How Do Plants Coordinate? Key Chemical Signals and Responses
Tropic movement in plants is a biological phenomenon that denotes growth or bending movements in plants due to environmental influences. Tropism is a three-step procedure that entails sensing the stimulus, signal transduction or response to the stimulus and the directional movement of plants. In plants, there are five tropism types: geotropism, chemotropism, phototropism, hydrotropism and thigmotropism. These will be discussed in detail later. Keep reading to learn more about tropic movement and coordination in plants and types of plant hormones.
Types of Tropism
Phototropism: Phototropism occurs in plants in response to sunlight, and the plants are reported to grow in the direction of light. The movement can be either towards or away from the sunlight.
What is Geotropism?
Positive geotropism is when a plant moves in the direction of gravity, causing its roots to grow downward. Similar to this, the process can be described as negative geotropism if its shoots continue to grow upward. The directional growth in plants occurs due to the uneven distribution of plant hormone auxin.
Chemotropism: In addition to environmental influences, there are also chemical elements that affect the tropic movements. Chemotropism example includes movement of pollen tube in the direction of the ovule. The transport of pollen grains to the female gamete releases certain chemicals which help in movement in seed plants.
Hydrotropism: Response of plant roots towards water is called hydrotropism. This phenomenon helps in the growth of roots in water.
Thigmotropism: The movement in plants that takes place in response to touch is called thigmotropism. An example includes the tropism that occurs in the touch me not plant.
Chemical Control and Coordination
All forms of life respond to environmental stimuli. Certain chemical reactions are reported to occur in plant cells in response to environmental stimuli. The capacity to use many plant components harmoniously and effectively is known as coordination. A plant chemical mechanism produces coordination, which is assisted by hormones and phytohormones.
Plant hormones, also known as phytohormones, are in charge of controlling and coordinating the chemical processes in plants. Cells are stimulated to produce these hormones that later diffuse throughout the plant. Plant hormones are involved in processes like fruit development, leaf drop, fruit ripening, plant ageing, cell division, cell enlargement, cell differentiation, and fruit ripening.
Plants display two types of movements: Growth-dependent movements/directional or Tropic Movements, and non-growth dependent/non-directional movements, also called the Nastic Movements.
Tropic movements are classified into 5 types. Phototropism (in response to light), Geotropism (in response to gravity), Hydrotropism (towards the water), Chemotropic movement (in response to chemicals), and Thigmotropism (in response to touch).
Nastic movements in plants are non-directional movements and are independent of stimulus. The example includes the leaves of Mimosa pudica, which immediately fold themselves when touched. These sorts of changes occur due to the alteration in the water content in the leaves. The leaves either swell up or shrink depending on the quantity of water. These changes are acquired in the opposite direction to the stimulus.
Types of Plant Hormones
The development and growth of the plant are affected by different hormones, namely, auxins, gibberellins, cytokinins, and abscisic acid.
Auxin is made at the stem's tip and aids in plant growth by lengthening cells. Auxins play a significant role in cell division, differentiation, development of embryos, roots, apical dominance, and flowering.
Gibberellin participates in flowering, seed germination, stem growth, and flowering. The environmental conditions must be favourable to initiate this process. The important environmental factors that need to be optimum for the processes to take place are the amount of water, oxygen, soil depth, and temperature. The fundamental process of seed germination is how various species of plants develop from a single seed into a young plant. The seed germination stages are water imbibition, root and shoot growth and formation of seedlings.
Seed Germination Diagram
In fruits and seeds, cytokinins are found where cells divide. It is primarily responsible for cytokinesis, but it also aids in the stomata opening, apical dominance, leaf senescence, and axillary bud growth.
Abscisic acid is in charge of stomata closing and limits the growth of several components, eventually inhibiting the plant's growth. Hence, the hormone is reported to encourage dormancy in seeds and buds. Plant hormones, also called phytohormones, are reported to regulate the directional or non-directional growth of plants.
Interesting Facts
Auxin was the first phytohormone to be identified, and it was found by biologist Charles Darwin.
Plant hormones are naturally occurring substances that impact a plant's growth and development.
Tropism in plants performs a protective role. They aid in creating an immediate reaction to a stimulus or change.
Key Features of Coordination in Plants
Plant hormones function as chemical messengers.
Plant hormones are reported to regulate all aspects of growth and development in plants. It also helps in the interaction that occurs between beneficial microbes and the plant.
Tropism promotes the development of pollen tubes that lead to ovules.
FAQs on Coordination in Plants Explained: Definitions, Processes & Examples
1. What is meant by coordination in plants?
Coordination in plants refers to the process where various physiological activities are synchronised to respond to environmental stimuli and regulate growth. Unlike animals, plants lack a nervous system. Instead, they rely on a chemical system involving plant hormones, also known as phytohormones, to transmit information and coordinate different functions smoothly and efficiently.
2. How do plants respond to external stimuli?
Plants respond to external stimuli like light, gravity, water, and touch through movements. These movements are broadly classified into two types:
- Tropic movements: These are directional growth movements that occur in response to a specific stimulus. For example, a plant shoot growing towards light (phototropism).
- Nastic movements: These are non-directional movements that are independent of the direction of the stimulus. For example, the folding of leaves in a 'touch-me-not' plant when touched.
3. What are the major types of plant hormones and their primary functions?
The major types of plant hormones, or phytohormones, responsible for coordination are:
- Auxins: Promote cell elongation, root formation, and are responsible for phototropism.
- Gibberellins: Promote stem elongation, seed germination, and flowering.
- Cytokinins: Promote cell division, help in breaking seed dormancy, and delay the ageing of leaves.
- Abscisic Acid (ABA): A growth inhibitor, it promotes dormancy in seeds and buds and causes the closing of stomata during water stress.
4. What is the importance of auxin in plant growth?
Auxin plays a crucial role in plant growth and development. Its primary importance lies in controlling cell elongation. This function is vital for tropic movements, such as phototropism, where auxin accumulates on the shaded side of the stem, causing those cells to elongate more and bend the stem towards the light. It also influences apical dominance, where the main central stem grows more strongly than the lateral stems.
5. How does coordination in plants differ from coordination in animals?
The coordination mechanisms in plants and animals are fundamentally different. Plants rely exclusively on a hormonal (chemical) system which is relatively slow. In contrast, animals have both a rapid nervous system for immediate responses and an endocrine (hormonal) system for slower, long-term regulation. Animals possess specialised cells, tissues, and organs like neurons and brains for processing information, which are absent in plants.
6. Why is abscisic acid often called the “stress hormone” in plants?
Abscisic acid (ABA) is called the stress hormone because its production increases significantly when a plant is under environmental stress, particularly water scarcity (drought). In response, ABA triggers the closing of stomata on the leaves to reduce water loss through transpiration. It also induces dormancy in seeds and buds, helping the plant conserve energy and survive unfavourable conditions, thus acting as a key signal in the plant’s defence and survival strategy.
7. Are all plant movements dependent on growth?
No, not all plant movements are dependent on growth. While tropic movements like phototropism involve cell growth, nastic movements are growth-independent. A classic example is the folding of the leaves of the Mimosa pudica (touch-me-not) plant upon touch. This rapid response is caused by a sudden change in turgor pressure in the cells at the base of the leaf, not by cellular growth.
8. Can you give an example of phototropism and geotropism?
Certainly. An example of phototropism is a houseplant on a windowsill bending its stem and leaves towards the window to maximise light absorption. An example of geotropism is seen when a seed germinates; the roots grow downwards into the soil (positive geotropism) to anchor the plant and find water, while the shoot grows upwards against gravity (negative geotropism) to reach the light.
9. How do farmers and gardeners apply the knowledge of plant hormones?
Farmers and gardeners use phytohormones to improve crop yield and quality. For example:
- Synthetic auxins are widely used as rooting hormones to stimulate root growth in cuttings for plant propagation.
- Gibberellins are sprayed on fruit crops like grapes and apples to increase their size and on sugarcane to increase stem length and sugar content.
- Cytokinins are used in tissue culture to stimulate cell division and differentiation, helping to mass-produce plants from a small tissue sample.
