In C3 plants the conservation of water promotes _.
a. photosynthesis
b. a shift to C4 photosynthesis
c. the opening of stomata
d. photorespiration
e. the light reactions
The Correct Answer and Explanation is :
In C3 plants, the conservation of water promotes b. a shift to C4 photosynthesis.
Explanation
C3 plants utilize the Calvin cycle for photosynthesis, wherein carbon dioxide (CO₂) is fixed into a three-carbon compound, 3-phosphoglycerate (3-PGA). However, these plants are prone to inefficiencies, especially under conditions of high temperature and low moisture. When water is scarce, C3 plants must close their stomata to reduce transpiration, the process by which water vapor exits the leaves. This closure limits the intake of CO₂, leading to a decrease in photosynthetic efficiency and an increase in photorespiration—a process where oxygen competes with CO₂ for the active site of the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO), resulting in the production of a two-carbon compound instead of the desired three-carbon compound.
Under prolonged conditions of water stress, some C3 plants may exhibit a shift toward C4 photosynthesis. This adaptation allows them to optimize carbon fixation even when stomata are partially closed. C4 plants have evolved a different pathway to initially capture CO₂ in a four-carbon compound (oxaloacetate) using a distinct enzyme (phosphoenolpyruvate carboxylase, or PEP carboxylase). This mechanism effectively concentrates CO₂ around RuBisCO, reducing the chances of photorespiration and allowing photosynthesis to continue efficiently even under conditions of high light intensity and low CO₂ availability.
While C4 photosynthesis is generally more efficient in hot, dry environments, the shift from C3 to C4 pathways is not immediate or universal across all C3 species. It reflects an evolutionary response that enhances water-use efficiency and photosynthetic output. Thus, in C3 plants, the conservation of water may eventually drive this shift to more efficient C4 photosynthesis under challenging environmental conditions.