The main function of bulliform cells in grasses is :

The correct answer is: to minimize water loss during water stress.

Bulliform cells are large, thin‑walled epidermal cells that occur in the upper side of grass leaves. Their unique shape and location give them a special role in leaf physiology.

When a grass leaf is well hydrated, bulliform cells are turgid (filled with water). Their expansion causes the leaf blade to roll outward, keeping the leaf surface flat and fully exposed to the environment. This exposure allows photosynthesis to proceed efficiently.

During water stress (drought or high evapotranspiration), the plant loses water and the turgor pressure inside bulliform cells drops. The cells become flaccid and shrink. As they shrink, the leaf blade rolls inward, forming a tubular shape.

This rolling action creates a micro‑environment inside the rolled leaf where the air is more humid and the leaf surface area exposed to the dry external air is greatly reduced. Consequently, transpiration (water loss through the stomata) is minimized.

Thus, the primary function of bulliform cells is to help the leaf conserve water during periods of water deficit by rolling the leaf and reducing water loss.