Wilting occurs when a plant loses more water through transpiration than it absorbs from the soil.To prevent further water loss, the stomata on the leaves close. When the stomata are closed,CO2 can't enter the leaf. CO2 is essential for photosynthesis, which produces glucose and oxygen.
The rate of photosynthesis decreases in a wilted herbaceous plant because the stomata close, preventing carbon dioxide (CO2) from entering the leaf.