The Gardens Trust
… Plants need water so plants in dryer location gain an evolutionary advantage if they can quickly maximise their use of sudden downpours, such as produced in a thunderstorm, before it soaks away. Thunderstorms carry an electrical charge and somehow plants have learned to read that as a signal that heavy rain is imminent. What lab experiments showed was that the optimum electrical charge to be applied to plants to increase yield turned out to be similar to the charge in a thunderstorm.
When it receives the charge a plant activates genes and speeds up its metabolism which includes increasing the rate at which roots can absorb water and thus encouraging growth etc.