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 primarily promotes photorespiration, which is a process that reduces the efficiency of photosynthesis.
C3 plants, such as rice, wheat, and barley, rely on the Calvin cycle to fix carbon dioxide (CO₂) into organic molecules. The enzyme RuBisCO is responsible for capturing CO₂ during this process. However, RuBisCO has a dual affinity—it can bind to oxygen (O₂) as well as CO₂. When O₂ levels are high and CO₂ levels are low inside the leaf, RuBisCO begins to fix O₂ instead of CO₂, leading to photorespiration.
When water is limited, C3 plants close their stomata to reduce water loss through transpiration. However, this also reduces the inward flow of CO₂ and causes CO₂ concentration in the leaf to drop, while O₂ accumulates due to ongoing light reactions. The altered CO₂:O₂ ratio favors the oxygenation activity of RuBisCO, which initiates photorespiration. This process uses energy and releases CO₂, undoing the work of photosynthesis and decreasing the plant’s productivity.
Photorespiration is essentially a wasteful pathway from the plant’s perspective, but it’s unavoidable in C3 plants under water stress. Some plants, called C4 plants, have evolved a different photosynthetic mechanism that minimizes photorespiration, but C3 plants do not have this advantage.
Therefore, in C3 plants, conserving water (by closing stomata) indirectly promotes photorespiration because it limits CO₂ availability and enhances O₂ binding to RuBisCO. This is one of the major limitations of C3 photosynthesis in hot, dry environments, where water conservation is crucial.
