The pneumatophores are specialized roots with negative geotropism growing out of the surface of the water. These roots have structures similar to pores or lenticels, whose function is to provide air to the roots typical of swampy and flooded places.
Hydrophytic species such as mangroves (Avicennia germinans and Laguncularia raecemosa) present pneumatophores, as well as bald cypress (Taxodium distichum) and tupelo (Nyssa aquatica). In the case of the red mangrove (Rhizophora mangle) the roots, in addition to support, allow the plant to breathe.
Pneumatophores. Source: flickr.com
This type of root develops in some plant species that grow in soils saturated with water and strongly compacted. The epigeal roots have many pores and spongy tissues, which facilitate gas exchange with the surrounding atmosphere.
Flooded areas or mangrove mud are anaerobic environments, so plants must adapt to these adverse conditions. In this case, the pneumatophores present wide intercellular spaces that facilitate the diffusion of gases to the roots that are submerged.
General characteristics
Pneumatophores develop as erect roots forming an ascending structure or prolongation of the underground root system. These roots are exposed during the day and remain on the surface of the water, facilitating the obtaining of oxygen from the environment.
Lenticels located along the surface capture oxygen through the spongy tissue, which is then spread throughout the plant. Species such as the mangrove develop pneumatophores, since the highly saline and anaerobic soils prevent the roots from carrying out gas exchange.
In the mangrove species Avicennia germinans and Sonneratia alba, pneumatophores develop as lateral, erect extensions of the longitudinal roots that grow underwater. Likewise, the horizontal roots expand considerably, fulfilling the anchoring function.
Mangrove pneumatophores have different sizes and morphological characteristics. In the mangrove Avicennia germinans the pneumatophores are finger- or pencil-like, while those of the Sonneratia alba species are coniform.
Pneumatophores are generally less than 30 cm in Avicennia sp. and less than 20 cm in Laguncularia sp. However, in Sonneratia sp. it grows slowly until it becomes woody and reaches a height between 30 cm and 3 m.
The presence of branching in pneumatophores is not common. However, bifurcations or epigeal extensions occur when tissue damage or cuts have occurred.
Avicennia germinans. Source: deskgram.net
The density of pneumatophores or number of aerial roots is relatively large. A fully developed mangrove of the Avicennia germinans species, with a height of 2-3 m, usually has more than 10,000 pneumatophores.
In the mangrove genera Avicennia and Sonneratia, pneumatophores contain chlorophyll in the subsurface layers. In fact, these structures have the ability to photosynthesize in chlorophyll layers under the cuticle.
Types of pneumatophores
Based on the nature of the surface, pneumatophores are differentiated into two types: smooth and rough or rough. Smooth pneumatophores are characteristic of young tissues, they are still under water, they have a smooth surface and they present fewer lenticels.
As for the rough pneumatophores, they are located mainly on the surface of the water and are the most developed structures. They are rough in surface and have numerous lenticels throughout the epidermal tissue.
Pneumatophores are air or respiration roots adapted to provide air to submerged parts of the plant, especially underground roots.
For this reason, pneumatophores show negative geotropism, so they grow vertically upwards until they reach an oxygen source.
Function
Functional pneumatophores have a gray or yellowish-green crust with various lenticels across the surface. Likewise, they are covered by a highly impermeable epidermal tissue.
Therefore, the main function of pneumatophores is related to gas exchange between internal tissues and the atmosphere, a process that takes place through lenticels that take in air and transfer it osmotically through the spongy tissue to the rest of the body. plant.
By transferring oxygen to subsurface roots, pneumatophores act as a specialized ventilation mechanism. In fact, this mechanism allows air circulation through the plant allowing its survival in an anaerobic environment.
Along the surface of the pneumatophores that remain underwater, a group of so-called feeding roots develop. These feeding roots adapted to high salinity conditions fulfill the function of absorbing nutrients from the aqueous medium.
Adaptation to the environment
Pneumatophores are specialized root structures that allow various species, such as mangroves, to live in anaerobic sediments.
In fact, mangrove trees are adapted to survive in oxygen-deficient soils through aerial roots.
Mangroves. Source: pixabay.com
Plants demand oxygen for the respiratory process through all living tissues, including underground roots. Therefore, in loose soils without water saturation, the diffusion of air between the pores of the soil makes it possible to satisfy the oxygen demand.
However, in flooded soils, the spaces become saturated with water with oxygen levels lower than air. Consequently, mangroves have developed an extensive aerial root system to the detriment of underground roots.
In this respect, these aerial roots, called pneumatophores, allow gas exchange towards the underground roots. Pneumatophores grow from underground roots to the surface of the soil or water.
In coastal areas where mangrove trees grow, pneumatophores take in the air at low tides through the lenticels. Later it transports the air through the spongy tissues to the rest of the plant, especially towards the underground roots.
In red mangroves, support roots are observed extending from the trunk and adventitious roots from the branches. On the contrary, in the black mangrove no supporting roots are observed, but there are small aerial roots that unfold vertically from the soils that surround the trunk.
References
- Everett Thomas H., Weber Lillian M. et al. (2018) Pneumatophores: Tree Structure And Growth. Recovered at: britannica.com
- Lim Kelvin K., Murphy Dennis H., Morgany T., Sivasothi N., Ng Peter K., Soong BC, Tan Hugh T., Tan KS & Tan TK (2001) "A Guide to Mangroves of Singapore." Volume 1: The Ecosystem and Plant Diversity. Recovered at mangrove.nus.edu.sg
- Pallardy Stephen G. (2008) Enzymes, Energetics, and Respiration. Physiology of Woody Plants (Third Edition), Pages 169-197.
- Pneumatophore (2016) A Dictionary of Biology. Recovered at: encyclopedia.com
- Purnobasuki, H., Purnama, PR, & Kobayashi, K. (2017). Morphology of Four Root Types and Anatomy of Root-Root Junction in Relation Gas Pathway of Avicennia Marina (Forsk) Vierh Roots. Vegetos-An International Journal of Plant Research, 30 (2), 100-104.