Oops, I forgot to water my plants. I realize this when I see the plants are wilted. Wilting is the very first reaction a plant has to drought stress. This is what happens at the cellular level when a plant wilts.
The process of photosynthesis requires the plant to exchange gases (carbon dioxide and oxygen) for water. This is called photorespiration (exchanging gas) and transpiration (water loss). Both processes occur at pores or opening on the bottom sides of the leaves called the stomata. Two specialized cells, called guard cells, open and close the stomata.
The guard cells work by water pressure through osmosis. When sunlight warms the plant leaf, chemical reactions begin and water moves into the cell by osmosis creating turgor pressure – the cell wall becomes rigid. Since one side of the guard cell’s wall is thicker than the other, the cell bends slightly opening the stoma.
When the sun goes down, the level of sucrose inside the cell increases, and osmosis moves the water out of the cells. Without the turgor pressure, the guard cells’ walls become flaccid, and the stomata pores close. Once the stomata are closed, water inside the plant is conserved.
If there isn’t enough water (or if the plant can’t move the water fast enough on a hot, windy day), osmosis moves the water out of the guard cells, which closes the stomata, conserving water inside the plant. This is a great survival mechanism for the plant! Visibly, it appears as wilting as the cells in the leaves are all flaccid.
Without water, osmosis continues to move water out of the cells until they become completely dehydrated. Gravity pulls water down. So the second thing we notice when we forgot to water is the tips of the leaves are crunchy and look burned.
Finally, without turgor pressure/water pressure in the stem and branches of the plants, those cells also become flaccid and the entire plant lists over to one side or the other. Further dehydration causes more cells to die, so more leaves are lost, which means less potential for the plant to photosynthesize and grow. When dehydration becomes fatal, it is called the permanent wilting point.
Conifers (evergreen plants with needles) utilize this exact same mechanism. Since the leaves/needles are stiff and waxy, we don’t notice wilting. It’s very hard to see drought stress on evergreens until you see a real substantial decline or even death.
When you notice a plant is wilting, check the soil moisture! If it’s dry, the sooner you can get water to it, the better. If the soil is also dehydrated, you will need to drench the plant or even put the pot into a bucket of water to rehydrate. Obviously, when a plant has drought stress its growth is also stunted; however, they can recover. Sometimes it even seems like a miracle when a plant that was shriveled up and dry, opens up and recovers.
For more information on providing water, and conserving water too, check these previous blog posts:
