A plant needs to open its stomata to let Carbon Dioxide in for photosynthesis. By doing this water vapour is let out. There is a higher water potential inside the leaf than outside - more a side effect of gas exchange.
Factors that affect transpiration rate:
1. Light - the lighter the faster the rate. Because the stomata are open when its light for gaseous exchange.
2. Temperature - higher temperature increases the rate. Warmer water molecules have more energy to evaporate from the cells inside the leaf, increasing the water potential gradient.
3. Humidity - lower humidity, increases the rate. If the air around the plant is dry the water potential gradient is decreased.
4. Wind/Movement of air - more wind, faster transpiration rate. Lots of…