted,[9] when the first pair of true leaves appear. This is often known aspricking out in the UK.[10][11] A shade may be provided if the area is arid or hot. A commercially available vitamin hormone concentrate may be used to avoid transplant shock which may containthiamine hydrochloride,1-Naphthaleneacetic acid and indole butyric acid.
