Understanding Growth and Development
Have you ever imagined that a small seed plant can be a tree with a very large size? A seed plant if planted will grow into sprouts then become tananam and large. Why are the seeds grown to germinate and grow into plants? The events of biological change in living things in the form of increasing size (volume, mass, height, etc.) are called growth. Growth is irreversible or irreversible.
Measurement of growth will produce an S-shaped graph that is known as the sigmoid graph. Based on this graph, growth can be divided into four phases: early phase (slow growth), log phase (maximum growth), slowing phase (slow growth), and stationary phase (stagnant growth). In the log phase there is a very rapid growth and followed by decreased speed of growth. An example of growth chart you can see in Figure above.
After the seeds germinate, will form a seed that is equipped with roots, stems, and leaves. The event occurs because of the process of differentiating meristem cells. Differentiation by plants aims to achieve the maturity level of plants. The process toward maturity in each individual is called development. The development is qualitative, meaning it can not be expressed in size (quantity, volume, and mass). Developments in plants generally go hand in hand with growth. Plants are said to be adults if they are ready to fertilize.
Factors Affecting Growth and Development of Plants
Growth and development of plants is influenced by two factors, namely internal factors (external) and external factors (external). What effect do these factors have on growth and development? In this section will be discussed about one of the internal factors and external factors that affect the growth and development of plants.
Hormones
Plant Hormones produce several types of plant hormones such as auxin, gibberellins, ethylene gases, cytokines, and abscisic acid. These hormones are produced in the body, but are affected by external conditions.
Auxins hormones
Auxins are a family of hormones found in plants. Auxins are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the shoots or roots. They change the rate of elongation in plant cells, controlling how long they become. Shoots and roots respond differently to high concentrations of auxins:
- cells in shoots grow more
- cells in roots grow less.
Gibberellins
Gibberellins are a group of plant hormones responsible for growth and development. Chemically speaking, gibberellins are actually acids. They are produced in the plant cell's plastids, or the double membrane-bound organelles responsible for making food, and are eventually transferred to the endoplasmic reticulum of the cell, where they are modified and prepared for use.
Ethylene gas
Ethylene gas fruits and vegetables is actually a plant hormone which regulates the plant’s growth and development as well as the speed at which these occur, much as hormones do in humans or animals. Ethylene gas was first discovered about 100 years ago when a student noticed that trees growing near gas street lamps were dropping leaves more rapidly (abscising) than those planted at a distance from the lamps.
Cytokinins
Cytokinins are a group of hormones that promote cell division in plant roots and shoots and the growth of buds. These hormones have been found in all complex plants as well as mosses, fungi, and bacteria. There are about 200 different natural and synthetic cytokinins known to botanists today.
Most cytokinins are produced in the meristem of the roots. Meristem is the name for a region of tissue within the plant that actively promotes cell division. In other words, the meristem is any place that's still growing (like the tip of the roots or the top of the stem).
Once the cytokinin has been produced in the roots, it travels up the xylem, or vascular tissue, to other parts of the plant where continued growth takes place (such as young leaves, developing fruits, and seeds).
Abscisic acid (ABA)
Unlike animals, plants cannot flee from potentially harmful conditions like :drought, freezing and exposure to salt water or salinated soil.
They must adapt or die. The plant hormone abscisic acid (ABA) is the major player in mediating the adaptation of the plant to stress.
Here are a few examples in closing of stomata
Some 90% of the water taken up by a plant is lost in transpiration. Most of this leaves the plant through the pores — called stomata — in the leaf. Each stoma is flanked by a pair of guard cells. When the guard cells are turgid, the stoma is open. When turgor is lost, the stoma closes.
Discussion of gas exchange in the leaf.
In angiosperms and gymnosperms (but not in ferns and lycopsids), ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration.
The mechanism:
- ABA binds to receptors at the surface of the plasma membrane of the guard cells.
- The receptors activate several interconnecting pathways which converge to produce a rise in pH in the cytosol and transfer of Ca2+from the vacuole to the cytosol.
- These changes stimulate the loss of negatively-charged ions (anions), especially NO3− and Cl−, from the cell and also the loss of K+ from the cell.
- The loss of these solutes in the cytosol reduces the osmotic pressure of the cell and thus turgor.
- The stomata close.
External Factors
External Factors As mentioned earlier, hormones are produced in the body, but are affected by external conditions (the environment). The environmental impact on the growth and development of plants is enormous. These environmental factors include air temperature, light, humidity, and the availability of groundwater and minerals.
Temperature
Temperature is an important environmental factor for plants because it deals with the ability to perform photosynthesis, translocation, respiration, and transpiration. Plants have the optimum temperature to grow and develop. The optimum temperature is the best temperature for ideal plant growth. In addition to the optimum temperature, the plant also has a maximum and minimum temperature that can be accepted by it. Maximum temperature is the highest temperature that allows plants to survive. Minimum temperature is the lowest temperature that allows plants to survive. Most plants require temperatures around 10° - 38°C for growth.
Light
Light plays an important role in the process of photosynthesis. If the food produced from the process of photosynthesis is reduced or even absent, the tissue becomes dead due to lack of food. However, the amount of light required by plants should not be too much. Excessive light will inhibit growth. Likewise the lack of light is also bad for plants. Examples of consequences of reduced photosynthesis include plants growing in dark rooms, the size of the stems much longer than plants that get enough sunlight. This plant is pale with a weak and thin trunk. Growth in such a dark place is called etiolation.
Humidity
Humid soils are well suited for growth, especially during seed germination. This is because moist soil provides enough water to activate enzymes in the seeds and dissolve food in tissues. The degree of influence of air or soil moisture in plants varies. There are plants that require high humidity and humidity, such as moss liver. In contrast, there are also plants that grow well in humidity and low humidity soils, such as Aloe vera and some orchid species.
Water and Minerals
Photosynthesis is what plants do to create their food, and water is critical to this process. Water enters a plant's stem and travels up to its leaves, which is where photosynthesis actually takes place. Once in the leaves water evaporates, as the plant exchanges water for carbon dioxide.
Plants need to build the complex molecules they need from minerals (elements and simple compounds), which they get from the soil. Animals on the other hand get their complex molecules ready made from their food – they need very few minerals to supplement their diet.
If a plant shows poor growth it may be due to a lack of one or more minerals in the soil. It's possible to do chemical tests to find out which minerals are deficient, but it's often more convenient to add a general fertiliser. This is sometimes called NPK fertiliser because it contains the elements nitrogen (N), phosphorus (P) and potassium (K). These elements, plus magnesium (Mg), are the minerals that plants need in the greatest quantities.
Availability of oxygen
From the Arctic to the tropics, the pattern of plant distribution bears the imprint of oxygen as a limiting factor, either directly by acting on plant metabolism or indirectly by altering the environment. This chapter compares the distribution and physiology of plants from a range of habitats. During the life cycle of most species of higher plants there are critical periods when oxygen is a resource that is frequently limiting for germination, growth, and survival. The tolerance of adversity is an important path to survival as resource acquisition and, despite the relative abundance of oxygen in the atmosphere, it too is a resource that frequently limits the survival and competitive ability of many species. The ecological advantages and disadvantages of the various adaptations such as glycolytic rate and alcohol dehydrogenase (ADH) induction and end–products of glycolysis in relation to anoxia tolerance are also discussed. The consequences of oxygen deprivation for survival and metabolism and climatic change for the vegetation of oxygen–deficient habitats are also reviewed. The future prospects for oxygen limitations for plants rooted in upland and oceanic areas are discussed in relation to possible changes in the hydrologic cycle as a consequence of the greenhouse effect.
Conclusion
Plant growth is influenced by genes, internal factors (plant hormones) and external factors (environment). Internal factors of plant hormones consist of auxin hormones, cytokines, gibberellins, abscisic acid and ethylene gases. External factors in the form of environmental influences consisting of, light, temperature, humidity, water and minerals and oxygen.