Science (086) — AI-generated practice question

During the day, stomata are open for photosynthesis, causing rapid transpiration. The loss of water vapour creates a tension (negative pressure) in xylem that pulls water upward — this is transpiration pull, the dominant force.

At night, stomata close, so transpiration virtually stops and transpiration pull disappears. However, root cells continue absorbing mineral salts by active transport, lowering their water potential. Water then enters root xylem by osmosis, building up root pressure (a positive pressure) that pushes water upward. With transpiration pull absent, root pressure becomes the primary driving force for water movement at night.

This question tests understanding of two mechanisms of water transport in xylem and the conditions that favour each. Key points examiners expect:

• Day: stomata open → transpiration → tension/pull in xylem (cohesion-tension mechanism).
• Night: stomata close → no transpiration pull → root pressure (active uptake of ions → osmotic entry of water) takes over.
• Clearly link the shift to stomatal behaviour.

Note: The source passages provided do not cover this topic directly — this answer is based on standard CBSE Class 10 Biology content on transportation in plants.