Exploring science

It is important to understand how different crops photosynthesize to respond differently under high CO2 levels.

Depending on the crops’ photosynthesizing ability, they are categorized into three types, C3, C4, and CAM (crassulacean acid metabolism).

Have you ever wondered why some plants perform well under certain climates (hot, dry, cool) and some may not thrive at all?

It is the enzyme Rubisco that grabs the CO2 from the atmosphere and puts in the assembly line in the Calvin-Benson cycle to produce carbohydrates that is essential for plant’s growth.

In C3 photosynthesis, the mechanism to concentrate the CO2 and transpiration, which is the loss of water, takes place through the same opening where the Rubisco is located. The second issue is that oxygen produced during the photosynthesis process can inhibit Rubisco enzyme, causing plants to experience too much light during the summer when high photosynthetic activity takes place, damaging the plants and causing water loss. Examples of C3 plants are rice and cool- season grasses. Most plants use the C3 mechanism to make carbohydrates through CO2.

In C4 plants, Rubisco is located in one cell and CO2 is transported through a different cell that is connected by openings between the cell called plasmodesmata. Unlike C3 plants, this unique mechanism allows C4 plants to concentrate CO2 in the vicinity of Rubisco, preventing oxygen inhibition. Consequently, C4 plants don’t experience high levels of sunlight during summer that can damage the plants. Examples of C4 plants include corn, sorghum, Bermuda grass, and foxtail. This type of mechanism is found in only one percent of plant species.

CAM photosynthesis, the third type, is found only in desert plants. Like all types of photosynthesis, CO2 is reduced to carbohydrates in Rubisco, but CO2 enters through the stomatal cell opening during the night and is stored in the plant vacuole as ‘malic acid’. In the morning, when the sun comes out, the stomatal opening is closed, and CO2 is removed from the malic acid and send to Rubisco to make carbohydrates. Examples of CAM photosynthesis are cactus and succulent plants.