Physical & Chemical Changes Impacting Photosynthesis & Respiration

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll talk about how physical conditions such as drought, heat, and salinity can be a problem for plants as they try to photosynthesize and respire.

Photosynthesis and Environment

You have a lot of control over your environment. If you are in an uncomfortable or dangerous place, you can use your muscles to get up and leave. By using your brain, you can also find a way to make things better. For the most part, you probably choose to eat things that you like or that you think are healthy; at the very least, you probably don't eat things that aren't food.

Most of us don't think of plants as having this kind of control over their environment. Despite not having brains or muscles, plants do regulate their internal environment in some very complex ways.

One of the things that almost all plants must do is to photosynthesize. Photosynthesis allows plants to convert carbon dioxide into sugars that they can use. Let's talk about some stressors that interfere with a plant's ability to efficiently photosynthesize.

General Stress

Many different stressors, including heat, drought, and salinity, have similar effects on some systems. For instance, stressing a plant causes it to produce more stress hormones, like abscisic acid. Concentrations of enzymes that are necessary for photosynthesis, such as Rubisco and pyruvophosphate dikinase, may decrease.

Heat and Drought

Plants need carbon dioxide for photosynthesis. They get this carbon dioxide from the atmosphere through little pores called stomata, but they also lose water through their stomata. So, if you're a plant, you want to open your stomata to maximize photosynthesis, but you also might want to close them if it's dry outside! Plants have the ability to open and close their stomata through 'lips' called guard cells. Guard cells that are full of water are puffy like water balloons and keep the stomata open. When guard cells dehydrate, they close.

A stoma
A stoma

So, plants that live in dry environments often photosynthesize less efficiently. Because heat increases evaporation rates, plants that live in very hot environments also photosynthesize less efficiently. Severe drought will wither other cells in the plant called mesophyll cells. This can also inhibit photosynthesis. Light regulates the production of many enzymes, so extreme light conditions like those in the desert also throw photosynthetic enzymes out of whack.

Salt

When a plant imbibes too much salt, photosynthesis is reduced in two stages: first, a stage of slow decline, then, a sudden, rapid decline of photosynthesis.

This first stage is probably caused by our old friends, the stomata. The osmotic pressure from salt causes water to rush out of the guard cells. As we've discussed, this causes the stomata to close, making photosynthesis slower because there may not be enough carbon dioxide.

In the second stage, salt causes a plant's chemical changes in a more direct way. Chlorophyll, the dye that plants use to collect energy from sunlight, is inefficiently made in environments that are high in salt. Ions like sodium collect in the chloroplasts, the organelles responsible for photosynthesis. This can be harmful to the chloroplasts.

Respiration

Respiration increases with temperature until about 40 or 50 degrees C. Then, it levels off for a bit, and after 50, respiration begins to decrease because the plant is damaged. Drought, on the other hand, decreases respiration.

However, in the field, these effects become entangled and unclear. For instance, drought conditions may decrease respiration directly, but also increase ground temperature, which may increase respiration.

If you suddenly increase the salinity of a plant's environment, it will increase respiration. This phenomenon is known as salt respiration. This is possible because the salt increases the demand for respiration, since plants need to move those salt ions around. However, in the long term, salinity will decrease respiration, because salt is toxic to plants.

Why Is This Important?

You already know that plants have ways of dealing with these kinds of stressors. Not all plants die in hot, dry desert conditions; cacti can be well adapted to these conditions. Not all plants die in saline conditions; mangroves tolerate salt well.

But you probably don't want to live on a diet of cacti and mangroves. It's also important that crop plants are able to photosynthesize efficiently so that we can have food to eat. Unfortunately, climate change is creating more extreme temperature and drought conditions. Our human population of over seven billion people means that we sometimes have to farm crops on less-than-ideal farmland. So what do we do to help crop plants photosynthesize efficiently?

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