Taproots are one of the three major types of root systems found in plants; the other two are adventitious root system and fibrous root system. Taproots arise from the radicle of a germinating embryo. The radicle of many plant species dies soon after maturation and, hence, the taproot gets converted into the fibrous root system in the later stages.
Morphologically, a taproot system consists of one thick, cylindrical main root known as the primary root. Branches of roots arise from the primary roots and these branches are known as secondary and tertiary roots.
At the tip of the tertiary root, a thin hair-like structure is present which is known as ‘rootlet.’ The rootlets are critical for water absorption from the soil. The taproot system is observed in dicots. Taproots are generally deep-rooted and difficult to uproot.
Taproots grow in acropetal succession, that is, younger roots are present at the growing end of the taproot while the older roots are present near the base of the stem.
In this lesson, we are going to learn about the types and modifications of the taproot system in detail and how they are beneficial for the plants.
Modifications of Taproot System
Roots are critical for absorbing water, minerals, and salts from the soil. In some cases, the structure of the roots gets modified to supply nutrients and water to different parts and carry out additional functions. The roots get modified to maintain the physiological and mechanical integrity of the plants. The taproots are modified into the following;
- Storage of Food
- Carry out Nitrogen-Fixation
1. Storage of Food
Taproots which are capable to store food are known as ‘storage roots.’ These roots are modified in such a way that they can store food prepared by the plants inside them. Later on, this food is utilised by the plant for nutritional purpose during unfavourable environmental conditions. Therefore, it is also called as reserve food. Humans usually get benefitted from the storage roots. These roots are included in our everyday meal, for example, carrot, sweet potato, radish, etc.
In storage roots, the cells of the primary taproot or the main root swell and start accumulating food particles. Due to the accumulation of food, such roots appear fleshy or swollen. However, the secondary or tertiary roots remain thin. In some plants, hypocotyl (the embryonic region between cotyledon and radicle) is responsible for storing the food.
Storage roots are categorised into three types based on their structure;
In this type of storage taproot, the primary root starts swelling from the base of the stem and gradually reduces towards the apex of the root. This type of root has one swollen end whereas the other end is tapered. This fleshy taproot resembles the cone. Many thread-like secondary and tertiary roots arise throughout the length of the conical fleshy taproot.
Example- Carrot (Daucus carota)
In this type of storage taproot, the primary root is tapered at both the ends; the ends being the base of the stem and the apex of the root. It appears like a spindle, thickest from the middle and narrow from the ends.
In conical roots, tapering occurs at only one end, while in fusiform, tapering is present at both the ends.
- Indian Radish: Its fusiform roots consist of swollen hypocotyl near the base and swollen taproot in the rest of the region.
- European Radish: In European Radish, the taproot only forms the terminal tapering fleshy part of the root while the hypocotyl forms the middle fleshy parts.
This fleshy root is very thick at the base of the stem and appears like a sphere.
The swelling of the taproot occurs from the base of the stem but does not reduce gradually. The taproot tapers at the base of the root.
- Turnip (Brassica rapa): Most of the swollen part of the turnip is the hypocotyl. The taproot appears at the tapered end of the turnip where secondary and tertiary roots are also present.
- Beet Root (Beta vulgaris): Both the hypocotyl and taproot form the swollen structure.
d. Tuberous Roots
These are the storage taproots which do not attain any particular shape. A tuberous root is an enlarged fleshy root modified to store food.
- Mirabilis jalapa (Four O’ Clock plant)
- Trichosanthes (Vern. Parwal)
- Echinocystis lobata
2. Nodulated (Tuberculate) Roots
Nodulated roots are the type of taproots that are irregularly swollen. In this, the primary, secondary, and tertiary roots are swollen and are called ‘root nodules‘ or ‘tubercles.’ These root nodules consist of millions of nitrogen-fixing bacteria of genus Rhizobium.
The nitrogen that is present in the atmosphere cannot be utilized by the plants directly, so, the Rhizobium present in the root nodules help to convert this nitrogen into the form which can be easily utilised. They pick up free nitrogen from the atmosphere and convert it into organic compounds of nitrogen, a process known as ‘nitrogen fixation.’
The leguminous plants take up some amount of nitrogen from their roots. This nitrogen is fixed by the bacteria in exchange for food and shelter. Such type of relationship in which both the organisms are benefitted is known as ‘mutualism.’
They mostly occur in leguminous plants such as-
- Pea (Pisum Sativum)
- Gram (Cicer arietinum)
- Groundnut (Peanut, Arachis hypogea)
- Methi (Medicago falcate)
- Soya Bean (Glycine max)
Non-leguminous plants containing root nodules are-
- Actinorhizal plants such as alder and bayberry
3. Pneumatophores (Aerophores)
Pneumatophores are the aerial taproots which come out of the soil and help the plant to breathe. Therefore, they are known as respiratory roots.
They are found in those plants which are growing in mangrove or swamps. The soil of the mangrove is clayey and sticky which does not allow the air to pass through it. Therefore, the roots of such plants come out of the soil for respiration.
Pneumatophores grow vertically upwards. They show negative geotropism. The surface of the root contains small pore-like structures known as the lenticels or the pneumathodes which help in the exchange of gases. The remaining surface of the root is covered by cork.
Examples of Pneumatophores include;
- Avicennia germinans ( The black mangrove)
- Heritiera (Vern. Sundri)
- Bald cypresses