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 (300+ words):
In C3 plants, the conservation of water—typically through closing stomata during hot or dry conditions—can lead to photorespiration rather than enhancing photosynthesis. This is a significant physiological trade-off in plant biology.
C3 Photosynthesis Basics:
C3 plants are named for the three-carbon compound (3-phosphoglycerate) that is the first product of carbon fixation in the Calvin cycle. The enzyme responsible for carbon fixation is RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). RuBisCO ideally binds with CO₂ to form sugars during photosynthesis.
Stomatal Closure and Water Conservation:
To prevent water loss under dry or hot conditions, C3 plants close their stomata. While this action helps reduce transpiration (the loss of water vapor), it also limits gas exchange, causing:
- A decrease in CO₂ concentration inside the leaf.
- A build-up of O₂, a by-product of the light reactions of photosynthesis.
Why Photorespiration Increases:
With less CO₂ and more O₂ in the leaf’s internal atmosphere, RuBisCO is more likely to bind with oxygen instead of carbon dioxide. This leads to photorespiration, a process where:
- RuBisCO fixes O₂ instead of CO₂.
- The resulting compounds are not useful for sugar production.
- Energy is consumed rather than generated.
- CO₂ is released rather than fixed.
Thus, water conservation in C3 plants ironically inhibits photosynthesis and promotes photorespiration, making it a wasteful process under such conditions.
Comparison with Other Options:
- Photosynthesis is hindered, not promoted, under water stress.
- C4 photosynthesis is a trait of specific plants (like corn), not a “shift” made by C3 plants.
- Light reactions still occur but are not the main concern in this scenario.
- Stomatal opening is reduced to conserve water, not encouraged.
Conclusion:
In C3 plants, water conservation through stomatal closure leads to photorespiration due to low CO₂ and high O₂ levels inside the leaf, reducing the efficiency of photosynthesis.