Water constitutes the large Central Vacuole in most plant cells. It is the medium for major transport processes in plants. The Oxygen we breathe comes from the splitting of water in Photosynthesis. Water is continuously absorbed and lost by plants. A plant may loose its entire fresh weight 100 times during its lifetime.
Transpiration is the process by which leaves regulate water loss.
Evaporation cools leaves and prevents them from overheating. Water is the most limiting factor for agriculture . It is also limiting for many natural ecosystems . The latter can be due to its absence or its over abundance.