Question

When air humidity is high does the plant transpiration increase or lower?

Answer 1

Transpiration is the movement of water within a plant body through its capillaries and evaporation through its aerial parts such as stem, leaves and flowers. Water is necessary for plant growth but only a small amount is used for its metabolism. The remaining 97-99 % is lost through transpiration.

Complete answer:
When there is higher humidity, a plant cannot make water to evaporate or draw nutrients from the soil. When it occurs over a prolonged time the plant body rots.
Relative humidity is the amount of water vapor in the air compared to the amount of water vapor that air could hold at a given temperature. Any reduction in water in the atmosphere creates a gradient for water to move from the leaf to the atmosphere. The lower the relative humidity of air, the less moist the air and higher will be transpiration.

Note:
The faster the transpiration the faster the plants take in water and nutrients. Humidity directly affects the transpiration rate. When relative humidity is high, then there is lots of water vapor in the atmosphere.