what is root pressure? and what are the factors affecting the rate of transpiration?

Root pressure is one of the the force involved in the water movement up the xylem.  This can be observed when a freshly cut root stump continues to exude sap from its xylem vessels. The process is inhibited by respiratory inhibitors such as cyanide, lack of oxygen, and low temperatures. The mechanism probably depends on active secretion of salts or other solutes into the xylem sap, thus lowering its water potential. Water then moves into the xylem by osmosis from neighbouring root cells.

The rate of transpiration is affected by Relative humidity, Temperature, Soil water, Light and Wind speed.