Stomatal pores cover only about 1% of a leaf surface, yet nearly all the CO₂ feeding the dark reactions has to squeeze through the slit between two tiny guard cells. The law of limiting factors rules everything: the rate is always pinned by whichever of light, CO₂ and water is scarcest, not simply by how bright the day is. Drought exposes the paradox most sharply — the plant clamps its stomata shut to save water, so photosynthesis nearly stops in full sun.
Stomatal pores cover only about 1% of a leaf surface, yet nearly all the CO₂ feeding the dark reactions has to squeeze through the slit between two tiny guard cells. The law of limiting factors rules everything: the rate is always pinned by whichever of light, CO₂ and water is scarcest, not simply by how bright the day is. Drought exposes the paradox most sharply — the plant clamps its stomata shut to save water, so photosynthesis nearly stops in full sun.
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