Basic plant physiology: How can it fail?
We’ve learned how simple the formula for photosynthesis seems. Just take some CO2, a little water, throw in some sunlight and there you have it, carbohydrates and oxygen. Not so fast, what happens when something goes wrong? Let’s look at the first part of the photosynthetic equation and talk about what could go wrong. Carbon dioxide is plentiful in our atmosphere but can still be a limiting factor in the plant’s ability to perform photosynthesis. CO2 enters the plant through small openings on the under side of leaves known as stomates. When conditions are right stomates open and CO2 enters the plant. Believe it or not a lot can go wrong in this process. For example when a plant is under drought stress, water is absent for photosynthesis and typically the plant will close all stomates preventing the flow of water from the plant to the atmosphere. Stomatal closure means water no longer flows out and CO2 no longer flows in. Another example is when the soil is compacted and gas exchange is reduced. The majority of the CO2 in the atmosphere used for photosynthesis is liberated from the soil as a byproduct of microbial activity and organic matter breakdown. Additionally when soil is too wet or too dry microbial activity suffers and CO2 evolution is limited.
The next component of photosynthesis is water. It’s obvious that drought is defined as a severe water deficiency. Lack of rain or irrigation is not the only way water can be limited for photosynthesis. Plants take in water with their root system. Compacted soils, excessive thatch, excessive organic matter, soil layering, and a poor root system are just some of the ways in which a plant may be challenged for water during the growing season. A condition called “wet wilt” occurs when the plant can no longer take in water even though there is plenty available in the root zone. Plants need oxygen to perform root respiration. Plants have to perform root respiration to take in water. If the soil is saturated a plant cannot perform root respiration and can no longer take in water even though it is plentiful.
“The sun will come out tomorrow”. Well not always, and if a tree is blocking the sunlight its rising is irrelevant. The putting surface of a golf course green consists of plants mowed below an eighth of an inch at times. That’s about the same thickness as two nickels stacked on top of each other. Not a very big solar panel!
We’re starting to discover there’s a lot more to photosynthesis and energy production than we thought. In the next Fuel Log we will look at the role nutrients play in photosynthesis.