Air Movement in the Growing Room – It’s Not Just About Transpiration

Let’s start out with a new word for a lot of people.

The Knights Who Say Ni! and who went on to become The Knights (or K’niggits) who say Ekke Ekke Ekke Ekke Ptang Zoo Boing! might just as well have adopted our word, thigmomorphogenesis. Seven syllables, and if you practice saying it, people will soon leave you alone at parties.

It's a good word for growers, because thigmomorphogenesis is good for plants. It is the term for the way plants respond to being annoyed. (Or amused, depending on how you look at it.) It’s a response to movement. It can be wind. Or rain. Or even animals rubbing it in passing. Plants can’t run away, so it’s a natural sort of ability to evolve. Each of the stimuli are things that can damage plants, so the response is to beef up to lessen future damage. Thigmomorphogenesis is not a tropism. Tropism is an organism turning in a particular direction in response to a stimulus. Thigmomorphogenesis is a change in the way the plant grows.

Thigmomorphogenesis induces plants to develop greater stem diameter, so they are less likely to be wind-damaged or broken by contact. Stems grow shorter for the same reason. Less leverage when something tries to bend it. Branches grow shorter. Again, less leverage to the point of breaking. Leaves become more numerous and denser, which I suppose is to compensate for the smaller stature. Chlorophyl content increases, also making the more compact plant more productive.

You might look at two plants and think there were genetic differences when it was just responses to being touched. It is what makes you pea plant wrap a tendril around a support it comes in contact with. And the flexibility changes with whether that part has found a support of is off seeking another and having to support itself.

Thigmomorphogenesis is effective in apparently most stalky plants, like tomato, beans and peas, eggplant and even celery and cauliflower. Commercial growers are interested in thigmomorphogenesis (keep saying it with me – it will become habit) because the daminozide they used to use to induce shorter, greener, stouter, more attractive plants became off limits for food production in 1989. Growers like to market stronger, more compact plants with greener leaves that are resilient and travel well. Those are all things mechanical stimulation does.

To study the phenomenon, the treatment used to induce it is often shaking. That is because the intensity and duration of mechanical shaking can be quantified and counting things is dear to the academic heart. Researchers need variables they can control. But there seem to be multiple variables affecting how much effect shaking has. Much of it is still to be learned. But in general, reduced light increases the effect. And it doesn’t reduce root growth as much as stem growth.

Some researchers have found greater nutrient uptake in shaken plants, but results are mixed. Plants conditioned by mechanical shaking also seem to handle water shortages better, recovering more quickly. The effect also works on small seedlings. Like many good things, it can be carried too far. Vigorous treatment seems to decrease yields.

Plants use touch response for many things. One is avoiding obstacles and their well-known ability to cling to supports. When subjected to need, plants immediately invoke a bewildering array of chemical responses and alter how some genes are expressed. Although Darwin observed and reported on roots avoiding barriers, it wasn’t until 1963 that our word was coined to describe mechanically induced effect.

And there’s almost no part of the plant that doesn’t respond. Some plants strengthen tissues in response to insult. Some make tissues more pliant and less prone to breaking. Younger and more fragile plant tissues have a stronger thigmomorphogenesis response. Probably because they are usually more tender. Plants assume that what has happened before will happen again, and they want to be ready for it. Of course, they’re just dumb plants and don’t think about what they want, but evolution has prepared them.

Active touching and shaking is probably more of a duty than most us want to pursue, especially as it seems easy to overdo it or to get results we would rather not get. For us, the most useful means of providing mechanical stress is artificial breeze from a fan. Not violent wind. Plants subjected to chronic strong winds yield to the wind and grow oddly. But enough wind to move the plant stimulates formation of more robust stems, shorter branches, more leaves and a host of improved functions. 

 

 

As a bonus, maintaining air movement facilitates transpiration. Many cases of edema can be reversed or avoided by playing a small fan on the growing area. Treatment can begin in the first week after germination. Countless chemical and hormonal changes take place.

Happily for us, thigmomorphogenesis is most effective under reduced light. For although we provide adequate artificial light, it is not often equal to the overall intensity of direct sunlight. We couldn’t ask for a better situation in which to use thigmomorphogenesis to our benefit. When deciding how much breeze to give plants, remember that growth may slow as the plant grows shorter and more stout but more robust. Make sure that’s what you want if you use a brisk breeze. And too much wind can cause stomata to close, reducing, rather than enhancing transpiration.

As in nature most of the time, the quality of breeze you want is light, just enough to slightly move the plants. It is even better if the breeze comes from different directions. A small oscillating fan provides something of the same effect. When you use an oscillating fan, the intermittent breeze can be more brisk than one from a constant direction.

Needless to say, an air conditioner is not a good source of breeze. The air from it is dry and may well be more harmful than no air movement at all. And during heating cycles, the plants directly in the bath of an A/C outlet may experience conditions of very unfavorable vapor pressure deficit. Any sign of drying should trigger rethinking.

Remember, you don’t have to be smart to sound smart. You can add this one to geotropism and gravitropism (sends roots down and foliage up), phototropism (growing toward light), aerenchyma (air channels plants develop to survive floods in nature), heliotropism (plants following the sun), hydrotropism (grow either toward or away from water), thermotropism, and related to our word is thigmotropism, growth away from a stimulus rather than a change in structure.