How to kill a plant: death by desiccation

Have you forgotten about a plant and come back to find it shriveled, dry and dusty? You are not alone. Plenty of people subject their plants to a bit (or total) neglect.  

Diagram of water traveling up a coleus plant from root to leaf. Image source.

Diagram of water traveling up a coleus plant from root to leaf. Image source.

Of all the resources plants need - carbon dioxide, oxygen, minerals, nutrients, - the need for water is paramount.  In my last post Death by Drowning, I wrote about plants sucking water up through their roots in order to quench this titanic thirst. To put the magnitude of this thirst in perspective, a mature tree can absorb hundreds of gallons of water on a hot summer day. The vast majority of this acquired water is pulled up through the plant’s xylem cells and transpires out from the surface of the leaves as carbon dioxide diffuses in. Only a measly three percent is used in photosynthesis and other metabolic processes. 

On its way through a plant, water is detrimental to building internal hydrostatic pressure, called turgor pressure, within plant cells. Plant cells, unlike animal cells, have tough, ridged cell walls. They need a certain amount of water in them to keep them turgid.  If there is too little water the pressure against the cell walls relaxes. Kind of like a helium balloon a few days after it’s blown up. A wilty plant is the response we see when plant cell walls relax.  

 
 

All is not lost if a plant wilts! For a plant, the Earths atmosphere is relatively dry, so plants have evolved mechanisms to absorb and retain water. A wilted plant can recover because these mechanisms allow plants to function briefly when there isn't enough water to maintain turgor.  

At the first signs of inadequate water, like a hot sunny day, a plant will close its stomata. These pores, on the undersides of leaves, are where water transpires out and carbon dioxide is absorbed into the plant. Most plants keep their stomata open during the day to collect carbon dioxide and maintain their metabolism. If the stomata are closed and the plant doesn't have a continual supply of carbon dioxide, photosynthesis slows down and metabolism stops resulting in a plant siesta. Once the temperature decreases, these plants can reopen the stomata and allow metabolic processes start chugging away.

This plant has been loosing it's lower, older leaves in order to redirect resources to younger leaves. Photo by Claire Collie.

This plant has been loosing it's lower, older leaves in order to redirect resources to younger leaves. Photo by Claire Collie.

Extending your roots is another mechanism for a plant to survive desiccation.  New soil area just outside of a roots reach might contain untapped sources of water that will help rejuvenate the plant.  However, this solution won’t always be successful. Often there just isn’t any more water to suck out of the soil, especially if you are a potted plant on the windowsill and rely completely on someone to water you.  

If a plant is watered intermittently, but not enough to be healthy, it may perform a plant version of senicide by abscising, or losing, its more mature leaves.  Without the oldest leaves to support, a plant can direct it’s remaining resources to the newer leaves. 

Am I scaring you off of houseplants? Does having a cat or dog or pot-bellied pig seem easier to care for? The balance between too much water and too little water for a plant is not that fine of a line to walk - you just have to understand the environment the plant is from. 

The desiccation tolerance of a plant varies based on what kind of plant it is and where the plant is from.  For example, trees are different than vegetables and the Great Plains biome is vastly different from the rainforest. Tapping into this information can allow you to be best plant owner you can be.  

For example, there are environments outside of your office windowsill that are dry most of the time.  Plants in arid, desert biomes have evolved more substantial mechanisms for coping without water.  

Some natives to the desert tough out dry periods by having a large network of roots, and thick resin covered leaves that don’t allow for much transpiration.  The trade-off for reduced transpiration is decreased photosynthesis, so these plants grow slowly.  Other plants don’t even try to live during periods of drought; perennial plants often loose their leaves during dry periods, and grow new ones when there is adequate water to support growth.  Annual plants, including many spring desert flowers, often complete their whole life cycle during the wetter seasons so only the seeds need to survive dry, hot weather.

The best-known drought tolerant plants are succulents and cacti. These plants store water in their swollen leaves and stems and use it sparingly.  During the day their stomata are usually closed to reduce transpiration. The spines cacti are known for aren't just to protect against predators, they also help reduce water loss. Dense spines (or hairs) can create enough shade to reduce the surface temperature of a plant. They also create a moist layer of non-circulating air around the surface of a plant. Lastly, if there is enough moisture in the air (a misty morning in the desert?) water that condenses on the vast surface area of spines will drip to the ground and be absorbed by roots.  

Moral of this story? Choose your houseplants wisely, and don’t forget to water them.

Just not too much.


There are lots of ways to kill a plant