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 :
The correct answer is: photorespiration
Explanation:
In C3 plants, water conservation can inadvertently promote photorespiration, a process that reduces the efficiency of photosynthesis. Here’s why:
C3 plants use the Calvin cycle to fix carbon dioxide, where the enzyme RuBisCO plays a central role. Under normal conditions, RuBisCO binds to CO₂ to produce a three-carbon compound. However, RuBisCO can also bind to oxygen (O₂) instead of CO₂, especially when the concentration of CO₂ inside the leaf is low. This alternative reaction leads to photorespiration, which consumes energy and releases fixed CO₂, thereby counteracting photosynthesis.
When water is scarce, C3 plants close their stomata (tiny pores on leaf surfaces) to prevent water loss through transpiration. But closing stomata also restricts the entry of CO₂ and limits its availability inside the leaf. At the same time, O₂—produced during the light reactions of photosynthesis—accumulates because it cannot diffuse out. The resulting low CO₂-to-O₂ ratio increases the likelihood that RuBisCO will fix oxygen rather than carbon dioxide, thus enhancing photorespiration.
Photorespiration is considered wasteful because it reduces the plant’s photosynthetic output without producing ATP or sugars. While it may have protective roles in certain contexts, such as detoxifying by-products or preventing photodamage, it generally reduces crop yield in C3 plants under hot, dry conditions.
In contrast, C4 and CAM plants have evolved mechanisms to concentrate CO₂ at the site of RuBisCO, thus minimizing photorespiration even when their stomata are closed. However, C3 plants, lacking such adaptations, suffer increased photorespiration under water-conserving conditions.
In summary, conservation of water in C3 plants typically leads to stomatal closure, reduced internal CO₂ levels, and a corresponding increase in photorespiration.