When environmental humidity decreases around a C3 plant, which specific mechanism initially reduces photosynthetic rate?

💡 Explanation

When humidity decreases, stomatal closure is triggered by guard cell turgor loss because of increased transpiration, limiting CO2 diffusion into the leaf. Therefore, reduced CO2 causes a decrease in the photosynthetic rate, rather than chloroplast degradation, Rubisco inactivation, or photosystem damage, which are longer-term responses.

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