[Campbell Biology P.252] When C3 plants close their stomata on hot, dry days, what primary c... | Practice Question

When C3 plants close their stomata on hot, dry days, what primary consequence occurs that leads to reduced sugar production, and how do C4 plants address this issue?

  • A: CO2 levels inside the leaf decrease, starving the Calvin cycle; C4 plants use PEP carboxylase with high CO2 affinity in mesophyll cells.
  • B: O2 levels inside the leaf decrease, inhibiting rubisco; C4 plants release O2 in bundle-sheath cells.
  • C: Water loss increases, causing dehydration; C4 plants have thicker cuticles to prevent water loss.
  • D: Light reactions cease due to heat; C4 plants have heat-resistant enzymes.

Explanation

On hot, dry days, C3 plants close their stomata to conserve water, which reduces CO2 intake and increases internal O2 levels. This leads to rubisco binding O2 instead of CO2, causing photorespiration and less sugar production. C4 plants circumvent this by initially fixing CO2 with PEP carboxylase in mesophyll cells, an enzyme with a high affinity for CO2 and no affinity for O2, allowing efficient carbon fixation even at low CO2 concentrations.