In C3 plants the conservation of water promotes:
photorespiration
photosynthesis
a shift to C4 photosynthesis
the light reactions
the opening of stomata
The Correct Answer and Explanation is:
Correct Answer: Photorespiration
Explanation
In C3 plants, the conservation of water often leads to photorespiration, a process that is generally wasteful for the plant. This occurs primarily due to the closure of stomata under hot and dry conditions, which plants do to reduce water loss through transpiration.
Stomata are tiny openings on the leaves that allow for gas exchange—CO₂ enters, and O₂ and water vapor exit. When the plant experiences water stress, it closes its stomata to conserve moisture. However, this also limits the intake of carbon dioxide (CO₂), which is essential for the Calvin cycle in photosynthesis.
As CO₂ becomes limited inside the leaf, the concentration of oxygen (O₂) increases because it continues to be produced during the light-dependent reactions of photosynthesis. The enzyme RuBisCO, which normally fixes CO₂ to start the Calvin cycle, starts binding O₂ instead of CO₂ due to the higher oxygen concentration. This alternative pathway is known as photorespiration.
Unlike normal photosynthesis, photorespiration does not produce sugars. Instead, it uses energy and releases CO₂, making it a counterproductive process for the plant. It is considered a metabolic flaw in C3 plants, particularly under stress conditions like drought or high temperatures.
C3 plants, named for the 3-carbon compound (3-phosphoglycerate) formed during the first step of the Calvin cycle, are the most common type of plants. However, under conditions favoring photorespiration, they become less efficient.
This is one of the reasons C4 plants evolved. They have a specialized mechanism to minimize photorespiration by spatially separating the initial CO₂ fixation and the Calvin cycle, thus maintaining high CO₂ levels around RuBisCO.
In summary, when water conservation leads to stomatal closure in C3 plants, photorespiration is promoted due to reduced CO₂ and increased O₂ inside the leaf, making this the correct answer.
