Have you ever noticed that when the lights are on, your cannabis plants are nice and perky, but a few hours before the lights go out, the leaves start to droop? They don’t look bad – they just droop. And then the next day, they are back up again. Why is this?
These movements are known as nastic movements. Nastic movements that occur in leaves, flowers or branches in response to the arrival of dusk and dawn are called “nyctinastic”. In the case of cannabis, where it is just the leaves that are moving, we refer to these movements as foliar (leaf) nyctinasty . Foliar nyctinasty is when there is pronounced changes in leaf angle on a daily basis, sometimes called the “sleep movements of leaves”. During the day, the leaves are in a more or less horizontal position and at night, the leaves take on a more vertical position. And to reach that vertical position, the leaf may rise up, fall down, twist to the side, or even fold in half . It’s not just cannabis plants that go through foliar nyctinasty. In fact, over 200 different genera of plants display leaf movements and most of them are in the legume (bean) family.
What are the Advantages of Leaf Drooping?
Why would a plant spend energy moving its leaves up and down? And yes – it does cost the plant energy to move the leaves up and down! Movement is, after all, a form of work, and work requires energy! Since most plant species don’t do these movements, some scientists think they are not critical for plant survival . Instead, leaf drooping is probably just a useful adaptation for those plants that have the ability. During the day, when the leaves are in a horizontal position, they optimize their ability to capture sunlight for photosynthesis . The real mystery is — why do the plants droop their leaves at night?
Over the years, scientists have suggested several theories for why plants droop their leaves at night. Darwin (yes, that Darwin) suggested that drooping could help keep plants warm at night. Drawing the leaves closer together can reduce the exposure of the leaf to the cold night sky, which can keep a plant about 1°C warmer than if the leaves were in a horizontal position . However, there is little experimental evidence to back up this idea, so it still needs further testing!
Another hypothesis is that drooping could help remove excess water stuck to the leaves. When a leaf moves into a vertical position, water droplets on the surface of the leaf will roll off. Research shows that water droplets on the surface of leaves can reduce photosynthesis during the very early morning before the leaf surface dries completely. Reducing leaf water may also help prevent the growth and spread of pathogens, like powdery mildew . However, once again, this theory needs further testing.
The theory with the most evidence says that leaf drooping could prevent herbivory from small animals and insects. A drooped leaf may be harder for an animal or insect to access and eat. A drooped leaf might also give less shelter for the herbivore and so the herbivore might become dinner for another animal .
Figure 1: Turgor change in cells at the leaf base causes the leaves to move up and down.
How does Leaf Drooping Happen?
Leaf drooping is regulated by internal (circadian clock) or external (environment – light) processes or both . A leaf droops when the cells at the base of the leaf change shape . The cells at the leaf base may swell or shrink, depending on how much water is inside the cells. This swelling and shrinking are known as turgor (Figure 1). Depending on the strain, the plant may use other methods to move the leaf. Plants use a variety of chemicals to signal to the leaves that they should start drooping, however, the exact mix of chemicals is strain-specific . As mentioned above, leaves can droop in a variety of ways, including twisting and folding. And each of these ways requires a completely different set of chemicals !
So if you notice your cannabis plants drooping a bit before bedtime, know that they are just going through their predictable leaf movements. You can go ahead and give them a goodnight kiss knowing they will perk back up in the morning.
- Minorsky, P. V. (2018). The functions of foliar nyctinasty: a review and hypothesis. Biological Reviews.
- Darwin, C. (1880). The Power of Movements in Plants. London: John Murray.
- Hart, J. W. (1990). Tropisms and Other Forms of Plant Movement. In Plant Tropisms and Other Growth Movements (pp. 1–22).