Question

On a very hot and dry day, a plant will likely close its stomata to save water. Explain what negative side-effect (trade-off) this creates for the plant's ability to make food.

Answer 1

Stomata are pores that let CO2 into the leaf for photosynthesis but also let water vapor out. When a plant closes its stomata to conserve water, CO2 entry is reduced, so the internal CO2 concentration falls. That limits the Calvin cycle (Rubisco has less CO2 to fix), so the rate of photosynthesis and sugar production drops. Lower CO2 also favors Rubisco’s oxygenation reaction (photorespiration), which wastes energy and carbon, and reduced transpiration raises leaf temperature, further increasing photorespiration. In short: closing stomata saves water but reduces the plant’s ability to make food and grow.