JoAnne Skelly: The science of water in plants

During the growing season we think about our plants’ water needs. Since plants can be up to 95 percent water, water is crucial to their growth, reproduction, and survival.

“Plants use water in three main ways: to maintain their turgidity, to transport water-soluble nutrients and to photosynthesize. A turgid plant is erect, not limp. When plant cells have enough water, they maintain their shape. When cells lack water, they shrink and the plant wilts, or loses turgidity. By the time a plant wilts, many plant processes have stopped and the plant will take some time to recover” (Plants in Drought https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=13978, Y. Rasmussen, UC Davis).

Turgidity happens through osmosis where water moves from an area of high concentration into an area of low concentration. When there is more water outside the plant cell than inside the plant cell, water will move into the plant cell until the two areas have the same concentration.

But the reverse is also true. If the soil around a plant is dry, the water will move out of the plant cell into the soil. Osmosis allows plants to absorb water from the soil and pull it up through the plant, eventually to the leaves like sucking water up a straw. If water moves out from the roots to the soil, the cells next to the root cells start losing water too. This loss of water continues up the entire plant until the plant wilts.

By the time a plant wilts, many plant processes have stopped. If turgidity is restored in time, a plant may recover. If not, a plant’s ability to transport nutrients is reduced or eliminated. It’s capability to photosynthesize is compromised because the release of oxygen out of the plant, which is a byproduct of photosynthesis, releases water vapor at the same time.

Normally, when the vapor goes out, osmosis and other properties pull replacement water into the roots from the soil and up the plant. If water is unavailable in the soil, the plant starts to cut back on “non-essential functions like growth and fruit production to essential processes that keep it alive. Root activity stops and cells stop growing and multiplying. A plant initiates senescence – the onset of old age – and foliage, branches and roots begin to die. Next, it prepares to drop these dead parts. First, it drops any fruit, then leaves and eventually branches. At some point, the whole plant may die” (Rasmussen).

For good growth and production, it is important to keep soil evenly moist through the growing season based on a particular plant’s water requirements.

JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email skellyj@unr.edu.