Question

Which one of the following is found in the coralloid root of Cycas?
(a) Algal zone
(b) Bacterial zone
(c) Fungal zone
(d) All of the above

Answer 1

Hint: Coralloid roots contain symbiotic cyanobacteria. These symbiotic cyanobacteria help in fixing nitrogen and in association with root tissues they produce beneficial amino acids like asparagine and citrulline. Organisms which are present in this zone help in fixing nitrogen.

Complete answer:
-The coralloid root of Cycas has an algal zone that harbors blue-green algae like Nostoc and Anabaena. Only a few species of cyanobacteria form associations with Cycads. This association is a mutual association as algae get dwelling place and Cycas use the food produced by the algae.
-The rhizosphere is the area around plant roots that is inhabited by a unique population of microorganisms attracted by the nutrients released by the plants in the soil around the roots. It’s the area where a mutualistic association between microbes and plant roots take place.
-Geotropism - This word comes from two words, “geo” meaning ground/earth and “tropism” meaning plant movement triggered by a stimulus in this case gravity. Negative geotropism is upward movement (stem growth) or moving away from gravity. Similarly, Positive geotropism refers to moving towards gravity that is downward movement (root growth).
-There are two kinds of nitrogen-fixing bacteria,
1) Free-living (non-symbiotic) bacteria like cyanobacteria, Azotobacter, Beijerinckia, and Clostridium.
2) Mutualistic (symbiotic) bacteria like Rhizobium, Frankia, and certain Azospirillum species.
- Free-living cyanobacteria are the prokaryotic autotrophs, which fix carbon by photosynthesis. On associating itself with selected plants it enters the symbiotic association and becomes chemoheterotrophic to fix atmospheric nitrogen using the enzyme nitrogenase.
-Similarly, Mycorrhizae or Mycorrhizal fungi are a class of fungus that can form a beneficial mutual relationship with $90\%$ of all land plant roots. The root extension with this fungi is known as mycelium. In this mutual association (mutualism) the plant provides the fungi with sugars produced through photosynthesis. In return, Mycorrhizal fungi help the plant in the uptake of nutrients trapped in mineral deposits, soil aggregates, and organic matter and water trapped in small soil pores (helpful during drought conditions and in disease and pest resistance).

So, the correct answer is ‘Algal zone’.

Note:
Cycas is a gymnosperm under the class Cycadopsida and Order Cycadales. Cycas plants have two types of roots, Normal and Coralloid roots. Normal roots usually develop from the radicle of an embryo and are less branched, they grow downward and show positive geotropism. The algal zone is absent in the normal roots of Cycas as they help in absorption and anchorage. Whereas, coralloid roots develop from normal roots and are highly branched. They grow upward and show negative geotropism. They have a broad algal zone present in the middle cortex that helps in nitrogen fixation.