Question

What is transpiration pull? How is it caused?

Answer 1

Transpiration is the process of exhalation of the water vapour through the stomata. Plants lose water as $90\%$ and above. The water vapour is released from the plants by the pore-like structure on the leaves. It is small and closable. Water molecules are cohesive in nature so it is pulled from the plant by the sunlight. This stream has lots of functions. Transpiration pull is the process that works against gravity. Normal transpiration is upward movement.

Complete answer:
Transpiration pull is the physiological process. The force that works against the gravity’s direction due to the regular process called transpiration pull. So this process helps to move the water and dissolved minerals in the upper parts of plants. Water is cohesive so it is lost from the leaf surface by transpiration but more water molecules also pull up to the leaves and parts in the upper surface. So the plant gets their water through a stem. Because of transpiration pull negative pressure is created which is used to move the water particles in the plants. Root pressure is also called the osmotic pressure. It raises water by the plant stem to the leaves within the root system. The transpiration pull is controlled by the stomatal aperture and small pores on the leaves for photosynthesis. Cohesive means capacity to rejoin. Transpiration pull is important because it pushes the water molecules from the plants. Transpiration pull is also called the sectional force. It means pulling water from the ground in an upward direction. During the photosynthesis a small amount of water is used and remaining will be vaporized. Adhesive force acts between the water molecule and xylem vessel. The transpiration pull theory is also related with that of the cohesion tension theory. The negatives of transpiration pull are it will reduce the growth of the plants and reduce the yielding. It will also produce the abscisic acid. It wastes the energy of the plants.

Note:
Transpiration pull is used to transfer the minerals and water to all parts of the plants and most importantly the top of the plants. Transpiration pull is called the negative driving force because it vaporizes water in upward direction. So the transpiration pull is helpful in the process called photosynthesis. This process used to avoid the excessive taking of water by the plants. It cools down the plants by water vaporization. Development of mechanical tissue may increase the transpiration.