The Question is from neet-botany photosynthesis-in-higher-plants factors-affect-photosynthesis, and it was created by the author D VINOD

Question

Under conditions of high temperature and drought stress, a plant closes its stomata to conserve water.  Which of the following is the MOST DIRECT consequence on the Calvin cycle?

D VINOD · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Photosynthesis and Stomatal Regulation
 **Related Topics**: Calvin Cycle, Carbon Fixation, Photorespiration, Stomata, Water Stress
 **Prerequisites**: Understanding of the Calvin cycle, the role of stomata in gas exchange, and the impact of environmental factors on plant physiology.
## SOLUTION PATHWAY
**Given Information**:
  Plants close stomata under high temperature and drought stress to conserve water.
  This directly affects the availability of CO2 for the Calvin cycle.
**Method & Steps**:
 1. Stomatal closure reduces CO2 uptake.
  Reasoning: Stomata are the pores on leaves through which CO2 enters and water vapor exits.  Closing them limits CO2 influx.
  Formula/Rule:  None directly applicable, but understanding the role of stomata is key.
 2. Reduced CO2 limits carboxylation in the Calvin cycle.
  Working: The Calvin cycle begins with the carboxylation of RuBP (ribulose-1,5-bisphosphate) by Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase).  Less CO2 means less carboxylation.
  Key Point:  The Calvin cycle requires CO2 as a substrate.  A shortage directly impacts the rate-limiting step.
 3. Decreased carboxylation leads to reduced RuBP regeneration.
  Result:  The products of carboxylation are eventually used to regenerate RuBP, which is essential for the continuation of the cycle.  If carboxylation is reduced, RuBP regeneration is also reduced, slowing down the entire Calvin cycle.
  Verification: The question asks for the *most direct* consequence.  Reduced CO2 availability directly affects the first step of the cycle, carboxylation.
 ## OPTION ANALYSIS
A :x: Increased photorespiration is a consequence of reduced CO2, but it's an indirect effect.  Higher O2 concentration relative to CO2 increases the likelihood of Rubisco binding to O2 instead of CO2, leading to photorespiration.
B :x: ATP synthesis is affected by light-dependent reactions, not directly by stomatal closure and CO2 availability in the Calvin cycle.
C ✓: Reduced CO2 directly limits the carboxylation of RuBP, which is the initial step of the Calvin cycle, slowing the regeneration of RuBP and the cycle's overall rate.
D :x: Rubisco activity decreases due to lower substrate (CO2) concentration, not increases. 
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing the light-dependent and light-independent (Calvin) reactions:  Stomatal closure impacts the Calvin cycle directly, not the light-dependent reactions.
  2.Focusing on indirect consequences:  While photorespiration increases, the most direct effect is the lack of CO2 for carboxylation.
 **Quick Tips**:
  1.Remember the role of stomata in gas exchange.
  2.Understand the first step of the Calvin cycle and its dependence on CO2.

D VINOD · Answer

Increased photorespiration due to oxygen accumulation.

D VINOD · Answer

Enhanced ATP synthesis due to increased proton gradient.

D VINOD · Answer

The correct Option is "Reduced availability of CO2 limits carboxylation, leading to a decrease in RuBP regeneration."

D VINOD · Answer

Increased activity of Rubisco due to higher substrate concentration.

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