A. Stress hormone ABA (Abscisic Acid) gives signals to stomata to get closed, whenever there is scarcity of availability of water to the plant. This prevents the loss of water through transpiration by leaves. It also increases the tolerance of plants to various kinds of stresses.

B. It (ABA) is released or transported from stem apices to leaves.

Plants can get rid of excess water by increasing the transpiration rate but upto a certain limit. Prolong water logged conditions for roots may lead to the death of the plant (as they can not respire).

Difference between diffusion and translocation in plants is as follows

Diffusion	Translocation
It is the movement of solute from higher concentration to lower concentration.	It is movement of organic compounds mineral salts from one part of the plant to another.
No energy consumption involved.	It is energy consuming process.

Difference between diffusion and facilitated diffusion is as follows

Diffusion	Facilitated Diffusion
In diffusion, molecules move in a random fashion, the net result being substances moving from regions of higher concentration to regions of lower concentration.	The diffusion of substance against a concentration gradient, which is facilitated by the proteins is known as facilitated diffusion without expenditure of ATP energy.
Diffusion is a slow process and is not dependent on a 'living system'. No expenditure of energy takes place.	The porins proteins that form huge pores in the outer membranes of the plastids, mitochondria and some bacteria allow molecules up to the size of small proteins to pass through.

Mass flow hypothesis was first proposed by Ernst Munch (1930). It explains the transport of solute or sugars, i.e., sucrose in the phloem tissue.

It involves the following three steps

(a) Phloem loading

(b) Translocation of solutes

(c) Phloem unloading

Loading of sieve tubes takes place in leaves. Photosynthetic cells make sugars, particularly sucrose, and other organic solutes. Companion cells use energy to collect solutes by active transport. As solute concentration increases in the companion cells, water enters by osmosis.

A pressure is created, which pushes the solutes through plasmodesmata into the sieve tubes. It is an active process.

Translocation of sucrose takes place through phloem column. Increase in hydrostatic pressure inside sieve tubes is greatest at the source and lowest at the sink. It pushes sucrose etc., from source to sink.

Unloading of the sieve tubes takes place at the sink sites.

It results in active transport of sugars out of the pholem converting them into complex sugars. Loss of solutes from the phloem create a high water potential, and water moves out of phloem, returning back to xylem.

Sinks are the regions where solutes i.e., sugars or food are being used, e.g., roots, fruits, storage organs and regions of growth.