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Saturday, 17 June 2017

External factors that affect growth in plants


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The externactors that affect plant growth include light, nutrients, temperature, relative humidity, water, carbon dioxide and oxygen, soil condition, biotic factors, and pollutants. Each of these factors is explained in detail below:https://edusportstz.blogspot.com/
  1. Light:The effect of light on growth can be studied under three headings: light intensitylight quality and duration of light. Growth is generally favoured by darkness, but light is necessary because of its role in the manufacture of food. Young plants growing in the absence of light develop elongated thin stems with narrow leaves and poorly developed shoot system. Such plants are said to be etiolated.In weak intensity of light the internodes are short and the leaves are expanded. In strong intensity of light, the plant assumes a normal height. Very low light intensity reduces the rate of overall growth of the plant, by lowering the rate of photosynthesis.Growth in full spectrum of visible light is found to be better than the growth in any one of the different colours of light. Red colour seems to be the most favourable for growth.The duration of light has a pronounced effect on the growth of vegetative as well as reproductive structures. The influence of duration of light is most marked in inducing or suppressing flowering. This phenomenon is termed as photoperiodism.
  2. Nutrients:Availability, quality and quantity of food substances will automatically affect growth. For growth to occur in living things, food must be broken down to release energy. In areas where nutrients and water are adequate, competition is reduced and population increases. In case of shortage of nutrients and water, competition sets in and most individuals die.There are different mineral nutrients required for optimum plant growth. These nutrients are classified as either macronutrients or micronutrients. Macronutrients are those nutrients required by plants in high doses while micronutrients are the nutrients required in small quantities. Examples of macronutrients include nitrogen, potassium, magnesium, calcium, phosphorous and sulphur. Micronutrients include iron, zinc, molybdenum, manganese, boron, copper, cobalt and chlorine.
  3. Temperature:Atmospheric and soil temperatures are very crucial for plant growth as it affects many plant processes such as photosynthesis, metabolism, respiration, transpiration, breaking of seed dormancy, seed germination, protein synthesis, translocation, and flowering. At high temperatures the translocation of manufactured food is faster so that plants tend to mature earlier.Growth can take place between 0°C and 50°C. But the optimum temperature for the growth is between 20° and 30°C. Low temperature, however, is necessary for many plants to flower. Different physiological processes such as photosynthesis and respiration are controlled by enzymes. The enzymes are affected by temperature and pH. Enzyme activity and the rate of most chemical reactions generally increase with rise in temperature. Up to a certain point, there is doubling of enzymatic reaction with every 10°C temperature increase. But at excessively high temperatures, denaturation of enzymes and other proteins occur.It follows, therefore, that drastic changes in temperature and pH will affect growth.
  4. Relative humidity:Relative humidity (RH) is the amount of water vapour in the air, expressed as the proportion (in percent) of the maximum amount of water vapour it can hold at certain temperature. For example, an air having a relative humidity of 60% at 27°C temperature means that every kilogram of the air contains 60% of the maximum amount of water that it can hold at that temperature.The relative humidity affects the opening and closing of the stomata which regulates loss of water from the plant through transpiration as well as photosynthesis. Transpiration is slower in humid conditions. This is because diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.
  5. Water:As mentioned earlier, water is a primary component of photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm of a cell. By means of turgor pressure and other changes in the cell, water regulates the opening and closing of the stomata, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water’s most critical roles is that of a solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage. Gradual evaporation of water from the surface of the leaf near the stomata helps stabilize plant temperature.
  6. Carbon dioxide and oxygen:The oxygen and carbon dioxide in the air are of particular importance to the physiology of plants. Oxygen is essential in respiration for the production of energy that is utilized in various growth and development processes. Carbon dioxide is a raw material in photosynthesis. However, a high concentration of carbon dioxide reduces growth because of its effect on the closing of stomata, and maintenance of dormancy. If the concentration of carbon dioxide in the plant leaf is higher than the surrounding air, the stomata will open to let in more of the gas from the surrounding air so as to balance the equilibrium of the gas between the two media (air and leaf air spaces). The opposite is the case if the concentration of the gas is higher in the air than in the leaf.
  7. Soil condition:The characteristics of soil play a big part in the plant's ability to extract water and nutrients. If plants are to grow to their potential, the soil must provide a satisfactory environment for plant growth.Plant growth is influenced by the soil properties such as texture or structure, salinity, acidity, waterlogging, or compaction.
  8. Biotic factors:Diseases, plant pests, weeds and harmful substances released by roots (allelopathy) affect plant growth drastically. Weeds compete with plants for moisture, nutrients, and light. Root knot nematodes reduce absorption, so more fertilizer is necessary. All of these have negative impacts on plant growth and development.
  9. Pollutants:Pollutants can hamper plant growth. Many pollutants composed of poisonous gasses (such as carbon monoxide, sulphur dioxide, hydrogen fluoride, hydrogen sulphide) are capable of restraining growth, even bringing plants to death. Pollutants from household or industrial wastes are also able to restrain plant growth.

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