Question

gives reason for developing aerial roots for gaseous exchange in mangroves​

Answer 1

Answer:

The respiratory gas exchange in the roots of two common mangroves has been investigated. Avicennia nitida has a system of radial, spongy, gas-filled roots running horizontally in the mud. These send up numerous air roots, often called pneumatophores. There is direct gas connection between these and the radial roots. The latter are embedded in mud which was found to be completely oxygen free. When the tide covers the lenticels on the air roots the pressure in the root system starts to drop and continues to drop until the lenticels are again uncovered by the falling tide. At low tide the oxygen concentration in the radial roots is usually around 10-18 per cent. Synchronously with the tidal drop in gas pressure the oxygen concentration in the roots falls. In comparison the CO2 rises very little. This respiratory gas shrinkage is the cause of the pressure drop. When the lenticels are freed by the falling tide the oxygen rises in the roots until they are covered by the next high tide. This tidal rhythm is not found in roots situated above the tidal action. In such roots a diurnal variation in the oxygen concentration may be apparent, with the oxygen lowest in daytime. Sometimes this diurnal rhythm may be seen to superimpose upon the tidal rhythm. On falling tide the gas pressure returns to atmospheric. This is caused by air being drawn in through the lenticels on the pneumatophores. When the pneumatophores are eliminated the oxygen in the roots drops, reaching in two days one per cent or less, proving that the pneumatophores in Avicennia are chimneys serving as ventilators for the root system in the anaerobic mud. The stilt roots of Rhizophora end in a bunch of spongy, gas-filled roots embedded in the mud which was found to be entirely oxygen free. The roots in the mud maintain a high oxygen concentration of 15-18 per cent, which showed diurnal variations with lowest oxygen during daytime, like Avicennia. There is direct gas connection between the mud roots and the lenticels on the stilt roots. When the lenticels were plugged by grease the oxygen in the buried roots fell and reached in two days 2 per cent or less. The high oxygen tension in the roots of Rhizophora is therefore maintained by means of a ventilation which takes place through the lenticels on the stilt roots .

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Answer 2

Aerial roots are roots above the ground. They are almost always adventitious. They are found in diverse plant species, including epiphytes such as orchids (Orchidaceae), tropical coastal swamp trees such as mangroves, banyan figs (Ficus subg. Urostigma), the warm-temperate rainforest rata (Metrosideros robusta) and pohutukawa trees of New Zealand (M. excelsa). Vines such as Common Ivy (Hedera helix) and poison ivy (Toxicodendron radicans) also have aerial roots.

Image of Mangrove roots

Mangrove roots

Image of Mangrove roots

Image of Mangrove roots

Image of Mangrove roots

For this purpose, mangrove species have specialized above ground roots called breathing roots or pneumatophores. In some species, these roots are pencil sized and peg like whereas in some other species they look like a knee. These roots have numerous pores through which oxygen enters into the underground tissues.

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