What are the tolerance limits of plants to a particular stimuli?
Deconstruction of The Problem: What are the tolerance limits of plants to particular stimuli? Type of Plant Factors That Affect the Growth of Plants Water Availability Light Temperature CO2 Availability Nutrient Availability Soil pH Salinity Options · Trees · Shrubs and bushes · Flowering plants · Fruits and vegetables · Low volume · High volume · Water quality · Light intensity · Light wavelength · Artificial light · Natural light · Low · High · Controlled vs natural environment · Low · Moderate · High · Level of nutrient availability · Low pH (acidic) · Neutral (pH 7) · High pH (basic) · High · Low · Solid vs liquid application of salt Questions to consider 1. Which of these plants can be grown in small spaces? 2. Of these plants, which grow at a fast rate? 3. Are they easy to grow? 4. Are they affordable and accessible? 1. How does the varying availability of water affect plant growth? 2. How does water quality affect plant growth? 1. How does varying light wavelength / intensity affect plant growth? 2. The effect of artificial vs natural light on plant growth. 1. How does varying temperature affect plant growth? 2. Is temperature in a controlled or natural environment more beneficial to plant growth? 1. What is the extent of the impact of CO2 availability on plant growth? 2. Do plants need any other gaseous components other than CO2? 1. What type of nutrients do plants need? 2. Why do plants need these nutrients and in what quantity? 1. How does the pH of the soil affect plant growth? 2. What is the optimal pH range of plants? 1. How does salinity effect plant growth and health? 2. What are the methods of changing salinity level? Findings The ability of the type of plant able to be grown in a small space is dependent on the size of the plant. This means that trees and shrubs and bushes are unsuitable options. The only options that are feasible to be grown in small spaces are flowering plants and fruits and vegetables. For the plant to be suitable for this investigation, the growth of the plant must be observable from day to day or at the very least, every few days. An example of plants that allow growth to be observed from day to day are bean plants and tomato plants.[1] Flowers generally show the most growth during germination which would be ideal to observe growth, however, it is not suitable for the investigation which focuses on plant growth after the germination stage.[1] Another option is to use herbs such as microgreen red radish, green onion and mizuna. Comparing between vegetable plants and herbs, herbs are much easier to grow with less specific requirements.[1] Therefore, herbs is the better choice for this investigation. Radishes, green onions and mizunas are all affordable with the pricing of these plants staying below the $10 mark. However, accessibility of radishes and green onions are much higher than mizuna. Moreover, due to the structure of the plants, mizuna plants also present the limitation of difficult measurement which is likely to be inaccurate. Water availability is a critical factor to plant growth as plants require water to carry out photosynthesis by which they gain energy for metabolic processes.[2] When a plant is not properly hydrated, there is not enough water pressure to make the leaves strong and sturdy, leading the stems and leaves dropping and eventually wilting. [2] Observable characteristics of wilting leaves are browning and/or drying of the edges of leaves.[2] However, if the plant is chronically underwatered but still receiving enough water to survive, the growth will be significantly slower.[2] Contrastingly, when a plant is overwatered, there is too much water in the plant’s root zone, decreasing the oxygen available to roots as there is not enough air pockets.[3] This is known as water logging and can be a major constraint to plant growth due to it causing changes in the chemical equilibrium of various elements found in soil, increasing the reduction potential of the soil.[3] Furthermore, water logging can cause the death of deeper roots resulting in plant death.[3] Water quality can be affected by a number of factors such as pollution or when water hardness is reduced.[4] Pollution of water have the potential to accelerate plant growth by providing the necessary nutrients.[4] However, it can also harm or kill plants by changing the growing conditions through raising or lowering the environment’s acidity.[4] Similarly, reducing water hardness through various processes such as chemical softening removes calcium and magnesium, which are important to plant growth, and increase the sodium content of the water.[4] Light is necessary for plant growth as it facilitates photosynthesis, the process by which light energy is converted to chemical energy for use by the organism.[5] Light energy is absorbed by special pigments in the chloroplasts of plant cells, called chlorophyll, causing a chain reaction known as the light-dependent reactions of photosynthesis.[5] The optimal wavelengths of visible light for photosynthesis is within the blue range (425-450nm) and red range (600-700nm).[5] Light wavelengths outside the blue and red range are generally not used by most plants and may contribute to heat build-up in plant tissue.[5] Another factor of light that needs to be considered is light intensity. As light intensity increases, the rate of photosynthesis increases until a second factor, such as CO2 availability becomes in short supply, known as a limiting factor.[6] At very high light intensities, it can cause a drop in the rate of photosynthesis and cause harm to the plant by targeting the water-splitting photosystem II, causing damage to the photosynthetic machinery, leading to eventual cell death.[6] Moreover, when light intensity is too high, it can stress the plant’s leaves to the point of dehydration, causing sunburn damage to leaves.[6] While artificial light can also be used to achieve successful growth of plants in growth chambers, natural light in the form of sunlight has been found to be the most effective for plant growth.[7] Sunlight generally has a higher intensity compared to artificial light and is relatively equally distributed among the different wavelengths of light that plants have evolved to survive best in. [7] The temperature of the environment in which the plant grows in plays a role in determining the rate of plant development and consequently affects the length and total growing period of the plant.[8] The optimal temperature of the plant varies considerably for different plants.[8] When temperature is low, the metabolism of the plant is slowed due to the enzymes that drive the biochemical reactions that result in plant growth are no longer working at optimal rate as they are not within their optimal temperature range.[8] However, if temperatures drop to a point where the plant experience a light, moderate or hard freeze, it can cause severe damage to most plants.[9] As temperature increases, rate of plant growth will increase as a result of increased biochemical reactions until optimal temperature is reached.[8] When air temperature rises above optimal range, the plant can experience instability of photosynthesis due to being under severe heat stress.[8] Additionally, when soil temperatures rise above optimum threshold, water and nutrient uptake of the plant can be impeded, causing damage to plant components.[10[ Constant temperatures in a controlled environment leads to results with higher accuracy. However, low nocturnal temperatures in the natural environment improves the water balance of the plant resulting in increased stem elongation and can also save energy for the plant.[11] Similar to light energy and water, CO2 is crucial for photosynthesis to occur. This means that if the plant had no access to CO2, the plant will then no longer be able to carry out photosynthesis, subsequently, the plant will not be able to generate chemical energy for metabolic processes and will eventually lead to plant death.[12] As CO2 availability and concentration increases, rate of photosynthesis will increase until a limiting factor such as availability of water or light energy become present.[12] In consequence, high levels of CO2 will cause plants to thicken their leaves which have been proven to have the ability to worsen climate change.[13] In addition to CO2, plants also require O2 to carry out respiration.[14] During photosynthesis, chemical energy converted from light energy is stored in molecular bonds of organic sugar molecules.[14] This energy is released during respiration when glucose is broken down to CO2 and H2O to release the stored energy in the form of ATP.[14] This energy is then used to facilitate metabolic reactions in the plant. Furthermore, if plant roots do not get enough oxygen, they become less permeable.[14] Nutrients are not absorbed properly and toxins will build up, resulting in stunted plant growth.[14] Plant growth and development is largely dependent on the availability of a range of nutrients available in the soil.[15] The 6 major nutrients required by plants are nitrogen, phosphorus, calcium, magnesium, sulphur and potassium.[15] Plants use nitrogen in the form of nitrate which assists in strong foliage growth by affecting the plant’s leaf development, increasing the leaf area, consequently increasing rate of photosynthesis.[15] Phosphorus assists with the growth of roots and flowers and increases the plant’s resistance to environmental stress.[15] Calcium aids in the growth and development of cell walls and is crucial to plant growth as well-developed cell walls help the plant resist disease.[15] Magnesium is required as it is the central core of chlorophyll molecules in plant tissue.[15] Magnesium deficiency leads to shortage of chlorophyll and stunted pant growth.[15] Sulphur contributes to plant growth as it is one of the molecular building blocks for various proteins, hormones and vitamins.[15] Moreover, it also contributes to the formation of seeds which allows the plant to reproduce.[15] Potassium is required as it strengthens the plants, contributes to early plant growth and helps retain water.[15] Soil pH directly affects the availability of nutrients in the soil to the plant.[16] In highly acidic or alkaline soils, the major plant nutrients nitrogen, phosphorus, sulphur, calcium, potassium and magnesium as well as the trace element molybdenum is reduced significantly.[16] Reduction of these nutrients in the soil reduced rate of plant growth.[16] In addition, nutrients may also be positionally less available to plants due to poor root growth in acidic or alkaline soils.[16] Moreover, extremes in soil pH levels also mean higher concentrations or more accessible forms of minerals such as aluminium (Al) which are toxic to plants.[17] Aluminium phytotoxicity blocks the cell division mechanism, thus inhibiting root growth.[17] Generally, the optimal soil pH range of most plants are between 5.5 to 7.0.[18] However, certain plants may require more acidic or alkaline soil.[18] Salinity poses adverse effects on plants by interfering with nitrogen uptake, reducing plant growth and inhibiting plant reproduction.[19] Some ions, especially chloride, are toxic to plants which could poison the plant and cause plant death if the concentration of such ions increases past tolerance level.[19] High salinity levels in soil can also decrease plant growth