n C3 plants the conservation of water promotes
a. photorespiration
b. photosynthesis
c. a shift to C4 photosynthesis
d. the light reactions
e. the opening of stomata
The correct answer and explanation is:
The correct answer is:
a. photorespiration
Explanation:
In C3 plants, water conservation often comes at the expense of efficient photosynthesis due to the physiological response of closing stomata under water-limited conditions. Here’s why:
1. Stomata and Water Loss:
Stomata are small pores on the leaves that facilitate gas exchange. When stomata are open, CO₂ enters the leaf for photosynthesis, and water vapor exits due to transpiration. Under drought or water-limited conditions, stomata close to reduce water loss.
2. CO₂ Limitation:
When stomata close, the internal concentration of CO₂ in the leaf decreases because CO₂ cannot enter. As a result, the ratio of CO₂ to O₂ in the leaf becomes imbalanced, favoring oxygenation over carboxylation.
3. Photorespiration:
The enzyme RuBisCO, which catalyzes the first step of the Calvin cycle, can use either CO₂ or O₂ as a substrate. When CO₂ levels are low (due to closed stomata), RuBisCO increasingly binds to O₂ instead of CO₂. This process is called photorespiration. Unlike photosynthesis, photorespiration:
- Produces no sugars.
- Wastes energy.
- Releases previously fixed CO₂.
4. Impact on C3 Plants:
C3 plants, which use the Calvin cycle for carbon fixation, are particularly vulnerable to photorespiration under conditions of low CO₂ availability. This process is a major inefficiency in C3 photosynthesis, especially under hot, arid conditions.
5. Significance:
While photorespiration is often seen as a wasteful process, it may help protect the plant from oxidative stress during drought. However, it reduces overall photosynthetic efficiency and crop productivity, which is a challenge for agriculture in water-scarce regions.
Thus, in C3 plants, conserving water by closing stomata indirectly promotes photorespiration due to reduced CO₂ availability inside the leaf.