Coleoptile
Coleoptile is the pointed protective sheath covering the emerging shoot in
Coleoptiles consist of very similar cells that are all specialised to fast stretch growth. They do not divide, but increase in size as they accumulate more water. Coleoptiles also have water vessels (frequently two) along the axis to provide a water supply.
When a coleoptile reaches the surface, it stops growing and the flag leaves penetrate its top, continuing to grow along. The wheat coleoptile is most developed in the third day of the germination (if in the darkness).
Tropisms
Early experiments on
The Cholodny–Went model is named after Frits Warmolt Went of the California Institute of Technology and the Ukrainian scientist Nikolai Cholodny, who reached the same conclusion independently in 1927. It describes the phototropic and gravitropic properties of emerging shoots of monocotyledons. The model proposes that auxin, a plant growth hormone, is synthesized in the coleoptile tip, which senses light or gravity and will send the auxin down the appropriate side of the shoot. This causes asymmetric growth of one side of the plant. As a result, the plant shoot will begin to bend toward a light source or toward the surface.[3]
Coleoptiles also exhibit strong
Physiology
The coleoptile acts as a hollow organ with stiff walls, surrounding the young plantlet and the primary source of the gravitropic response.[4] It is ephemeral, resulting in rapid senescence after the shoot emerges. This process resembles the creation of aerenchyma in roots and other parts of the plant.[5] The coleoptile will emerge first appearing yellowish-white from an imbibed seed before developing chlorophyll on the next day. By the seventh day, it will have withered following programmed cell death. The coleoptile grows and produces chlorophyll only for the first day, followed by degradation and water potential caused growth. The two vascular bundles are organized parallel longitudinally to one another with a crack forming perpendicularly. Greening mesophyll cells with chlorophyll are present 2 to 3 cell layers from epidermis on the outer region of the crack, while non-greening cells are present everywhere else. The inner region contains cells with large amyloplasts supporting germination as well as the most interior cells dying to form aerenchyma.
The length of the coleoptile can be divided into an irreversible fraction, length at turgor pressure 0, and reversible fraction, or elastic shrinking.[6] Changes induced by white light increase water potential in epidermal cells and decrease osmotic pressure, which resulted in an increase in the length of the coleoptile. The presence of the expanding coleoptile has also been shown to support developing tissues in the seedling as a hydrostatic tube prior to its emergence through the coleoptile tip.
Anaerobic germination
In a small number of plants, such as rice, anaerobic germination can occur in waterlogged conditions. The seed uses the coleoptile as a 'snorkel', providing the seed with access to oxygen.[8]
References
- ^ "Coleoptiles - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2021-05-04.
- ^ Darwin, C. R. (1880). The Power of Movement in Plants. London: Murray.
- PMID 10677441.
- S2CID 20664660.
- PMID 12199518.
- PMID 15045666.
- ISBN 9781891127557.
- PMID 18660495. Retrieved 27 March 2022.
External links
- Media related to Coleoptiles at Wikimedia Commons