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, a process that can reduce the efficiency of photosynthesis. This outcome is rooted in the physiological adaptations of C3 plants, which differ from those of C4 and CAM plants.
C3 plants carry out photosynthesis using the Calvin cycle, where the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) fixes atmospheric CO₂ into a 3-carbon compound, 3-phosphoglycerate (3-PGA). RuBisCO, however, has a dual affinity — it can also bind to O₂, especially when CO₂ concentrations are low.
When water conservation becomes necessary, especially under hot or dry conditions, C3 plants close their stomata to reduce water loss through transpiration. This closure reduces the influx of CO₂ while O₂ continues to accumulate due to ongoing light reactions. The resulting low CO₂:O₂ ratio inside the leaf favors the oxygenase activity of RuBisCO. Instead of fixing CO₂, RuBisCO binds O₂ and initiates photorespiration, a process where oxygen is consumed and carbon dioxide is released without producing ATP or sugars.
Photorespiration is generally considered wasteful, as it counteracts the benefits of photosynthesis. It consumes energy and leads to the loss of fixed carbon, reducing the plant’s overall productivity. C4 and CAM plants have evolved mechanisms to minimize photorespiration, but C3 plants do not have these adaptations.
To summarize, when C3 plants conserve water by closing stomata, they inadvertently create internal conditions (low CO₂, high O₂) that promote photorespiration. While this may help the plant survive drought conditions, it does so at the cost of photosynthetic efficiency.
Therefore, the correct answer is “photorespiration” — it is the process that is promoted in C3 plants when water conservation leads to stomatal closure.
