Pollution's Impact On Germination: A Toxic Environment's Effect

Germination is the process by which a plant grows from a seed into a seedling. It is affected by various environmental factors, including light, temperature, water, humidity, and nutrition. However, pollution can also have a significant impact on germination. Heavy metals, such as lead, nickel, cadmium, and mercury, are known to cause toxic effects on plants, reducing crop yields and posing threats to agro-ecosystems. These metals can contaminate soil, water, and air, disrupting the germination process and seedling growth. For example, nickel can retard seed germination and growth, while lead can inhibit seed morphology and physiology. Additionally, copper and cadmium can induce oxidative stress and impair food reserve mobilization. Understanding the effects of pollution on germination is crucial for developing effective strategies to mitigate its impact on agriculture.

Heavy metals like nickel, cobalt, cadmium, copper, lead, chromium and mercury can cause abnormalities and decrease germination

Heavy metals like nickel, cobalt, cadmium, copper, lead, chromium, and mercury are important environmental pollutants that can cause toxic effects on plants, thereby lessening productivity and posing dangerous threats to agro-ecosystems. They act as stressors on plants, affecting their physiology and causing abnormalities and a decrease in germination.

Nickel (Ni) is toxic to most plant species, affecting enzyme activity and retarding seed germination and growth. It also affects the digestion and mobilization of food reserves like proteins and carbohydrates in germinating seeds, reducing plant height, root length, chlorophyll content, and enzyme activity, while increasing malondialdehyde content and electrolyte leakage.

Lead (Pb) strongly affects seed morphology and physiology, inhibiting germination, root elongation, seedling development, and plant growth. It causes alterations in chloroplasts, obstructs the electron transport chain, inhibits the Calvin cycle, and impairs the uptake of essential elements like magnesium and iron.

Copper (Cu) induces oxidative stress in plants via the generation of reactive oxygen species (ROS) and decreased catalase (CAT) activity. It reduces the germination rate and induces biomass mobilization by releasing glucose and fructose, inhibiting the breakdown of starch and sucrose in reserve tissues.

Cadmium (Cd) causes a delay in germination, induces membrane damage, and impairs food reserve mobilization, increasing lipid peroxidation products and reducing germination percentage, embryo growth, and biomass distribution. It also inhibits enzyme activities, impairs membrane integrity, reduces water content, shoot elongation, and biomass.

While cobalt (Co) has been reported to induce DNA methylation in seeds, the specific effects of chromium (Cr) and mercury (Hg) on seed germination were not found in the sources provided. However, it is known that mercury can cause toxic effects on plants, and both chromium and mercury are considered important environmental pollutants.

Sulphur dioxide can stop germination at the right concentration

Sulphur dioxide is a widespread air pollutant that is toxic to both plants and animals. However, it has been found to have a positive effect on seed germination in certain conditions.

However, sulphur dioxide can also promote germination in certain conditions. One study found that pre-treatment with a sulphur dioxide donor alleviated the inhibitory effect of cadmium stress on wheat seed germination. Sulphur dioxide pre-treatment was found to moderately increase the activities of amylase and esterase and cause higher levels of reducing sugars and soluble protein in germinating wheat seeds under cadmium stress. Another study found that sulphur dioxide treatment significantly promoted seed germination and seed vigour in maize.

The effect of sulphur dioxide on germination, therefore, depends on various factors, including the type of seed, the presence of other pollutants, and the concentration of sulphur dioxide.

Temperature affects germination and growth

Temperature is a critical factor in the germination process, and its deviation from optimal ranges stimulates antioxidant enzymatic activity. The ideal temperature for germination varies depending on the plant species. For example, the optimal germination temperature for wheat seeds is 20-22°C, while for African leafy vegetables, it ranges from 29°C to 36°C.

Temperature affects the rate of seed deterioration, with higher temperatures accelerating the deterioration process. Additionally, temperature influences the dormancy of seeds. Dry seeds continuously lose dormancy at a rate dependent on temperature, while hydrated seeds respond differently, with high temperatures generally reinforcing dormancy.

The rate of germination also has a positive linear relationship with temperature up to a certain point, after which it declines. For instance, the germination rate of wheat seeds initially increases with temperature but decreases at temperatures above 30°C.

The impact of temperature on seed germination is closely linked to the activation of antioxidant enzymes. Higher temperatures induce the production of reactive oxygen species (ROS), leading to oxidative stress. In response, plants activate antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX) to counteract the oxidative stress.

Overall, temperature plays a crucial role in seed germination and early seedling growth, and its effects vary depending on the plant species and environmental conditions.

Germination is triggered by the presence of water

Water triggers the seed to begin converting starch to sugar, providing energy for the embryo during germination. The seed absorbs water through its seed coat, causing the seed coat to soften and activating enzymes that begin the growth process. The seed's cells start to elongate and divide, and the radicle, or primary root, is usually the first part of the embryo to break through the seed coat.

However, it is important to note that water is not the only factor influencing germination. Oxygen, temperature, and light or darkness also play a role. Seeds need oxygen for metabolism and as part of aerobic respiration until they can produce green leaves of their own. The ideal temperature for seed germination is around 25-30°C, although this varies depending on the species of plant and its environment. Some seeds require lower or higher temperatures, and some need fluctuations in temperature.

Most seeds do not require light for germination and germinate best in dark conditions. However, some seeds, like carrots and certain lettuce varieties, need light to germinate.

Salinity can prevent germination by reversing the osmotic flow of moisture into the seed

Salinity is a major environmental stress that reduces germination rates and delays the initiation of germination and subsequent seedling establishment. Salinity affects seed germination through osmotic stress, ion-specific effects, and oxidative stress.

Salinity increases the external osmotic potential, reducing water uptake during imbibition. This is known as osmotic stress. Salinity also creates ionic stress, which is the toxic effect of excess sodium and chloride ions on embryo viability. This toxicity disrupts the structure of enzymes and other macromolecules, damages cell organelles and the plasma membrane, and disrupts respiration, photosynthesis and protein synthesis.

Salinity may also adversely influence seed germination by decreasing the amounts of seed germination stimulants such as gibberellins (GAs), enhancing abscisic acid (ABA) amounts, and altering membrane permeability and water behaviour in the seed.

Salinity can also affect the germination of seeds by disrupting the structure of enzymes and other macromolecules. This can lead to a decrease in the activity of α-amylase, which is an enzyme that breaks down starch to provide energy and nutrients for the growing embryo and radicle.

In summary, salinity can prevent germination by reversing the osmotic flow of moisture into the seed through the combined effects of osmotic stress, ion-specific effects, and oxidative stress.

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