Like other living beings, plants glean a lot of information from their environment. Not only can they detect these cues, but they are also able to respond to them and adapt their behaviours based on them. The fact that plants can respond to their environment shows how sophisticated many plants are – much more sophisticated than plants were initially thought to be!
Of course it makes sense that plants would grow towards the sun – but how do they know which way that is? We’ll walk through the answers to these questions, and use some locally-found invasives as examples. This blog is part of a series of posts called How Do Plants Work?
Plants Respond to Environmental Stimuli
As we explained in How Do Plants Get Energy from the Sun, the timing of different steps in photosynthesis depends on the time of day. Plants sense light’s direction, intensity, duration, and quality and perform their energy harvesting under the most ideal conditions possible.
Many plant species have flowers or leaves which track the sun through the sky over the course of the day. This is called solar tracking. These plants sense in which the direction the light hits them and slightly alter their growth to ensure they face the sun, so they can get to get the most energy possible.
Plants can also sense the intensity of the light they’re receiving from the sun and sense if it’s too intense and burning them. If they sense that the UV rays are too intense, they can make compounds called phenolics and flavonoids, which act as sunscreen for the delicate molecules inside the leaves.
Environmental stimuli (cues that the plant can sense in the environment) control many different aspects of plant growth, reproduction, and evolution. Some of these processes are:
- Seed germination
- Fruit development
- Programmed cell death (ex. leaves dying in Autumn)
- Directional growth
Photoreceptors
Plants sense light cues using photoreceptors. Photoreceptors absorb light photons and trigger different responses in the plant based on the photon’s wavelength. Photoreceptor molecules are similar to chlorophyll in that they interact with photons, but chlorophyll either reflects or passes along the photons while photoreceptors absorb them.
There are several different types of photoreceptors, so plants can sense light quantity, quality, intensity, and duration.
Phytochromes
Phytochromes are one of the most common types of photoreceptor. They’re able to absorb blue, red, and far-red light, and are responsible for many different vegetative and reproductive processes. Blue light is responsible for causing the opening of stomata, the pores in the leaves that allow water vapour to leave and CO2 to enter.
Plants use the ratio of red to far-red light to tell whether or not other plants are shading them. The chlorophyll in leaves absorbs red light easily, so if a plant is shaded (for example, by another plant’s leaves), it will receive more far-red light (compared to the amount of red light it receives). Because they can use many light colours, phytochromes switch back and forth between their red and far-red forms based on the light they receive. The ratio of each form of phytochrome in the plant triggers certain processes in the plant.
Some processes controlled by this ratio are shade avoidance, seed germination, and competitive interactions. Responses in the plant can take anywhere from seconds or months to take place. In the case of changing their growth, the response can take seconds. However, in the case of constant new shade, it can result in the plant changing which genes it expresses, which can take months to years.
How invasive plants utilize their photoreceptors
Yellow Lamium is an example of a species that has adapted to prefer shade or a high ratio of far-red light to red light. Most species don’t thrive in shaded areas since they can’t absorb the red light first; however, Yellow Lamium has evolved to utilize the light that it does receive and takes over shaded areas.
Night-Flowering Catchfly utilizes environmental cues to tell when it’s day and night. Once the sun goes down and red light levels decrease, this species closes its flowers for the night. At sunrise, when red light levels increase again, it opens its flowers back up.
Bull Thistle is an example of a species that does not tolerate shade. This invasive requires full sun and cannot function with too much far-red light. While many species struggle in constant direct light, Bull Thistle has adapted to withstand these extreme conditions. This allows it to outcompete many other species in bare or exposed areas.
Tropisms
Changes in the direction a plant grows as a response to an environmental cue is called a tropism. There are three major tropisms in plants:
- Gravitropism: plants sense and respond to gravity. Roots grow down while shoots grow up.
- Phototropism: plants sense and respond to light. This allows leafy shoots to grow toward the sun.
- Thigmotropism: plants sense and respond to touch. This allows roots to grow around obstacles and for vines and tendrils to wrap around things.
How invasive plants utilize tropism to their advantage
Following their distribution, some Yellow Flag Iris seeds find themselves submerged underwater. This plant uses gravitropism and phototropism to sense which way is up and sends leafy shoots up towards the sun while its rhizomes grow outwards in mats.
This invasive species has mastered thigmotropism. English Ivy has rootlets that form along its vines that excrete an adhesive substance when they touch a surface. This allows them to grow up and over virtually any obstacle in their way. They also wrap around and choke other plants surrounding them.
References
- Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. S. (2015). Plant Physiology and Development (6th ed.). Sinauer Associates Inc.
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Species mentioned in this post
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