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Photosynthesis is the reaction of carbon dioxide and water in the presence of sunshine and chlorophyll to produce glucose and oxygen.

The photosynthetic reaction is summarized as follows:

$6{\mathrm{CO}}_2+12{\mathrm{H}}_2\mathrm{O}→{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$

Stomata are small apertures on a plant's leaf surface that aid in the exchange of gases.

They are typically found beneath the leaf's surface. These apertures let carbon dioxide to enter the plant and be used in photosynthesis.

Stomata's guard cells keep the closure and opening in check.

Plants lose water through their stomata, therefore on a hot, dry day, the openings are closed to conserve water.

Because carbon dioxide is a driving force for photosynthesis, as the stomata close to conserve water, the rate of photosynthesis will slow.

Because there will be no openings for carbon dioxide to pass through, the rate of photosynthesis will be slowed.

On a hot, dry day, the rate of transpiration increases, causing the plant to lose more water through evaporation.

As a result, stomas must be closed to conserve water, which will eventually reduce photosynthetic rates due to low carbon dioxide levels.