Air pollution can affect plants in many different ways depending on the pollutunt. Which brings us to our first pollutants. SO2 and NOx.
There are two different routes acid rain can take to affect a plant. Through the leaves or through the soil. But how did the rain become an acid?
Acid rain can be formed from two different types of gases ,SO2 and NOx. They are both formed from the combustion of fossil fuels. To turn the gases into acid rain first each gas must be combined with oxygen (which there is plenty of in the air).
2NO + O2 –> 2NO2 and for SO2 2SO2 + O2 –> 2SO3
The rection above makes the gases perfect to form an acid. All they need to do is find a water molecule and then…
SO3 + H2O –> H2SO4 and for NOx 2NO2 + H2O –> HNO3 + HNO2
Sulphuric acid (H2SO4) and nitric acid (HNO3) are both very soluble in water and you guessed it easily dissolves into the rain. All the reactions above occur way up high in the atmosphere so the acid has a supply of water in the form of clouds on the reg.
So what does this all mean…
To explain that we must cast our minds back to around 30 seconds ago when I mentioned the two paths acid rain takes to hurt a plant. Either the leaves or the soil. First, we will start with the leaves.
Leaves. One of, if not the most, important organ in a plant. I like to say the leaves are the heart of the plant because without them (unless it is Winter) the plant is pretty much well dead. So you can imagine the types of issues that may arise with acid rain landing on leaves.
The damage done by this pollution can be a bit of a nuisance as it weakens the leaves. Weakening the leaves of a plant will not kill them immediately, but the consequences of acid rain will. As the leaves are not at their optimum health, the leaves will not be able to perform gas exchange as well as before. Lower gas exchange rate will cause the plant to not be able to produce glucose during photosynthesis and thus will not be able to produce energy during aerobic respiration. Maintaining the plant’s metabolic functions and any growth and repair or even fighting a disease, maybe compromised from the lack of ATP (energy). This limits the plant, and slowly but surely the plant will die. 🙁
Soil is how a plant acquires its nutrients and water, which are both very important for the plant’s health. Most soil has a pH of neutral (7), but when acid rain comes in contact with soil it changes the pH to an acidic one.
Some plants can thrive in acidic conditions; for example, Hydrangea turns a beautiful shade of dark purple in acidic soil. However, most plants, in acidic conditions tend to eventually die. Here is why…
For a plant to absorb nutrients, the nutrients must be able to dissolve in the soil solution. If a nutrient can not dissolve into the soil solution it will not be absorbed by the plant. In acidic levels nutrients such as magnesium, calcium and phosphorus become less available while aluminium and manganese become more available. But why is this bad? Well, you see aluminium and manganese are toxic to plants. Aluminium has been seen to reduce a plants growth resulting in a stunted much smaller plant than it should be. For information about how different soil pH effect plants visit https://www.esf.edu/pubprog/brochure/soilph/soilph.htm
Aluminium (Al) stops cell division, mainly in the roots, this can cause a shortage of nutrients in the plant. High levels of Al will cause phytotoxicity ( basically a build-up of a toxic chemical in the leaves making the leaves yellow stopping the leaves from photosynthesising). Which -as mentioned before- will lead to the death of the plant. 🙁
Ok moving on to manganese which will be referred to as Mn from now on …:)
Mn interferes with biological mechanisms in plants. It can be very difficult to pinpoint which mechanism it is affecting but the easiest one to spot is on the leaves. Mn turns the leaves yellow which will have the same effects as phytotoxicity as Mn causes phytotoxicity. For more on Mn and plants visit https://www.tandfonline.com/doi/abs/10.1080/01904169809365409.
Little side note about Mn:
Mn in humans and mammals can cause many neurological disorders like Parkinson’s and can be linked to causing some cancers.
To sum up, acid rain changes the pH of the soil which leads to Al and Mn being more readily available to dissolve, leading to the leaves turning yellow.
How high levels of CO2 affect plants:
High levels of carbon dioxide can postively and negativly affect plants….
Carbon dioxide is an essential gas for photosynthesis so increasing carbon dioxide will increase the rate of photosynthesis. This will increase crop yield because…
To explain why carbon dioxide increases yield I first must tell you about stomata. Stomata in greek means pore which very much so fits with its function. Its’s function is to open and close like a pore to control gas exchange. Specifically, carbon dioxide. The stomata are located underneath the leaf and are made up of two guard cells. Guard cells are curved and look like a pair of lips and they open and close like a mouth. In the day, stomata are almost always open whereas, in the night it is the opposite (closed). Stomata are closed at night in order to minimise water loss in the plant but are open in the day to obtain CO2 for photosynthesis.
Now we know enough about stomata we can continue with positive.
If there are higher levels of CO2 in the atmosphere more CO2 will pass through the stomata (diffuse) into mesophyll cells (photosynthesising cells). This will increase the rate of photosynthesis; making the crop grow quicker. The crop yield increases because there is an increase in the plant’s water efficiency. There is more carbon dioxide so the stomata will close and, as mentioned before, this will reduce water loss. Thus the crops will transpire less ( lose water through leaves and petals) so water can be used in photosynthesis.
Yes, plants growing quick is great it means more fruits and veggies, but these fruit and veggies are not always the best quality. According to
CFAES, plants growing in high levels of CO2 on average will have more starch and fewer proteins and vitamins e.g. iron, zinc and vitamin C.
Photosynthesis produces glucose (stored as starch) and as the rate of photosynthesis rapidly increases in high CO2 climates, the plant will create starch primarily and vitamins and proteins when it respires ( the plant would respire less as of the high rate of photosynthesis). Meaning that the food produced by the crop will not be as nutritious. Luckily, some plants are not affected by this cycle, for example, corn and sugar cane.
Acid rain weakens the leaves and changes the pH of the soil these do not directly kill plants but will eventually if the rain is regular. CO2 can be useful for growing crops quick but these crops will be less nutritious. Quality over quantity.